Table of Contents - GitHub€¦ · [GCC 4.2.1 (LLVM, Emscripten 1.5)] on linux2 This is just...

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1.2.10

1.2.11

1.2.12

1.2.13

1.2.14

1.2.15

1.2.16

1.2.17

1.2.18

1.3.10

1.3.11

1.3.12

1.3.13

1.3.14

1.3.15

1.3.16

TableofContentsIntroduction

1.0-Programmingbasics

1.1-Interactivecoding

1.2-Strings

1.3-Nilandvariables

1.4-Usingfunctions

1.5-Commentsincode

1.6-Scriptingandprinting

1.7-Makingfunctions

1.8-Booleans

1.9-Flowcontrol

1.10-While

1.11-Typechecking

1.12-Firstgame

1.13-Tables(part1)

1.14-Tables(part2)

1.15-Forloops(part1)

1.17-Scopes

1.18-Chapterreview

2.0-IntroducingLÖVE

2.1-Upandrunning

2.2-LÖVEstructure

2.3-Geometry

2.4-Gameloop

2.5-Deltatime

2.6-Mapping

2.7-Theworld

2.8-Readingdocumentation

2.9-Modulesandorganization

2.10-Collisioncallbacks

2.11-Breakout(part1)

2.16-Binaryandbitmasks

1.3.181.3.17

2.17-Networking(part1)

3.0-Programmingin-depth

3.01-Primitivesandreferences

3.02-Higher-orderfunctions

3.03-Mapandfilter

3.04-Stackandrecursion

3.05-Reduce

learn2loveCurrentprogress:

Chapter1-Programmingbasics✔Chapter2-IntroducingLÖVE✔Chapter3-Programmingindepth(inprogress)Chapter4-LÖVEindepth(todo)

Viewasawebpage:link

Downloadinebookformat:pdf-epub

Whatisthisbook?ThisbookteachesprogrammingfromthegroundupinthecontextofLuaandLÖVE.Itteachesbasiccomputerscienceandsoftwarebuildingskillsalongtheway,butmoreimportantly,teachesyouhowtoteachyourselfandfindouthowtogoaboutsolvingaproblemorbuildingasolution.Toolscomeandgo,sothegoalistoteachthingsofvaluewithlessfocusontheprogramminglanguageandothertoolsusedtobuildthesoftware.Ihavebeenprogrammingsince2007,focusingonteachingmyselfbestpractices.AlongthewayIhavefoundalotofgoodandbadtutorialsontherightandwrongwaytobuildthingsandIwanttohelpothersavoidgettingstucklikeIdid.

Whoisthisfor?Anyagegroup.Kidstoo,withabitofdemonstration,helpandencouragement!Anybodythatwantstolearnbasiccomputerscience.Thisbookwilltouchonseveralcomputersciencesubjectsinordertobuildprograms.Anybodythatwantstolearntoprogram.Nopriorskillsorknowledgerequired.Anybodythatwantstolearntomakeagame.Makinggamesarefunandrequirelearningmanythingsalongtheway.We'llbuildafewthroughthisbook.AnybodythatwantstolearnLua.Althoughwewon'tdiveintotheadvancedfeaturesofthelanguage,wewillgainalargeunderstandingonhowthelanguageworksinordertoactuallybuildsomethings.Therearealreadyonlineguidesandreferencescoveringsomeofthemoreadvancedtopics.ForexperiencedprogrammerswantingtolearnLua,theProgramminginLuabookmaybesufficient.

Authorandcontributorsjaythomas:OriginalauthorJimmyStevens:Editsandsuggestionsinchapter1&2rm-code:Chapter2gettingstartedValentinChCloud:Chapter3primitivesandreferences

ContributingIssues,comments,andsuggestionscanbemadeusingtheGitHubissuespage.Todownload,build,andrunthebookoranycodeexamplesusethe"Cloneordownload"buttononthemainrepositorypage.

Introduction

Fordevelopersandthecurious

Feelfreetosubmitapullrequest.ThedocumentationisbuiltusingNodeJS.Ifyouwishtorunthedocumentationforlocaldevelopmentpurposes,installnodejsthenrunthesecommandsfromwithinthe learn2lovedirectoryyoudownloaded:

npminstall#Downloadsbuildtoolstothea"node_modules"folderinsidethedirectory

npmstart#Createsalocalwebservertowhereyoucanvisitthelinkhttp://localhost:4000

Oncethelocalwebserverisrunning,anyeditsyoumaketothepageswillrebuildthebookandreloadthepageyou'reviewing.

Introduction

Chapter1:ProgrammingBasicsThegoalofthischapteristoteachthemostnecessarybuildingblocksofprogramming.Bytheendofthechapteryouwillbebeabletobuildbasicprogramswhichwewillapplywithexercisesinthefollowingchapters.

1.0-Programmingbasics

Interactivecoding

What'saREPL?Programmingdoesn'ttakemucheffortbeyondloadingupaREPLandjusttyping.WhatisaREPL?It'saninteractivewindowyoucantypecodeintoanditspitsouttheresultsonscreenwhenyouhitenter.ItstandsforRead-Evaluate-Print-Loop.Thesearethe4thingstheREPLdoes:

1. Readthecodethatwasjusttyped2. Evaluate,orprocessthecodedownintoaresult3. Print,orspitouttheresult4. Loop...doeverythingagainandagainuntiltheprogrammerisdone

It'sactuallysimplerthanitsounds.Let'sgotoawebsitewithaREPLandtryitout:https://repl.it/languages/Lua

Youwillseetwowindowpanesonthewebsite:alightsideontheleftanddarksideontheright.Theright-sideistheREPLandiswhatwe'reinterestedinfornow.Ithasalotofinformationthatisn'tnecessarilyusefultousatthemoment.Somethingsimilartothis:

[GCC4.2.1(LLVM,Emscripten1.5)]onlinux2

ThisisjusttellingyouwhatprogramminglanguagethisREPLisloading,inthiscase,Lua.Ifyouclickinsidethewindowpaneandstarttypingyouwillseeyourtextappear.

Let'strytypingsomecodefortheREPLtoRead.Youalreadyknowsomecodeifyouknowarithmetic.Type:

ThenhitENTERandimmediatelytheREPLwillPrintout:

Alothappenedveryquickly.AfterhittingENTER,theREPL,Readtheline 2+2,itEvaluatedthevalueofthatstatementtobe 4,itPrinted4onthescreenforyou,thenLoopedbacktoanewlinetoawaityournextcommand.Tryoutsomemorearithmetic.Multiplication:

Subtraction:

Division:

Youcanuseparenthesistotellitwhichordertodotheoperations:

(2+2)*(3+1)

Whichgivesdifferentresultsthan:

2+2*3+1

IfyougivetheREPLasinglenumber:

Itwillgiveyouback 12,becausethiscan'tbesimplifieddownanyfurther.

Youcanalsodoexponentsusingthe ̂ (caret)symbol:

Numbersareatypeofdata,and +, -, /, *, ̂ , %areoperators.Statementssuchas 2-2and 23*19arealloperations.

Onelastarithmeticoperationwe'llcoverismodulo,whichisdonewiththemodulusoperator.Themodulusoperatorisrepresentedinmostlanguagesasa %(percent)symbol:

Modulusoperationsaren'tseeningradeschoolclassroomsasoftenastherest,butarequitecommoninsoftwareandcomputersciences.Thewayitworksisyoutakethe2ndnumberandsubtractitfromthebiggernumberasmanytimesaspossibleuntilthe2ndnumberisbiggerthanthe1st.Theresultiswhat'sleftofthe1stnumber.With 8%3,ifyoukeepsubtracting 3from 8thenyouendupwith 2left.

Arealworldexampleistimeelapsingonananalogclock.Imaginethefaceofaclockwiththehourhandonnoon.If25hourspassthenthehourhandgoesallthewayaroundtwiceandendson1.Thatwouldbeequivalenttowriting:

Thehourhandresetseverytimeitpasses12,so 13%12, 25%12,and 37%12wouldallequal 1.Likewise, 10%4resultsin 2because4goesinto10twice,andleavesaremainderof2.

ExercisesTrytypingdifferentmodulooperationsinandguessingwhattheanswerwillbe.Tryusingnegativenumbers( -3+-2).Tryusingasetofparenthesisinsideanothersetofparenthesis.Doesitbehaveasyouexpect?Afterrunningthroughalltheexercisespressthe'up'keyintheREPL.Whathappensandhowcanthisspeedupyourwork?

StringsNumbersareonetypeofdatathatcanbeoperatedon.Let'sexploreanotherdatatypewithintheREPL.TakeasetofquotesandputsometextinitandhitENTER:

"hello"

TheREPLwillprint hellobacktoyou.Thisisastring.Astringisasetofcharacters(lettersandsymbols)stringedtogetherasonesinglepieceofdata.Thisstringismadeof9characters:

"H-E-L-L-O"

Likenumbers,thereareoperatorstomakestringsplaywitheachother.Theconcatenateoperator( ..)combinesstringstogether:

"hello".."world"

What'stheresult?Noticethattheresultingstringhasnospacebetweenthetwowords.Ifyouwantedaspace,youwouldhavetoputoneinthequotestobepartoftheoperation:

"hello".."world"

Youcouldevenmakeaseparatestringwiththespaceinit:

"hello".."".."world"

Stringscanhaveanycharactersinthemthatyouwant.

"abc".."123"

"Япо́нский".."ロシア語!!"

ExercisesTryusinganarithmeticoperatoronstrings "hello"/"world".Whathappens?Tryusingtheconcatenateoperator( ..)onnumbers( 1..1).

1.2-Strings

Nilandvariables

Data,orthelackthereofHumanshavedifferentwaysofrepresentingalackofdata.Iftherearenosheeptocountthenwehavezerosheep.Iftherearenowordsonapagethenthepageisblank.Inacomputerwemayrepresentthenumberofsheepas 0orthemissingwordsonapageasanempty "".Thesearestilldatathough...anumberandastring.Insoftwarewhenyouwanttorepresentalackofdatawehave:

Sometimescalled nullor undefineddatainotherlanguages.It'sseeminglyuseless.Youcan'tuseoperatorsonnil.

nil+nil

Thiswillprintanerrorlikeitdidwhenyoutrieddoingarithmeticonstrings.Let'stakealookatvariablesandwe'lldiscoverthepurposeof nil.

VariablesSometimesyouwanttowriteoutdata,butyouwantthatdatatobeeasytochange.Variablesletyougivedataanametoreference.Here'sanexampletotry:

name="Mandy"

"hellomynameis"..name

Sinceyoutolditwhat nameis,itknowswhatvaluetoaddtothestring "hellomynameis".Ifyoutype:

...andhitENTER,itwillprintoutthevaluethatbelongstothisvariabletoremindyou.The =(equal)signtellsLuathatyouwanttoassignavaluetothegivenname/variable.Youcanchangethevalueofavariableandgetdifferentresults:

name="Jeff"

"hellomynameis"..name

Assignmentisn'tthesameasitisinAlgebra.Youcanchangethevalueofavariablemultipletimes.Wecantell namethatitequalsitselfwithsomeadditionalinformationconcatenatedtoit:

name="abc"

name=name.."def"

Youcanassignanytypeofdatatoavariable,includingnumbers:

name="Jeff"

1.3-Nilandvariables

age=16

"hellomynameis"..name.."andIam"..age.."."

Youcanchangenumbersafterassignmenttoo:

age=16

age=age*2

"myagedoubledis"..age

So,whatifyoutypeinamadeupvariablename?

noname

Youwillseeithas nil,ornodatayet.Ifyoutrytouse nilinyourstringoperationyouwillgetanerror:

"hellomynameis"..nil

[string"return"hellomynameis"..nil"]:1:attempttoconcatenateanilvalue

"hellomynameis"..noname

[string"return"hellomynameis"..noname"]:1:attempttoconcatenateglobal'noname'(anilvalue)

Tryassigningavaluetoavariablename:

best_color="purple"

thenassigningthatvariabledatatoanother:

worst_color=best_color

worst_color

You'llseethatbothvariablesnowhavethevalue "purple".

Variablescanhavenamesmadeupofletters,numbersandunderscores( _).Variablenamescannotbeginwithanumberthough,otherwiseitwillthinkyou'retryingtotypeinnumberdata.Here'ssomeexamplesofvalidvariables:

my_dog="Poe"

myDog="Zia"

DOG3="Ember"

ExercisesTryoutdifferentvariablenames.Tryafewinvalidvariablesnamestoojusttoseewhattheerrormessagelookslike.It'simportanttoseeerrormessagesandunderstandthem.Theyhelpyouunderstandhowaprogrambreakssoyoucanfixit.

1.3-Nilandvariables

UsingfunctionsMostprogramminglanguagescomewithsomevariablesalreadydefinedforus.Luahasmany,solet'stypeoneinandhitENTERtoseewhatthevalueis:

string.reverse

=>function:0x2381b60

Ohmy.So"function"isanotherdatatype,butwhatis 0x2381b60?It'sjusttellingyouwhereinthecomputer'smemorythatfunctionexists,justincaseyouwantedtoknow.Functionsworkverydifferentlythannumbersinstrings.Essentiallyfunctionsarepre-definedinstructionsthattelltheprogramhowtododifferentthings.Theytakedataandreturnbackdifferentdata.Let'sseehowtogivethisfunctiondata:

string.reverse("hello")

=>olleh

Attheendofthefunction'svariablename, string.reverse,wetypeasetofparenthesis, string.reverse(),andputinsidetheparenthesissomedatawewantchanged( string.reverse("hello")).Makingthefunctionrunisoftencalledinvokingthefunction.Havingafunctionthatreversestextinastringforuscanbeuseful,andwecancapturethereturnvalue(theresults)ofthefunctionusingavariable.Tryitout:

greeting="hello,howareyou?"

backwards_greeting=string.reverse(greeting)

backwards_greeting

=>?uoyerawoh,olleh

Itshouldbeobviousfromthenamewhatthatfunction'spurposeis.Howaboutthisone?

string.upper("hello,howareyou?")

Nowtrycapturingthatvaluebyassigningittoavariable:

greeting="hello,howareyou?"

shouting_greeting=string.upper(greeting)

crazy_greeting=string.reverse(shouting_greeting)

Wecangetcrazier.Howaboutinvokingafunctionwheninvokinganotherfunction??

string.reverse(string.upper("hey"))

What'shappeninghereisthestringisbeinguppercasedby string.upperbutthenthevaluefrom string.upperisbeingreversedby string.reverseassoonasitisdone.It'sjustlikeinarithmeticwhenyouhavenestedparenthesis.Theinner-mostparenthesisareresolvedbeforedoingtheouter-mostparenthesis.

Let'stryonemorefunction.Thisfunctionhastwoparameters,meaningitacceptstwopiecesofdatawhichitrequirestoworkproperly.

1.4-Usingfunctions

Let'stryonemorefunction.Thisfunctionhastwoparameters,meaningitacceptstwopiecesofdatawhichitrequirestoworkproperly.

math.max(7,10)

Whengivingmorethanonepieceofdatatoafunction,youneedtoputacomma( ,)betweentheparameters

Thesearegreatfunctions,butwouldn'titbegreatifwecouldmakeourown?We'llgiveitashotinjustafewpages.

ExercisesSeeifyoucanfigureoutwhat math.maxdoes.Giveitdifferentnumbersandexaminetheresult.Thereisanotherfunctioncalled math.minthatalsotakestwonumbers.Whatdoesitreturn?

1.4-Usingfunctions

CommentsincodeSometimeswemightwanttowriteacommentinourcode–anexplanationtoafriendorourfutureselvesonwhatthepurposeofsomecodeis.Perhapswewanttowriteanotetoourselvestochangesomethinglater.Commentsworkverysimilarlyindifferentlanguagessothey'reprettyeasytoreadevenifyoudon'tunderstandtheprogramminglanguageorthecodeitself.Luadenotesacommentas --andanytextthatfollowsit:

--Thisisacodecomment

--Thisisanotherlineofcomments

Thesecommentswillbecompletelyignoredbythecomputerandaremeantforthehumantoread.Commentscanalsobeonthesamelineascode.Thecomputerwilljustignoretherestofthelinewhenitseesacommentstarting.

1+1--Thisismycomment.Thiscodeaddssomenumberstogetherincaseyoudidn'tknow!

Youwillseecommentsappearinfutureexamplecode,sodon'tletitsurpriseyou!

1.5-Commentsincode

ScriptingandprintingLookingbackatthewebsite,(youbookmarkedit,right?)wehavebeenusingtheREPLwindowpaneontheright,buthaven'ttalkedaboutthepaneontheleft.Thiswindowisjustatexteditor.Insteadofrunningtheprogramwitheachlineyoutype,itallowsyoutowritemultiplelinesofcodebeforeexecutingitall.Let'strytypingsomethinginit.Onceyouaredonetypingallthecodeyoucanclickthe"Run"button.

number=4

number=number+1

Butwhenyouclickrun,nothinghappens.Okwellnowthatyouarewritingaprogram,youneedtoreturndata.Solet'sprovideanotherstatementtoourprogram.

number=4

number=number+1

returnnumber

NowwhenyouclickRun,thetext =>5appearsintheconsole.Whenyoutoldittorun,itreadandevaluatedeachlineofthecode,thenwhenitgottothelinewith returnonit,theprogramstoppedandreturnedthevalueyouaskedittoreturn(inthiscase 5).Ifyouwriteanycodeafterthereturnstatement,itwillcauseanerrorwhenwetrytoruntheprogram,sothereturnstatementshouldbethelastthinginourfile.

number=4

number=number+1

returnnumber

--Thislinewillcauseanerror:

number=10

Youcanreturnanytypeofdata,notjustnumbers:

return"hello"

Rememberthoseotherfunctionsweusedbefore?Youcanwritethoseaspartofthereturnstatement.

returnstring.reverse("hello")

Sometimeswhenwritingprograms,wewanttopokearoundandseevalueswhiletheprogramisrunningandnotwaituntilitisdone.Thisissocommonthatthereisafunctionthatprovidesthisforus.

print("hello")

return"world"

The printfunctiontakesanydataandprintsthevalueinthewindowpaneontheright.

"hello"

=>"world"

So "hello"isbeingprintedand =>"world"isbeingreturned.Youcaninvokefunctionsonthesamelinewhenprintingifyoureallywanttogetcrazy:

print(string.reverse("hello"))

return"world"

The printfunctionand returnstatementwillbothcomeinhandywhentestingthefunctionswe'regoingtowrite.

ExercisesWhenwepassdataintoafunction,itiscalledanargument.Wepassed1argumentinto printbutitcanpassintwo,orthree,ormore.Whatdoesitlooklikewhenyouprintmultiplearguments?Funtip,whenusingatexteditoralong-sidetheREPLyoucanrunthecodewithoutthemousebypressing'command+enter'onMacand'Alt+enter'onWindows.Doesthisspeedupyourlearning?

MakingfunctionsFunctionsarethethirddatatypewe'veseen.We'veaccessedsomevariableswherefunctionsweredefinedforusandhadablastusingthem(IknowIdid).Functionsarethebuildingblocksofsoftware.YoucancomposethemthensnapthemtogetherlikeDanishplasticblocks.Ittakestimetounderstandhowtheyworkandmuchlongertomastertheirinnerpower.Sowithoutfurtherado,let'sseewhattheyactuallylooklike:

function()

return4+4

Typeitoutinthetexteditorwindowandletusbreakthisdownlinebylineandwordforword.Wheneverwetypefunction()wearebeginninganewfunction.The2ndlineisthebodyofourfunctionwherethingshappen.Thebodyofthefunctioncanbemanylineslong.Thebodyofthefunctioncouldalsobeempty(butthat'snotveryuseful).Onthelastlineofthefunctionbodywereturndata.Thisreturnstatementwon'tendourentireprogramthough!Itwillonlytellthecomputerwe'refinishingupwithourfunction.Thenonthethirdline,we'retellingthecomputerwe'redonewritingourfunction.Inordertousethisexamplefunction,weshouldprobablyuseavariabletogiveitaname:

add=function()

return4+4

Thefirstbitshouldbeunderstandable.Wedeclaredavariablecalled add,thenweassignedsomedatatoitontherightoftheequalsign.Inthiscase,ourfunction.Nowitisreadytouse.

add=function()

return4+4

result=add()

returnresult

We'vemadeourveryownfunctionwithourveryownnameforitandeveninvokeditandgotbackdata!Ifyouinsteadgotanerrormessage,doublecheckwhatyoutypedthatnothingismissing.Errormessagesgiveyoualinenumberofwheretofindtheerrorthatcrashedtheprogram.

Takealookforaminuteathowweinvokedourfunction:

Wetypedoutthevariablenamethatourfunctionisassignedto,followedbysomeparenthesis.Inthoseparenthesisisthedatathatwepassedintoourfunction...waitaminutetheparenthesisareempty.Wedidn'tpassanydataintoourfunction.Wheneverwecalledthoseotherfunctionswepassedindata,likewhenwepassed "hello"intostring.reverse("hello").Whatifwemodifyourlinewhereweinvokeourfunctionandgiveitsomedata?

add=function()

return4+4

result=add(16)

1.7-Makingfunctions

returnresult

Itseemsitalwaysreturns =>8nomatterwhatargumentswetrytopassin.Weneedtorewindtothefirstlineofourfunctionandtakeacloselookatthisbit:

add=function()

The ()attheendof function()iswherewetellourprogramhowmanyargumentsweareaccepting.Iftheparenthesisareempty,thenourfunctionisignoringallargumentsandwilllikelyalwaysreturnthesameresult.Let'stweakthefunctionslightlyandgiveitoneparameterwiththename a.Let'salsotweakthesecondlinewhilewe'reatit:

add=function(a)

returna+4

result=add(16)

returnresult

Nowwhenwepassindifferentnumbers,wegetdifferentresults:

add=function(a)

returna+4

print(add(16))

print(add(12))

Tocompletethisfunction,let'sgiveitasecondparameterof bandmodifythereturnstatementinthefunctionbody:

add=function(a,b)

returna+b

print(add(16))

print(add(12))

Ifwetryandrunthecodenow,we'llgetanothererror:

[string"add=function(a,b)..."]:2:attempttoperformarithmeticonlocal'b'(anilvalue)

Let'sreadthiserrorcarefully.Itissayinginsidethesquarebracketsthatanerroroccurredwhenusingthefunctionwedefined( add=function(a,b)...).Totherightofthesquarebracketsitissayingline2( :2)ofourtextistheparticularlocationofthecrash.Totherightofthelinenumberiswhathappenedthatmadeitcrash.Ittriedtoperformadditionwith a+bbutthevalueof bwasnil.Westatedthatourfunctionrequirestwoparametersnow, aandb,andourprogramwillcrashifwetryandinvokethefunctionwithonlyoneparameter.Let'smodifythelineswhereweinvokethefunctiontogiveittwoargumentseachtimeweinvokeit:

add=function(a,b)

returna+b

print(add(16,10))

1.7-Makingfunctions

print(add(12,2))

Great,everythingisworkingagain!Withtheexperienceofourfirst,fully-functionalfunction,wecannowstarttreadingthewatersofthisgreatworld.

ExercisesTogetusedtowritingfunctions,trywritingsomecomplimentaryfunctionsnamed subtract, multiply, divide,or modulate(modulus).Makea concatenatefunctionthataccepts2stringsandreturns1combinedstring.Trymakingafunctionthattakes3ormoreparameters.

1.7-Makingfunctions

BooleansDatatypesarelikeelementsontheperiodictable.Themoreelementsyouhavethemorechemicalscancreate.Luckilytherearen'tasmanydatatypesasthereareelements.Infactwe'velearnedalmostallofthem.Thereareonlytwopossiblebooleans:

That'sright.Andyoucanassignthemtovariablesjustlikenumbers,strings,nil,andfunctions:

myboolean=true

print(myboolean)

Thecoolthingwithnumbersandstringsisyoucanusethemtocreatestatementsthatcanbeevaluatedas trueorfalse.Letmegiveanexamplebyintroducingsomenewoperators.TrytheseoutintheREPL:

=>true

=>false

"5isgreaterthan3"isatruestatementsoitreturnsa trueboolean.Naturally,"5islessthan3"isafalsestatementandreturns false.Wecanchecktoseeiftwonumbersareequalinvalue:

number=5

number==5

=>true

Byusingadoubleequal( ==)wecancomparetheequalityoftwonumbers.Thisalsoworksforstrings:

"hello"=="hello"

=>true

"hello"=="HELLO"

=>false

1.8-Booleans

Forstrings,oftentimeyouwillseesinglequotes ''(apostrophe)usedinsteadofregularquotes(sometimescalleddoublequotes)wrapperaroundthetext.Luadoesn'tcareaslongasthetextinsidebothstringsareidentical.Wecanprovethatwithanequalitycheck:

'hello'=="hello"

=>true

Anyways,youcanalsodotheinverseofanequalitycheckandcheckforinequality(iftwothingsarenotequal):

=>true

"HELLO"~=string.upper("hello")

=>false

Nowlet'sdiginalittledeeperwithtwomoreoperators.Firstisthe andoperator:

3<4and4<5

=>true

ThisreadsoutalmostasplainEnglish.3islessthan4and4islessthan5.Thisisalogicallysoundstatementsoitevaluatestotrue.Justtobeclearonwhat'sactuallygoingonherethough,let'sbreakitdown.Whatwesaidisbeinggroupedinto3separateoperations:

(3<4)and(4<5)

Thetwosetsofparenthesisareevaluatedfirstandinternallythecomputerbreaksthesetwooperationsdownto:

(true)and(true)

Trueandtruearebothtrue.Thissoundssilly,butitisindeedlogicallysound.Let'stryonemorejusttogetthehangofit:

"hello"=="hello"and6>10

Finally,let'stryonemoreoperatortoputabowonthings.Sometimeswedon'tcarethatbothoperationsarecorrect.Weonlycareifone ortheotheriscorrect.

4==10or4~=10

=>true

1.8-Booleans

1>100or12==12or"hello"=="bananas"

=>true

Aslongasoneoftheoperationsiscorrect,theentirestatementislogicallytrue.Withtheintroductionof trueandfalsewe'vebroughtinalotofnewoperators:"greaterthan"( >),"lessthan"( <),"equal"( ==),"notequal"( ~=),"and"( and),and"or"( or).

TriviaBooleansgettheirnamefromGeorgeBoolewhoinventedbooleanalgebra,whichwe'vejustseenalittlebitof.

ExercisesTrywritingdifferentstatementswithallthenewoperators.Tryusingtwo andoperatorsinthesamestatementandseeifyoucanmakeitevaluateto true.Tryoutthesetwobonusoperatorswithsomenumbers:"greaterthanorequalto"( >=),and"lessthanorequalto"( <=).

1.8-Booleans

FlowcontrolTypicallythecomputerstartsatthetopofourscriptandreadseachlinedowninasequence.WemaketheprogramsjumparoundwithfunctionsinthemixTrythisoutinthetexteditor:

print("I'mcalled1st")

add=function(a,b)

print("I'mcalled5th")

returna+b

subtract=function(a,b)

print("I'mcalled3rd")

returna-b

print("I'mcalled2nd")

subtract(16,10)

print("I'mcalled4th")

add(12,2)

Wehaveafunctionthatissavedtothevariable addbutitisn'tinvokeduntilfurtherdowninthecode.Soinasenseourprogramhasworkeditswaydownthepagethenjumpedbackuptothefunctionandworkeditswaythroughthebodyofthefunctionthenpickedbackupwhereitwasbefore.Inasimilarfashion,wecanmakeourprogramtakeonepathoranotherdependingifthedatais trueor false.

noise=function(animal)

if(animal=="dog")thenreturn"woof"end

return""

print(noise("dog"))

print(noise("rabbit"))

Let'sanalyzethisfunctionlinebyline.Thefunctioniscallednoiseandtakesananimalname(string)asaparameter.Onthenextlineitsaysif"animalisdog"istruethenreturnsomethingspecial.Weputan endattheendofourstatementtomakeitobvioustothecomputer.Ifthestatementwasfalse,then "woof"doesnotgetreturned.Insteadanemptystring( "")getsreturned.Whenweinvokethefunctionwiththeargument"dog"thenwegetback"woof!".With"rabbit"wegetbacksilence.Maybetherabbitdoesn'twantthedogtohearwheresheis.Let'smakeourfunctionmoreversatilebyaddingmoreanimals:

noise=function(animal)

ifanimal=="dog"oranimal=="wolf"thenreturn"woof"end

ifanimal=="cat"thenreturn"meow"end

return""

print(noise("dog"))

print(noise("cat"))

print(noise("wolf"))

1.9-Flowcontrol

Wehavebranchingpathshappeningwithinourfunction.Ifweweretomapoutthesebranchesitmaylooksomethinglike:

+-->"woof"

+-->"meow"

+-->""

There'snorequirementthatastatementhastobeallwrittenoutononeline.Sometimeswhendoingmultiplethingsinsideanifstatementwemaywanttoputitonmultiplelines:

ifmy_age>17then

print("You'reanadult!")

print("Getajob!")

Similartofunctionshavingbodies,everythingbetween thenand endisconsideredthebodyoftheifstatement.Sometimesitisnecessaryforourbranchestohaveforkswithinthem.Let'ssayourfunctiontakesalanguageasasecond,optionalparameter:

noise=function(animal,language)

ifanimal=="dog"oranimal=="wolf"thenreturn"woof"end

ifanimal=="cat"thenreturn"meow"end

ifanimal=="bird"then

iflanguage=="spanish"thenreturn"pío"end

return"tweet"

return""

print(noise("dog"))

print(noise("bird"))

print(noise("bird","spanish"))

Theifstatementforcheckingiftheanimalisabirdis4lineslong.Oncewefindoutthattheanimalisabird,whilestillinthebodyoftheifstatement,westoptocheckandseeifthelanguageissettoSpanish.Ifitis,weendupinsideanifstatementwithinanifstatement!Otherwisewe'llreturn "tweet"ifthelanguageisn'tSpanish.Maybemappingoutthepathswillclearthingsup:

+-->"woof"

+-->"meow"

+----->"pío"

|+-->"tweet"

+-->""

Ourcodecangetunreadableveryquicklyifwestartnestingifstatementsinsideeachother.Fortunatelydoingsoisn'tusuallynecessary.

Let'stalkaboutanotheraspectofifstatements.SupposeIhavetwobranchesofcodethatareoppositeofeachother:

ifdaytime==truethen

thermostat=71

ifdaytime==falsethen

1.9-Flowcontrol

thermostat=68

Ratherthanwritingthisoutastwoifstatementsandcheckingthevalueofdaytimetwice,Icantakeadvantageofthekeyword else:

ifdaytime==truethen

thermostat=71

thermostat=68

Thatwayif daytimeisnot true,itwilldefaulttothesecondbranch.Youcouldreadthisoffalmostlikeasentence:"Ifdaytimeistruethensetthethermostatto71,otherwisesetthethermostatto68."Nothavingtocheckthingstwicewhendoingcomputationssavesustimeandmakesourprogramrunmoreefficiently.Since daytimeisabooleaninthiscase,wedon'tneedtocheckifitistrueorfalse.Wecanjustpassittotheifstatementtobecheckedfortrue/ falseandmakeouroperationevensimpler.

ifdaytimethen

thermostat=71

thermostat=68

Better."Ifdaytimethensetthermostatto71,otherwisesetthermostatto68."There'sonemorefeatureofifstatementsweshoulddiscuss.Ifthereisanotherconditionyouneedtocheck,maybeseveralmore,youcanusetheelseifkeyword.Itlookssomethinglikethis:

color="green"

ifcolor=="blue"then

print("That'smyfavoritecolor!")

elseifcolor=="green"then

print("Verysubtlechoice.Ilikeit.")

elseifcolor=="pink"then

print("Nice,boldchoice.")

print("Idon'tthinkthatcolorwouldmatchyourshoes.")

Tryitout!

Thebeginningoftheifstatement... ifcolor=="blue"then...isfalse.Thiscodegetsskippedover.Thenthenextpartoftheifstatement... elseifcolor=="green"then...istruesothatsectionofcodeunderneathit... print("Verysubtlechoice.Ilikeit.")isran.Therestoftheifstatementisskippedwithoutcheckingifitstrueornot.So elseifcolor=="pink"then/ elseareneverprocessed.

ExercisesWriteoutafunctionthattakes1parameternamed"sides".Makethefunctionreturnthenameoftheshapedependingonthenumberofsides(forinstance,"triangle").Trytomaketheifstatementincludean elseattheendtoaccountforeverythingelsethattheifdoesn't.

1.9-Flowcontrol

WhileAnotherwaytocheckconditionsiswiththe whilekeyword.

while1+1==2do

print("Mymathiscorrect!")

Whileaconditionistrue,thebody(everythingbetweenthe doand end)willberunrepeatedlyandnotstop.Soifyoutriedtorunthatbitofcode,yourscreenprobablywentcrazyprintingoverandoverinanever-endingloop.Weneedtomakesuretheconditioncangetchangedsowe'renotstuckinanever-endingloop.Let'swritealoopwecanescapeoutof.

boolean=true

--Thisconditionwillgetcheckedtwice.Thefirsttimeit

--ischeckeditwillbetrueandthebodyofthewhile-loop

--willberun.Thesecondtimetheconditionischecked,

--ourbooleanwillbefalseandthewhile-loopwon'tberunagain!

whilebooleando

print("Switchingbooleantofalse.")

boolean=false

print("Booleanhasbeensettofalse.")

print("Wemadeitoutoftheloop!"

Understandingthatwecanchangethewhileconditionfrominsidethebodyoftheloop,wehavethepowertowriteprogramsthatendexactlywhenwewantthemto.Canyouguesswhatthiswilldowhenwerunit?

countdown=10

whilecountdown>1do

print(countdown.."...")

--Thislineiscriticaltomakeournumbershrink.

countdown=countdown-1

print("Blastoff!")

...Andremembertousea >andnota <,oryourloopmayneverrun.

ExerciseComeupwithyourownideaforawhileloop.

1.10-While

TypecheckingLuadoesn'tcarewhattypeofdataavariablehas.

data=12

data="hello"

data=true

Tothisend,wecanusethe typefunctiontocheckwhatkindofdataavariableisholding.

type(data)

=>boolean

Wecancheckthetypeoffunction:

type(string.reverse)

type(type)

Wecanalsouseittocheckwhattypeofdataafunctionisreturningbacktous:

type(string.reverse("hello"))

=>string

type(type(12))

=>string

ConvertingdatatypesWe'vealreadyseendatatypeconversionpreviouslywhenwetooknumbersinandoperationandtransformedthatintoatrueorfalsestatement.

type(12>3)

=>boolean

Therearealsowaystoconvertbetweennumbersandstringsusing tonumberand tostring.

number=tonumber("24")

print(type(number))

string=tostring(number)

print(type(string))

number

1.11-Typechecking

string

Interestingbutmaybelessuseful,youcanconvertotherdatatypestostring:

print(tostring("alreadyastring"))

print(tostring(true))

print(tostring(nil))

print(tostring(tostring))

ExercisesWhichofthesestringscanbeconvertedtoanumbersuccessfully? "001", "7.12000", "5", "1,943"

1.11-Typechecking

FirstgameLet'slearnaboutafewnewfunctionsandthenwe'llbeabletowriteourfirstgame!

ReadinginputNotonlycanourprogramprintoutdata,butusingthefunction io.readitcantakedatatoo.Thisfunctiondoesn'tneedanyargumentsbecauseitwillpromptusinthewindowontherightforustotypeindata.

print("Enteryourname:")

name=io.read()

print("Yournameis"..name..".")

Afteryouclick"Run",theprogramwillpausewhenitruns io.read().TypeyournameandhitENTERandlook,theprogramprintsbackoutthenameyougaveit.Noticethelastprintstatement.Wecombinedthenamewithtwootherstringstoformasentence.Youcanprompttheusermultipletimesifyouneedtogetadditionalinformation:

print("Enteryourname:")

name=io.read()

print("What'syourfavoritefood?")

food=io.read()

print("Yournameis"..name.."andyourfavoritefoodis"..food..".")

Onelimitationwithdoingthisisthedatawillalwayscomeinasastring:

print("What'syourfavoritenumber?")

data=io.read()

print(type(data))

string

Inthelastsectionwetalkedaboutconvertingdatabetweendifferenttypes.Ifwewantedtofindoutwhetheryourfavoritenumberisoddoreven,wewouldneedtoconvertittoanactualnumbertoperformoperationsonit.Typethisinyourtexteditorandrunit:

print("What'syourfavoritenumber?")

data=io.read()

number=tonumber(data)

--Iftheusergaveusananswerthatisn'ta

--number,thenthevalueof"number"isnil.

ifnumber==nilthenreturn"Invalidnumber."end

ifnumber%2==0thenreturn"Yournumberiseven."end

return"Yournumberisodd."

Randomnumber

1.12-Firstgame

Manylanguagesgiveusaccesstoarandomnumbergenerator.Randomnessishowwegeneratesecurepasswordsandkeysintherealworld.TogeneratearandomnumberinLua,weuse math.random:

math.random(100)

Thisgeneratesarandomnumberbetween1and100.Except,ifyouruntheprogramrepeatedlyyoumaynoticethatitspitsoutthesamenumber.That'sbecausenothinginthecomputerworldisrandom.Ifwefedinrandomnoisesthroughaspeakerorwhitenoisefromanoldtelevisionsetthenourcomputercouldusethistogeneraterandomnumbers.Sincewedon'teasilyhaveaccesstothosethings,weneedtoseedLuawithsomeperceivedrandomness.

Ifwerun os.timewewillgetthecomputer'scurrenttimeinintegerform:

os.time()

=>1.529098167e+09

Thisnumberishardenoughtoguessthatitwillworkasaseedforourprogram.Let'stakethesystemtimeandfeeditinusing math.randomseedthenfromthere,Luawillbeabletogeneratea"random"numberintherangewewant(1-100).

seed_number=os.time()

math.randomseed(seed_number)

returnmath.random(100)

Success!Itisgenerateddifferentnumberseachtimewerunit,withnopattern.

PuttingitalltogetherIshouldprobablyexplainwhatthisgameis.It'squitesimple.Wewantthecomputertomakeupanumberandtheuserhastoguesswhatthenumberis.Ifthey'rewrong,thenweshouldgivethemahintandmakethemguessagain.Wecantakeadvantageofthewhilelooptomakethemcontinueguessingwhiletheirguessisincorrect.

--Thecomputer'ssecretnumber

math.randomseed(os.time())

number=math.random(100)

print("Guessmysecretnumber.Itisbetween1and100.")

guess=tonumber(io.read())

--Whiletheuser'sguessisnotequalto

--thenumber,repeatthebodyoftheloop.

whileguess~=numberdo

--Givethemsomehints

ifguess>numberthen

print("Yourguessistoohigh.")

ifguess<numberthen

print("Yourguessistoolow.")

1.12-Firstgame

--Makethemguessagainandagainuntiltheygetit

print("Guessagain:")

--Winningmessage

print("Youguessedcorrectly!Thenumberwas"..number..".")

Let'sre-factoronebitofthiscodetomakeiteasiertoread.Whenwetalkedaboutifstatements,rememberthekeyword else?

--Givethemsomehints

ifguess>numberthen

--Winningmessage

Nowthatthingsarecleaner,let'saddonefeaturetoourprogram.Itwouldbemorefunifthegamekepttrackofhowmanyguesseswemadesowecouldgivethemaspecialmessage.Let'screateavariablecalled guess_countthatwillstartat 1andincrementeverytimetheusermakesanotherguess.We'llalsogoaheadandaddsomemessagestothebottomtopraisetheuseriftheydiditinareasonablenumberofguesses.

--Ourstartingnumberofguesses

guess_counter=1

--Incrementtheguesscounterby1

guess_counter=guess_counter+1

--Givethemsomehints

ifguess>numberthen

1.12-Firstgame

--Winningmessages

ifguess_counter<=5then

print("Amazing!Itonlytookyou"..guess_counter.."tries.")

print("Ittookyou"..guess_counter.."tries.Notbad.")

ExercisesTryaddingmoremessagestothe guess_counterfordifferentscores.Tryaddingamessagetotheifstatementwiththehintsforwhentheuserguessesaninvalidnumber.Howwouldyoudothat?Maketheconditionexitif guess_countergoesabove10andtelltheusertheylostthegamebutshouldtryagain.

1.12-Firstgame

Tables(part1)Tablesarethelastdatatypewe'lldiscussinthischapter.Otherlanguageshavedifferentnamesforthisdatatypelike"object","hash","map"and"dictionary",andthefeaturesmayvaryfromoneprogramminglanguagetoanother.Tablesareusedtobuildcompositedatatypeslikelists,trees,orabiggreenorcrunningacrossthescreen.Compositedatatypesarehigherorderdatastructurescreatedfrommoreprimitivedatatypeslikenumbersandstrings.Thenumberofdatastructuresyoucancreateareendless.Weneedtolearnaboutafewtonotonlyunderstandhowtableswork,buttobeabletobuildanymodernsoftware.

Thebasicsyntaxfortablesistomakeacurlybrace {(samekeyasthesquarebraceonmostkeyboards)tostartthetable,writesomedatainthetable,thenputaclosingcurlybrace }toendthetable.Soanemptytablewouldlooklikethis:

my_cool_table={}

ListsListsareusuallystartedbywritingthefirstitem,thenthesecond,andsoon.Ifwewantedtomakeagrocerylistinsoftware,itmaylooklikethis:

groceries={

[1]="beans",

[2]="bananas",

[3]="buns"

Okmaybeyourtypicalgrocerylistlooksdifferent.Whatdowedowiththisdatanowthatwegotit?Wecanaccessandmodifythedataasiftheywerestoredintheirownvariables.

returngroceries[1]

=>beans

Firstwespecifythevariablenameofthetable,theninsquarebracketsweputthenumberwewant.Youcanaccesstheminanyorderandmodifythemasneeded:

print(groceries[3])

groceries[1]="coffeebeans"

print(groceries[1])

coffeebeans

Theorderyoudefinethemindoesn'tmatter:

groceries={

[3]="beans",

[1]="bananas",

[2]="buns"

1.13-Tables(part1)

Thenumberinsquarebracketsisthekey.Akeythatispartofanumericsequenceofkeyssuchasthislistisoftencalledanindex.So "bananas"hasanindexof 1.Thepluralofindexisindices.

Don'tforgetthecommasbetweeneachiteminyourlistoryouwillgetquitetheerrormessage:

[string"groceries={..."]:3:'}'expected(toclose'{'atline1)near'['

Whenyouaremissingacommabetweenitems,itthinksithasreachedtheendofthetablebutthenerrorsoutwhenitgoestoclosethetablebutseesanotheriteminsteadoftheclosecurlybracket }.

Anotherissueyoumayrunintoisifyoutrytoaccessakeythathasnodata.Thereisno4thiteminourtablesoifwetrytoaccessit:

print(groceries[4])

Wegetback nil,thesamewaywewouldifwetriedtoaccessavariablenamethathasnodataassignedtoit.

Writingoutlargelistscanbecomeaheadachewhenwehavetomanuallynumbereachiteminalist:

groceries={

[1]="beans",

[2]="bananas",

[3]="buns",

[4]="blueberries",

[5]="butter",

[6]="broccoli",

[7]="basil"

Whatifweremoveanitemorwewanttomovesomethingtoadifferentpositioninthelist?Whatapaintohavetore-indexeverything.Thankfullythereisshorthandwayofwritinglists:

groceries={

"beans",

"bananas",

"buns",

"blueberries",

"butter",

"broccoli",

"basil"

Thisisidenticaltothecodewrittenabove,exceptnowtheindicesareauto-generatedforme. "basil"hasanindexof 7sinceitisthe7thiteminthelist,butifIcutandpastedittothetopofmylist,it'sindexwouldbe 1andeverythingbelowitwouldberenumberedaccordingly.

LoopingoverlistsIfwewantedtoprintourgrocerylist,wecouldsaysomethinglike:

print(groceries[1])

print(groceries[2])

print(groceries[3])

print(groceries[4])

--andsoon...

1.13-Tables(part1)

Butthatisquiterepetitiousandrequiresupdatingifthesizeofourlistchanges.Luckilywealreadyknowaboutwhileloops.

index=1

whilegroceries[index]~=nildo

print(index,groceries[index])

--Gotothenextindexinthelist

index=index+1

Seehowinsteadofaccessingeachitemas groceries[1], groceries[2]...wecanjustuseavariableinthesquarebracketsinsteadofanumber.Theninsidetheloopwebumpthenumberupandaccessthenextiteminthelist.Theloopstopswhentheindexgoesbeyondthelastiteminthelistandthereisnothingthere.Sowhenindex8isread,groceries[8]isnilandthewhileconditionisnolongertrue.Whileconditionsdon'tevenneedabooleanexpression.Itcanknowwhetherornottocontinuesimplyifthegivenitemhasdataorisnil.Itcanbesimplifiedtoread:

index=1

whilegroceries[index]do

index=index+1

Again,itknowstoexitwhenitsees falseor nil.Thecaveattothiswouldbeifyoumakeaspeciallistwith falseinit:

groceries={

"beans",

"bananas",

false,

"blueberries",

"butter",

"broccoli",

"basil"

Whenthewhileloopgetstothethirditeminthelistandsees false,itwouldstoploopingbeforeitreadstherest.It'snottypicallyagoodideatomixandmatchdifferentdatatypesinalistbecauseofissueslikethis,however,wecouldworkaroundthisifweneededto.Thereisaspecialoperatorfortablestogetthesizeofthelist.

print(#groceries)

Aneasywaytorememberthe #operatoristorememberthatitreturnsthe#ofitemsinalist.Usingthisoperatorwecouldwriteourwhileloopinadifferentway.

index=1

whileindex<=#groceriesdo

index=index+1

1.13-Tables(part1)

Youcouldeventweakthisslightlytoreadthelistbackwardsifyouwantedto:

index=#groceries

whileindex>0do

index=index-1

Noticewearesubtractingfromtheindexwitheachloopinordertoaccomplishthis.

ExercisesTrytomodifythewhilelooptoonlyprinteveryotheriteminthegrocerylist.(Hint:insteadofincrementingby1oneachread,youwanttoincrementmore.)Writeawhileloopthatcountsto10andpopulatesanemptytablewiththesameitem10times.(Hint:youassigntoindicesjustlikevariables, list[index]="hi".)

1.13-Tables(part1)

Tables(part2)Inthelastsectionwesawhowsimpleitwastomakealist.Workingwiththelistwasalittletrickyatfirstbuthopefullynottoobad.Ifwerewindback,wecanrememberthatwecreatedatablebyassigningsomekeysvalues.

boxes={

[1]="JohnDoe",

[2]="AmandaParker",

[3]="TylerReese"

Thinkofitlikepostofficeboxesandwelabeleachboxwithauniquenumber.Wheneverwereferenceapostalbox,wedosobyreferencingthenumberwithinthearray(list)ofboxes: boxes[2].Thelabel,orkey,isultimatelyarbitrarythough.Formakingalist,welabelthingsinanincrementalordertomakethemeasiertoloopoverandtogiveusasenseoflinearsequence.Keysdon'tneedtobenumbers.Theycouldjustaswellbestrings:

coins={

["half"]="50cents",

["quarter"]="25cents",

["dime"]="10cents",

["nickel"]="5cents",

["penny"]="1cent"

Whichwouldbeaccessedjustthesameway:

print(coins["nickel"])

5cents

Thiscanbereallyusefulfordoingalookupifweinsteaduseavariableforthekey.Trythisoneout:

coins={

["half"]="50cents",

["dime"]="10cents",

["penny"]="1cent"

print("Whichcoindoyouhave?")

response=io.read()

print("Yourcoinisworth"..coins[response]..".")

Thisisn'tfarofffromhowcertaindatabasesanddigitalserviceswork.Itemsarestoredinauniquekeythatcanbereferencedforgettingadefinitionoutoflater.That'swhythisdatastructureissometimescalledadictionary.Remember,wecanadditemstoatableafteritisdefined:

coins["silverdollar"]="1dollar"

AnothershortcutLuagivesusiswedon'tneedtousethesquarebracesorquoteswhenaddingkeysthatarestrings.

1.14-Tables(part2)

coins.nickel="5cents"

Thelimitationwithdoingthisisthekeysdefinedthiswaycan'thavespacesorspecialcharacters.Theymustbevalidinthesamewayvariablenamesarevalid.

coins.silverdollar="1dollar"--INVALID

coins.silver_dollar="1dollar"--Valid

coins.100="1dollar"--INVALID

Youcanusevariablenamesforkeyswhencreatingthetabletoo:

color="purple"

description="thebestcolor"

colors={

[color]=description

print(colors.purple)

print(colors[color])--printsthesamething

Byconvention,stringsaretypicallyusedfordictionary-liketableswhilelistsarenumbers.Don'tmakethemistakeofthinkingthesearethesame:

list={

1="someitem",

["1"]="auniqueitem"

Youcoulduseotherdatatypesaskeys,butyoumightfindyourresultstobeveryunexpected:

crazy_list={

[true]="works",

[false]="works",

["true"]="notthesame",

["false"]="notthesame"

print(crazy_list[true])

print(crazy_list[false])

print(crazy_list["true"])

print(crazy_list["false"])

crazy_key={}

crazy_list={

[crazy_key]="works"

print(crazy_list[crazy_key])

crazy_list={

[nil]="doesn'twork!"

print(crazy_list[nil])

Throwsanerror:

[string"crazy_list={..."]:2:tableindexisnil

1.14-Tables(part2)

Valuesinatablecanbeanytypeofdata,includingfunctions:

color="gray",

smelly=true,

make_sound=function()

print("meyuow!")

cat.make_sound()

ExercisesRemembertheearlyfunctionwemadethatreturnedtheanimalsounds?Makeafunctionwithatableinit,whereeachkeyinthetableisananimalname.Giveeachkeyavalueequaltothesoundtheanimalmakesandreturntheanimalsound.Tryinvokingthefunctionandseeifyougetbackthecorrectsound.

1.14-Tables(part2)

Forloops(part1)Wesawpreviouslythatwecouldusewhileloopsformanythings,butwealsosawhoweasyitwastomakeawhileloopthatdidn'trunproperly.Theprogrammerhastomakeavariabletopasstothecondition,makesuretheconditioniswrittenoutcorrectly,andthenmakesuretheconditioncanbechangedsotheloopcaneventuallyend.Thismanystepseachtimewewanttowriteasimpleloopleavesuspronetoerrorsandwastingourtime.Withforloops,wecantellaloopexactlyhowmanytimeswewantittorunandskipallthesesteps.

Numericforloops

fornumber=1,10do

print(number)

Onthefirstlinewearesaying"For[startingnumber]through[endingnumber]dothefollowing".The number=isavariableyouareassigningthestartingnumberto.Thevariablenamecanbewhateveryouwant.Thesecondlineisthebodyoftheloopandthethirdlineendstheloop.Ifyourunthisprogram,itwillprintthenumbers 1, 2, 3...through 10as numberisbeingincrementedby1witheachloop.Thisvariableisabitpeculiarthough,notonlybecausewedefineditinthemiddleofastatementbutbecauseitdisappearsafterwearedonewiththeloop.

fornumber=1,10do

print(number)

returnnumber

Thisiscalledalocalvariable,becauseitonlyexistslocallywithintheforloop.

Forloopsactuallyhave3parameters:

startnumber-weassignthevariabletoitandthevariablewillincrementwitheachloopstopnumber-thelastnumbertoincrementtobeforestoppingtheloopstep-howmuchtoincrementbywitheachloop.Ifwedon'tspecifyastepsizeitwilldefaultto1.

Let'ssaywewantedtoprintoutonlyevennumbers.Wecouldchangethestartingnumberto2andsetthesizeofthestep(3rdparameter)to2:

fornumber=2,10,2do

print(number)

Ifwewantedtoiterate,orloopoverandreadeachiteminalist,itwouldlooksimilartoawhileloop.Let'slookatthewhileloopexampleagainjustforcomparison:

items={'a','b','c','d'}

index=1

whileindex<=#itemsdo

print(items[index])

index=index+1

items={'a','b','c','d'}

forindex=1,#itemsdo

print(items[index])

Wecouldalsocountdownbychangingtheparametersaroundandsettingthesteptoanegative1.

items={'a','b','c','d'}

forindex=#items,1,-1do

print(items[index])

Inthiscasetheindexstartsatthepositionofthelastitemandstopswhenitgetstothestopnumber,1.

ExercisesModifythepreviousloopsothatitonlyprintseveryotheriteminthelist.

Forloops(part2)Wecancreateadifferentstyleofforloopusingfunctions,butinordertodothat,weneedtounderstandanotheraspectoffunctionswehaven'tyetcovered.Functionscanreturnmultiplevalues.

sort_numbers=function(a,b)

ifa>bthen

returna,b

returnb,a

bigger,smaller=sort_numbers(12,18)

print(bigger)

print(smaller)

Thisfunctiontakestwonumbers,checkstoseewhichisbigger,thenreturnsboththebiggernumberfirstthenthesmallernumbersecond.Noticewedidthisbyputtingacommainthereturnstatementthenprovidingasecondvalueafterthecomma.Likewise,wewereabletocapturebothvaluesintovariablesbyputtingthefirstvariablename,acomma,thenthesecondvariable( bigger,smaller=).Wedon'tneedtocaptureeverythingreturnedfromafunction.Wecouldhavejustaseasilycalledthefunctionandonlycapturedthebiggernumberifthat'sallwewantedfromit.

bigger=sort_numbers(12,18)

GenericforloopsLet'stakealookatthesiblingtothenumericforloopcalledthegenericforloop.It'scalledgenericforloopbecauseittakesafunctionthatmakesitbehaveindifferentwaysfordifferentsituations.Itdoesn'tdoanythingonitsown.Itreliesonthefunctiontotellithowtobehave.

ipairsHere'swhatgenericforloopslooklike:

list={'dog','cat','mouse'}

forindexinipairs(list)do

print(index,list[index])

ipairstakesourforloopandmakesititerateovereachiteminthelistandgivesusan indexvariabletoworkwithinsidetheloop.Butwait,there'smore! ipairsprovidesuswithanothervariablethatholdsthevalueoftheitematthatindex.Tryitoutyourself:

forindex,valueinipairs(list)do

print(index,value)

Ahyes,soconvenient!Thereisonegotchawithdoingthis.Ifyouwantedtoeditthetablefrominsidetheloop,youneedtoaccessthetabledirectly:

list[index]=string.upper(value)

print(list[1])

Ifyoutrytojusteditthevalue:

value=string.upper(value)

print(list[1])

thelistwon'tbemodified,because valueisjustacopyofthedatathat'sactuallyinthelist.You'reeditingatemporarycopy.

pairsAnotherfunctionforprogrammingforloopswithspecialfunctionalityis pairs.Thiswilliterateovereverykeyinatable:

table={

cat='meow',

dog='bark'

forkey,valueinpairs(table)do

print(key,value)

Evenindices:

table={

cat='meow',

dog='bark'

print(key,value)

Nosneakingpast pairsforanyofthesekeyseither:

table={

[1]='a',

[2]='b',

[3]='c',

cat='meow',

dog='bark',

[true]=false,

[{}]='what?'

print(key,value)

Aneasywaytorememberthedifferencebetween ipairsand pairsisthe"i"in ipairsstandsforindex.Surethere'sadifferencewhenworkingwithweirdtablesliketheoneabove,butwhycan'twejustuse pairsforregularlist-styletables?

table={

[2]='b',

[3]='c',

[1]='a'

print(key,value)

Asyoucansee,theorderoftheitemsisn'tguaranteedwith pairs. ipairsisalsooptimizedtohandlenumerickeysandwillgenerallyperformfaster,soit'sgoodtoknowthedifference.

Underthegeneric-for-loophood

ipairsand pairsarejustregularfunctionsthatweinvoke.Theyreturnafunction(yes,afunctionthatreturnsafunction!)andthisreturnedfunctionprogramsourlooptobehavehowwewant.

forkey,valueiniterator,list,start_numberdo

print(index)

Sothisiswhatagenericforloopreallylookslikewithoutthehelpof ipairsor pairs.Itrequires3parametersthatipairs/ pairsprovidesdatabacktothekeyandvaluevariablesthatwecanuseinsidetheloop. iterator, list,start_numberareallvariableswewouldotherwisehavetodefinewithouttheirhelp.

iteratorwouldbeafunctionweprovidetothelooplistwouldbewhatwewanttoiterateoverstart_numberwouldbethestartingindexinthelist

list={'a','b','c'}

iterator,list,start_number=ipairs(list)

forindex,valueiniterator,list,start_numberdo

print(index)

ipairsgivesusaniteratortopasstotheforloop,aswellasourlistwealreadyhad,andastartingnumber.Wecanprinttheresultsofipairsandseethe3thingsitgivesus:

print(ipairs(list))

function:0x156a3f0table:0x1572aa00

Sotosayitagain,genericforloopsrequire3things:aniteratorfunction,ourlist,andanumber.Inordertonothavetowritethemourselves,wegeneratedthose3thingsbyinvoking ipairsthenpassingthemintotheforloopparameters.Don'tfrettoomuchifthisseemsconfusingrightnowbecausewe'renotgoingtoneedtowritecustomforloopsorcustomiterators.

Numericversusgeneric:whichtouse?

Numericforloopsaregoodforsimplecountingbutperformjustaswellormaybeevenbetterthangenericforloops.Genericforloopsaremoreadaptable.Ifyouhaveasituationwhereeitherwouldwork,justusewhicheveryouwant.Itreallywon'tmakeanydifference.

ExercisesMakealistandthenwritebothanumericforloopandgenericforloopthatiterateoverthelistandprinteachitem.Comparethetwoapproaches.Makeatablewithanimalsforkeysandthesoundstheymakeforthekeyvalues.Makeaforloopthatusespairstoiterateovereachandchangethenoisestoallcapitalletters.

ScopesWhendefiningfunctions,wedefineparametersforthosefunctionswhichworklikeregularvariables.Ifwetrytoaccessaparameteroutsideafunctionwewillseethatitis nil.

addition=function(a,b)

print(a,b)

returna+b

addition(1,2)

print(a,b)

Theparameters aand barelocalvariables.We'veseenlocalvariableswithforloops,wherethevariablecounting_numbercouldn'tbeaccessedoutsidetheforloop:

forcounting_number=1,4do

print(counting_number)

Functions,forloops,andwhileloopscreateascopeeachtimetheyareran.Thingscreatedinthescope,includinglocalvariables,aredestroyedwhenthatlooporfunctioninvokeisdone.Thisishowtheprogramtidiesupafteritselfandkeepsthecomputerfromrunningoutofmemory.Theprocessofremovingunuseddatafrommemoryandreleasingcontrolofthatmemoryiscalledgarbagecollection.Luadoesthisforussowedon'thavetothinkaboutit.Variableswecreatenormallydon'tfollowthesamerules.Theywillcontinuetoexistafterthescopetheywerecreatedinhasbeendestroyed.

text="I'mnotgoingaway."

returna+b

addition(1,2)

print(text)

Eventuallyallthesevariableswemakewillfillupmemoryunnecessarily.Thiscanalsobeproblematicifweaccidentallymaketwovariablesbutusethesamename.

--Thismodifiesthexatthetop!

returna+b

print(x)

result=addition(x,y)

print(x)

1.17-Scopes

Whenyouwritealargeprogram,you'llinevitablymaketwovariableswiththesamename,sothiscouldbeahugeissue.Thesolutionistomakeourvariableslocalvariablesbyputtingthekeyword localbeforeallourvariableswhenwecreatethem.

localtext="I'monlyaccessibleinsidethefunction."

returna+b

addition(1,2)

print(text)

Now textisonlyinthescopeofthefunctionandnotgettingintootherpeople'sbusiness.Ifyoudon'twrite localbeforeavariable,thenwhatyouarecreatingisaglobalvariable.It'sashamethatvariablesareglobalunlessweexplicitlytellthemnottobe.Thereisneverareasontocreateglobalvariablesifyouhaveenoughknowledgetoknownotto.Soasabestpractice,allcodeexamplesgoingforward,onlylocalvariableswillbecreated.Let'sseeafewmoreexamples:

localnumber=12

--Thisfunctionhasnoparameters

localprint_numbers=function()

--Thisworks.Youcanseevariablesoutsidethefunction

print('number:',number)

--Thisdoesn'twork.Thevariabledidn'texist

--atthetime"print_numbers"wascreated.

print('number2:',number2)

localnumber2=18

print_numbers()

--Wealready"declared"number.Wedon'twrite"local"again.

number=13

print_numbers()

Noticewhenitprintedthatitknew numberwasupdatedto13butcouldn'ttrack number2.Aslongasavariablewascreatedbeforethescope(function'sscopeinthiscase)wascreatedthenitwillalwaystrackthelatestvalue.

Asareminder,wealreadysawwiththeforloopandwhileloopthatyoucanmodifyvariablesintheouter,orparent,scope:

localnumber=1

whilenumber<10do

number=number+1

Thisalsoworkswithfunctions:

localnumber=1

localmutate_number=function()

number=7

print(number)

mutate_number()

1.17-Scopes

print(number)

Whatifyoumaketwovariableswiththesamenameintwodifferentscopes?Tryrunningthisone:

localnumber=18

localshadowing=function()

localnumber=6

print(number)

shadowing()

Theinner numberdoesnotaffecttheouter numberinanyway.Theouter numberisnotaccessibleinsidethefunctionaslongastheinner numberexists.Ifavariablehasthesamenameasanothervariableinaparentscopethentheparentscopevariablebecomesinaccessible:thisiscalledshadowing.Typicallyyouwouldwanttoavoidshadowingifatleastforthereasonthatusingthesamevariablenametwiceinthesamefilecanmakethecodehardertoreadandmorepronetoerrorsbeingintroduced.

Onemoreinterestingthingswithscopes.Normallyafunctioncannotseeitself:

localself_reference=function()

print(self_reference)--Thiswillbenil!

self_reference()

Itdoesn'tseeitselfbecausethevariableisstillbeingcreatedwhenthefunctionisbeingcreated.Butrememberthatifavariableexistsbeforethefunctiondoes,itcanseethelatestup-to-datecontentofthatvariable.Sohere'sthetricktomakethatwork:

localself_reference=nil

self_reference=function()

print(self_reference)

self_reference()

Thevariableisdeclared,eventhoughweassigned niltoit.Assigningniltogetavariabledeclaredisprettycommon,soLuaincludesashorthandwayofdeclaringemptyvariables:

localself_reference

self_reference=function()

print(self_reference)

self_reference()

Thismayseemsillythatafunctionwouldneedtoaccessitself,buttherearesomeverypowerfulapplicationsforthisthatwewillseelateron.

ExercisesDeclareaglobalvariableinsideafunction, x=5(nolocalkeyword)thentrytoprintthevariablefromoutsidethefunction.Canitbeprinted?How?

1.17-Scopes

Chapterreview

TerminologyOperatorandOperation-Operatorsaresymbolsthatcauseanoperation,orinteractiontohappenbetweentwopiecesofdata.Anexampleoperationwouldbe 5+5. (5+5)*2wouldbetwooperations.

ModuloandModulus-Moduloisaspecialtypeofarithmeticoperationbetweentwonumbersusingamodulusoperator.Themodulusisrepresentedbya %(percentsymbol).Example: 24%2==0

VariableandValue-Variablesarenamesthatreferenceacertainpieceofdata.Thevalueiswhatisstoredinsidethevariable: variable="value"

Statement-Thisiswhenyoudosomething,likeanoperation(orgroupofoperations),declareavariable,orinvokeafunction.Forinstance,thisisaprintstatement: print("hello")

Invoke-Run/callafunction.

ParameterandArgument-Functionstellyouwhatandhowmanyparameterstheyhave.Argumentsarethedatathatgetspassedintothoseparameters.

Boolean- trueor false

Equality-Whetherornottwothingsareequal.Thisisusuallydonewithanequal( ==)comparison.

Loop-Codethatrepeats.

KeyandIndex-Keyisthenamedreferenceinatablewheredatacanbefound.Itissimilartoavariable.Indexisakeythatcomesinanorderedsequence,suchasnumberedkeysinalist.Thepluralofindexisindices.

Iterate-Loopoveralistanddosomethingwithit.

Scope-Anareawherevariablescanbecreatedthataren'taccessiblefromtheoutside.Scopesarecreatedbyfunctionsandloops.

LocalandGlobal-Localdescribesthingsaccessibleonlyinthecurrentscope,suchaslocalvariables.Globalthingsareaccessiblefromanywhereintheprogram.

Shadowing-Whenalocalvariablehasthesamenameasavariableinaparentscopeandpreventsyoufromaccessingtheparentscopevariable.

1.18-Chapterreview

Chapter2:IntroducingLÖVEThegoalofthischapteristoapplyallthebuildingblockswelearnedinthefirstchapterandmakethemconcretethroughpractice.Bytheendofthechapteryouwilllearnreal-worldskillssuchashowtointeractwithotherpeople'ssoftwareandbasicprinciplesfordesigningandbuildingyourownprograms.

2.0-IntroducingLÖVE

UpandrunningLearningbymakingisfunandeffective.Learningtointerfacewithotherpeople'ssoftwareispartofbeingaprogrammerandisanecessaryskilltohaveasone.LÖVEisaframeworkformakinggames.Aframeworkisjustasetoftoolsorfunctionalitycombinedtogethertoservealargerpurpose.InthecaseofLÖVEthisincludes,butisnotlimitedto:

Functionsforloadingimages,audio,andtextFunctionsforcreatingandmovingobjectsonscreenParametersformakingtheobjectsinteract

InstallingyourdevelopmentenvironmentTheLÖVEwebsitehaslinkstoinstallthesoftwareonyoursystem.IfyouhaveLÖVEinstalledalready,makesurethatyouatleastversion11(mysteriousmysteries)assomefunctionalitywe'llcoverheredoesn'texistinolderversions.Formobiledevicesyoucanfindacopyintheappstore.

AlongwithinstallingLÖVE,youwillneedatexteditorforcreatingLuafilesonyoursystem.I'mnotgoingtomakeanyrecommendationshere,becauseintheenditallcomesdowntopersonalpreference,butyoucancheckthislistbytheLÖVEcommunityifyouneedastartingpoint.Itfeaturesdifferenteditors(andrecommendedplugins)forLÖVEandLuadevelopment.Simplypickone.

TestthatLÖVErunsWhenyoulaunchLÖVE,(seeinstructionsbelowonhowtodothat)youwillbegreetedwithafriendlygraphicandthetext"NOGAME",meaningyouarerunningtheenginebutdidn'tgiveitagametoload.

macOSOnceyouhavedownloadedtheLÖVEbinaryformacOS(64-bitzipped),proceedtothe"Downloads"folderandunzipthearchive.Youshouldnowseeanapplicationcalled"love".

macOSmightshowyouawarningmodal,becauseyouaretryingtoopenanapplicationbyanunverifieddeveloper.Ifso,rightclickontheapplicationandchoose"open"and"open"againinthefollowingdialog.Youshouldnowbegreetedbytheno-gamescreen.

Addendum:Homebrew

IfyouarefamiliarwithdevelopmentonamacOSmachine,youmighthaveheardofHomebrew.Itisapackagemanagerwhichallowsyoutoinstallalotofprograms,librariesandsoondirectlythroughyourTerminal.

Ionlyrecommendthisapproachforadvanceddeveloperswhoknowwhattheyaredoing.ForcompletenesssakeherearethestepstoinstallLÖVEviaHomebrew.

/usr/bin/ruby-e"$(curl-fsSLhttps://raw.githubusercontent.com/Homebrew/install/master/install)"

brewtapcaskroom/cask

brewcaskinstalllove

2.1-Upandrunning

Oneofthebenefitsofthisapproachis,thatyoudon'thavetosetupyourownterminalalias,becauseHomebrewalsotakescareofthat.

Windows

IfashortcutforLÖVEdidn'tappearinthestartmenu,youshouldbeabletotype"love"inthesearchandseeit.

Ubuntu

Openthe"UbuntuSoftware"applicationandsearch"love2d".Clickonthetopresultandyoushouldseeafamiliarapplicationdescription:

Clickthe"Install"buttontoinstallit.Onceinstalled,youcansearchforthe"terminal"application.Oncethatisopen,type lovetolaunchtheapplication.

OtherGNU/LinuxoperatingsystemsMostdistroshaveLÖVEintheirrespectiverepositories:

Archlinux-basedsystems- sudopacman-SyloveFedora-basedsystems- yuminstalllove

Onceinstalledfromyourpackagemanager,openaterminalandtype lovetotestthatitruns.

Ifyourdistrodoesn'thaveLÖVEinthepackagemanageranalternativewaytogetitistodownloadedtheAppImageversionfromthehomepage(https://love2d.org/).AppImagefilesarelikeauniversalexecutablethatworksacrossLinuxsystemssimilartothewayan"exe"fileworksonWindows.Oncedownloaded,opentheterminal,changetothedirectorywhereyoudownloadedtheAppImageandtypethecommands:

chmoda+xlove-11.1-linux-x86_64.AppImage#Marksthefileasasafeexecutable

2.1-Upandrunning

./love-11.1-linux-x86_64.AppImage

love-11.1-linux-x86_64.AppImageshouldbechangedtomatchthenameofthedownloadedAppImagefile.

CreateaprojectfolderFindasafeplacetocreateafolderandgiveitthename"hello".Withinthefolder,createanewtextfilenamed"main.lua".Thiswillbewhereourgame'scodegoes.

NoteforWindows:Inordertocreateafilewiththename"main.lua",youmayneedtofirstcreateanew"TextDocument",right-clickonit,click"Properties"thenfromthepropertiesmenuchangethefileextensionfromreading"main.lua.txt"tojust"main.lua".ToavoidhavingtodothisforeveryLuafileyoucreateinthefutureyoucantellWindowstoalwaysshowthefullnameoffiles,includingtheirextension.Toenablethis,type"ControlPanel"intheprogramsearchandopenthe"ControlPanel"result.Withinthecontrolpanel,select"FileExplorerOptions".Clickthe"View"tab.Removethecheckmarkfromthe"Hideextensionsforknownfiletypes"andpressApply/OK.

CreateatestgameWithin"main.lua",writeoutthefollowingfunction:

love.draw=function()

love.graphics.print('HelloWorld!',400,300)

Nowlet'sfigureouthowtorunitandseewhatitdoes.

RunthegameThiswillbedifferentfordifferentoperatingsystems.

macOSStartingyourgame

ThesimplestwaytostartaLÖVEgameistodragthewholefoldercontainingthegame'ssourcefiles(notjustthemain.luafile!)ontotheapplicationfile.

2.1-Upandrunning

Thisalsoworkswith.lovefiles.

Usingtheterminal

IfyouarefamiliarwiththeTerminal,youcanuseitasamoreconvenientmethodofstartinggames.

Assumingthedownloaded"love"applicationisstillinyour"Downloads"folder,openanewTerminalandtypethefollowinglines(youneedtopressreturnaftereachline):

#SwitchestotheDownloadsfolder

cd~/Downloads/

#StarttheLÖVEapp

openlove.app

ThisobviouslystartsLÖVEwithano-gamescreensincewedidn'tspecifywhichfoldertoload.Let'sfixthisbytypingthefollowingcommand:

#Inmycasethefullcommandtostartthegalaxydemowouldbe:

#open-alove.app~/Downloads/galaxy

open-alove.app<path-to-your-game>

Usingaterminalalias

Wecanstillimproveonthepreviousmethodbyusinganalias.Beforewedothis,wemovethe"love"applicationbundletotheApplicationfolder.

#MovetheappfromDownloadstoApplications.

~mv~/Downloads/love.app/~/Applications/love.app

2.1-Upandrunning

Nowtrythefollowingcommand:

#StartLÖVEbyusingthescriptinsideoftheapplicationbundle.

~/Applications/love.app/Contents/MacOS/love<path-to-your-game>

AsyoucanseewenowcanrunLÖVEwithoutusingthe opencommand,whichalsohastheaddedbenefitofshowingthegame'sconsoleoutputdirectlyinourterminal.

Ofcourseitwouldberatherinconvenientifwehadtospecifythefullpatheachtimewewanttorunourgame,sowe'llnowsetupanaliasinyour.bash_profile(whichbasicallyactsasaconfigurationfileforyourbashsessions).

Sinceitisahiddenfileyoumightnotbeabletospotitinyourfinder,butwecansimplyedititthroughourTerminal.

#Appendsthealiasdefinitiontoanexisting.bash_profile

#orcreatesanewone.

echo"aliaslove='~/Applications/love.app/Contents/MacOS/love'">>~/.bash_profile

#Usetheupdated.bash_profileforthecurrentsession.

source~/.bash_profile

#Startyourgamethroughthealias.

love<path-to-your-game>

Andthat'sit:Youcannowquicklyrunyourgameswiththe lovealias.Thisisespeciallyhandyifyouareinsideofthegame'sdirectory,becauseallittakesnowisaquick love.tostartthegame.

Windows

FindtheshortcuttoLÖVEanddraganddropitinthegamefolderlikeso:

Thenright-clickontheLÖVEshortcutandyouwillseea"Properties"dialogwindowsimilartothis:

2.1-Upandrunning

The"Target"fieldmaybethesameorslightlydifferentdependingonyoursystemversion.Withoutdeletingthetextstringcurrentlyinthe"Target"field,appendthepathtoyourgamefolderinquotestotheend.Youcancopyandpastethispathfromthefolder'saddressbar.Forinstancethetargetpathinthepictureshouldgofromreading:

"C:\ProgramFiles\LOVE\love.exe"

"C:\ProgramFiles\LOVE\love.exe""C:\Users\IEUser\Desktop\hello"

Nowpress"OK"toclosethePropertiesdialogandclickingtheshortcutwilllaunchthegame.Ifthegameransuccessfully,youwillseeablackwindowwiththetext"Helloworld!"insmallprint.

GNU/LinuxIfyouknowthelocationofyourfolder,youcanopenaterminalandtypethecommand:

love<path-to-your-game>

Where <path-to-your-game>hasbeenchangedtotheactualfolderpathwhereyourgameresides.

Ifyouarealreadynavigatedintothegamefolder,youcanrunaterminalcommandwithinthatdirectory:

2.1-Upandrunning

The"."simplymeans"thisfolderthatIamcurrentlyin".

Congratulations!You'vesetupyourdevelopmentenvironmentforwritingagameinLua.Ifyouhadissuesgettingthroughthis,reachouttomeeitherthroughaGitHubissueormycontactinformationandIwillupdatethisguidetoincludinganyadditionaltroubleshootingstepsforfutureusers.

Nowthatourdevelopmentenvironmentissetupandourfirstgameisrunning,trymodifyingthecodesothestring"HelloWorld!"readssomethingdifferent.It'sprettyapparentthatrunningthisfunctionprintstothescreenwhateverstringwegiveit.Butwhatarethe2ndand3rdparametersfor?

love.graphics.print('HelloWorld!',400,300)

Trymodifyingthosenumbersandseewhatitdoestothetext.

2.1-Upandrunning

LÖVEstructureOpenup"main.lua"andtakealookatourfirstline.Wedefinedafunctioncalled love.draw,whichimpliesthereisatablecalled loveandwecreatedakeyinitcalled draw.Indeedthisisthecase,butsomehowthefunctionwasinvokedwithoutusinghavingtowrite love.draw()andinvokeitourselves.Thisrequiresahigh-levelexplanationofwhattheengineisdoingwithourfile.

WhenLÖVEisrun,beforeourmain.luafileisran,atablecalled loveisdefinedasaglobalvariable.Wecanassignfunctionstothistable( love.draw)andaccessfunctionsalreadydefinedinit( love.graphics.print).love.graphics.printhastwodotsinit,sothatmeansthelovetableprobablylookssomethinglike:

love={

draw=nil,

graphics={

print=function()...end

The lovetablehasaplentyofothertablesnestedinit,anditputssimilarfunctionsintablestogether.Soallthefunctionsrelatingtographicsareinsidethe love.graphicstable.

Once"main.lua"isdonerunning,we'veaccessedandmodifiedthe lovetableandaddedsomenewfunctionalitytoit,tellingithowtodrawtothescreenbydefiningour love.drawfunction.Ifwedefineafunctionwiththisname,thegameenginewillseeitandinvokeit.Infact,itcontinuouslyinvokes love.drawmanytimesasecond.Toprovemypoint,let'smodifymain.luaandmakeitprintanumber.

localnumber=0

number=number+1

love.graphics.print(number,400,300)

Eachtimewegotoprintthenumber,weincreaseitby1.Runthisprogramandseehowquicklythenumberclimbs.

The lovetableisaseeminglycomplexstructureoftablesinsidetablesandfunctionsinsidethose,butwe'llgraduallylearnthestructureandpurposeofeachthingoverthecourseofthischapter.Inthenextsection,let'stakealookatthe2ndand3rdparametersin love.graphics.printandseehowtheywork.

2.2-LÖVEstructure

GeometryIfyoumodifiedthenumbers 400and 300inmain.luayouwillhaveseenthattheymovethetext.Realizingthatthey'resomekindofcoordinates,let'stalkaboutgraphs.

Whenlearningaboutgraphsingeometryclass,welearnedaboutanx-axisandy-axisandlabeledplottedpointsalongthegraph.Ifyouwantedtomark(-2,-4)thenyouwouldfindwhere-2onthex-axisintersectswith-4onthey-axis.KnowingthatXishorizontalandYisvertical,ifwehad(-2,-4)and(1,2)wecoulddrawitoutlikethis:

Thesetwopointscouldevenbeconnectedtoforma2-dimensionalline:

Beforewegettoofarondrawingpointsandlines,let'slookbackatourfunction:

love.graphics.print("HelloWorld!",400,300)

The 400istheXpositionandincreasingitwillmovethetextfurthertotheright.DecreasingtheXpositionwillmovethetextfurthertotheleft.The 300istheYposition,onedifferencebetweencomputersandgeometryclassisdataiscalculatedfromtoptobottom,soincreasingtheYpositionmovesthetextdownanddecreasingitmovesthetextup.Let'stakealookatwhatourgame'sgraphlookslikewiththepoint(5,3)highlighted:

Noticethatthetop-leftcornerofourscreenis(0,0),soifyoutriedtodrawanypointswithnegativenumberstheywouldbedrawnoffscreenwherewecan'tseethem.Anotherthingtonoteisinthegame,thecoordinatesrepresenthowmanypixelsdownandtotherightwewanttodraw.Sincecomputerscreensaremadeofsomanypixels,youneedtouselargenumberstomakeanoticeabledifference.

Ifwewantedtodrawapolygon(shape)suchasatriangleonthisgraph,wewouldhavetogivethreepoints:

Inthesameway,wecanplotoutpointsinourcodeandtellittodrawalinetoconnectthedots.Let'suselargernumbersthough.Rewritemain.luatolooklikethis:

love.graphics.polygon('line',50,0,0,100,100,100)

Thenumbersinthiscodecanbereadoffinpairstoidentifythecoordinates:(50,0),(0,100),(100,100)LÖVE'sphysicsenginetakesthecoordinates,startingatthefirstpointandconnectsthemwithalinesequentially.Onceitreachesthelastpoint,itdrawsalinefromthelastpointbacktothefirsttoclosetheshape.

Let'stryarectangletogetsomemorepracticein:

localrectangle={100,100,100,200,200,200,200,100}

love.graphics.polygon('line',rectangle)

Noticethistimeinsteadofpassingthenumbersdirectlyinto love.graphics.polygon,weputthemintoalistandpassedthelistin.Passingincoordinatesbothwayshasthesameeffect.

Anotherimportantthingtothinkaboutisifyoudrawapolygonwith4ormoresides,youneedtomakesurethepointsarelistedinthecorrectorder.Considerthefollowingexample:

Ifwefedthepointsintothefunctioninaclockwiseorcounter-clockwise/anti-clockwiseorder,therectanglewouldbedrawnthesameeitherway.Ifwefedthepointsinfromcrossdirections,wemayaccidentallydrawabowtie:

Creatingshapeswithunclosedsidesdon'tplaywellwithLÖVE'sphysicsengineassuchshapesarenotphysicallypossible.Ifyoutrytodothis,youmaynotseetheshapeyouexpect,andperhapsnothingwillbedrawn.

2.3-Geometry

Creatingshapeswithunclosedsidesdon'tplaywellwithLÖVE'sphysicsengineassuchshapesarenotphysicallypossible.Ifyoutrytodothis,youmaynotseetheshapeyouexpect,andperhapsnothingwillbedrawn.

ExercisesTakealookatthedocumentationforthefunctionlove.graphics.polygon.Theexampleshowstheargument'line'isastringandrepresentsthe"drawMode".Trychangingthe"drawMode"from 'line'tooneoftheotheravailableoptions(seetheexamplesorclickthedrawModelinkonthewikipage).Whatotheroptionisthere?Howdoesitwork?Tryitoutandseehowitworks!Trymakingapolygonwith5sides/points.Hint:usethesquareexampleaboveasareference.

2.3-Geometry

GameloopAnotheraspectcommonwithgameenginesisthatthereisaloop(likeawhileloop)thatcontinuouslyrunsandkeepsthegamegoing.Theorderthatthingshappeninvaries,butthecontentsmoreoflesslooklike:

1. Gameisstarted.Loadgamefiles.2. Beginloop.3. Checkforinputfromkeyboard,joystick,orotherperipherals.4. Ticktimeingame.5. Redrawthescreen.6. Gobacktostep2.

Duringstepsinthegameloop,LÖVEinvokescertainfunctionsinsidethe lovetable.Forinstance,everytimethescreenneedstobere-drawn,thegameloopinvokes love.draw()ifyoudefinedit.InthestepwhereLÖVEchecksforuserinput,ifthereisuserinput,itinvokes love.keypressed(PRESSED_KEY)ifwedefinedit.The PRESSED_KEYthatispassesinofcoursedependsonwhatkeytheuserpressed.Whendefining love.keypressed,itmaylooksomethinglikethis:

love.keypressed=function(pressed_key)

print('keywaspressed:',pressed_key)

Let'smodifymain.luatohaveacontrivedexample:

localcurrent_color={1,1,1,1}

localsquare={100,100,200,200,100,200,100,200}

--Initializethesquarewiththedefaultcolor(white)

love.graphics.setColor(current_color)

--Drawthesquare

love.graphics.polygon('fill',square)

ifpressed_key=='b'then

--Blue

current_color={0,0,1,1}

elseifpressed_key=='g'then

--Green

elseifpressed_key=='r'then

elseifpressed_key=='w'then

--White

Whenyoupressanyofthekeys,"b","g","r",or"w",ourfunction love.keypressedwillbeinvokedandthevariablepressed_keywillbeastringmatchingoneofourletters.Thischanges current_color,whichischangingthecolorbeingdrawnin love.draw.

Inthenextsection,let'sseehowLÖVEhandlesthe"4.Ticktimeingame."stepofthegameloop.

2.4-Gameloop

ExercisesTryaddingafewmorecolorstotheprogram.Tounderstandhow love.graphics.setColorworks,seethedocumentation.Makeissothatiftheescapekeyispressed,thefunction love.event.quitisinvokedandthegameexits.Thestringtousefortheescapekeycanbefoundonthewiki'sKeyConstantpage.Spoilers:thesolutioncanbeseeninthenextsection.

2.4-Gameloop

DeltatimeHere'swhatwe'velearnedaboutthegameloopsofar:

1. Gameisstarted.Loadgamefiles.main.luaisloadedandthe lovetableisupdatedwithourmodifications.

2. Beginloop.3. Checkforinputfromkeyboard,joystick,orotherperipherals.

Iftherewaskeyboardinputandwedefined love.keypressed,invokeit,passingitinformationaboutthepressedkey.

4. Ticktimeingame.???

5. Redrawthescreen.Invoke love.drawifwedefinedit.

6. Gobacktostep2.

Let'stakealookatthefunction love.update:

love.update=function(dt)

print(dt)

NoteforWindows:UnlessyouarerunningLÖVEfromtheconsole,youwon'tseeanythingprintedout.Putthisfileinthegamefoldernexttomain.luaunderthename"conf.lua":

--LÖVEconfigurationfile

love.conf=function(t)

t.console=true

Thisconfigurationfilewillletussetspecialparametersforourgame.Don'tworrytoomuchaboutwhatalltheoptionsare,butifyou'recuriousthenyoucanfindthedocumentationhere.EssentiallythiswillopenaconsolewindowonWindowstoseeprintedvalues.

Nowrunthegameandifyouweren'tseeingthe print(dt)messagedisplayanythingyoushouldnowseeitbeinginvokedmanytimesasecond,printingoutadecimalnumber. dtstandsfordeltatimeanditrepresentstheamountofsecondsthathaspassedsincethelastgameloop.Ifthegameloops4timesasecond,thatmeans love.updateand love.drawgetinvoked4timeseachsecondaswell.Thedeltatimeinthiscasewouldberoughly 0.25asroughly1/4asecondhaspassedbetweeneachtime love.updatewascalled.Somecomputersarefasterthanotherssothenumberofgameloopspersecondwillbedifferent.Youarelikelyseeingnumbersaround 0.01orsmaller,meaningthegameisrunningroughly100framesasecond.Let'saddacountertothescreenlikebefore,butnowusingdeltatime.

localseconds=0

localsquare={100,100,100,200,200,200,200,100}

--Printacounterclock

localclock_display='Seconds:'..seconds

love.graphics.print(clock_display,0,0,0,2,2)

2.5-Deltatime

--Blue

--Green

--White

ifpressed_key=='escape'then

love.event.quit()

--Addupallthedeltatimeaswegetit

seconds=seconds+dt

Imagineifwewantedtomoveacharacteracrossthescreenbutwedidn'tusedeltatime.Thecharacterwouldrunfasteronsomecomputersandsloweronothers.Computerswouldkeepgettingfasterandthegamewouldrunsofastitwouldnolongerbeplayable.Deltatimesolvesthisissueandwe'llbetakingadvantageofitforeverythingtime-basedinourgame.

ExercisesChangetheline localclock_display='Seconds:'..secondssothat secondsisformattedtodisplaywholenumbers.Hint:youwillneedtouseLua'sbuilt-in math.floorfunctiontoformat seconds.Changethexpositionoftheleftsideofthesquarefrom 100to (seconds*10)andwatchwhatthesquaredoes.

2.5-Deltatime

MappingLet'ssidetrackfromLÖVEforaminutetolearnaboutaconceptcalledmaps.Nottobeconfusedwithoverheadmapsaplayerwouldwalkaroundoninagame,butdatamaps.Weactuallydidmappingbackinchapter1whenwelearnedabouttables.

coins={

["half"]="50cents",

["dime"]="10cents",

["penny"]="1cent"

print("Whichcoindoyouhave?")

response=io.read()

print("Yourcoinisworth"..coins[response]..".")

Whenevertheusertypedinacoin,wemappedthecoinnametoavaluebylookingupthecoinnameinthetable,ordictionary.Sowhat'sthedifferencebetweentables,dictionaries,andmaps?

tablesarejustadatatypeinLuathatcanbeusedtobuilddatastructureslikelistsanddictionariesdictionariesarekey-valuestoragesusedtocentralizesimilar-purposedatainoneplaceandmakeiteasiertolookthedataupmapsaredatastructuresusedtotranslateonetypeofinformationtoanother,andadictionaryisonetypeofmap

Dictionariesaretheonlytypesofmapwe'llbeconcernedabouthere,butknowthatmapsgenerallyrefertoinstancesofdatastructuresthatdomapping.Thereareoftendiscrepanciesinterminologybetweenmathematicsandthevariousfieldsincomputerscience.Don'tbesurprisedifyouseedictionariesandmapsbeingusedsynonymouslyinothercontextslaterinlife.

Let'sdosomemappingonourcodewepreviouslywrotetogetabetterfeelforthem.Rememberallthoseif/elseifstatementsinmain.lua?

--Blue

--Green

--White

love.event.quit()

Wecanputallthatfunctionalityinamaplikethis:

localkey_map={

b=function()

current_color={0,0,1,1}--Blue

2.6-Mapping

g=function()

current_color={0,1,0,1}--Green

r=function()

current_color={1,0,0,1}--Red

w=function()

current_color={1,1,1,1}--White

--Closethegame

escape=function()

love.event.quit()

Thisdoesn'tlookanymoreconcisethanourpreviouscode,butourgoalistokeepthe love.keypressedfunctioncleaninthiscase.Whenakeyispresseditwillbemappedtoakeyfunctionwedefinein key_map.Anotherimportantthingisthesefunctionscouldbemodularandmovedanywhereweneedthemtobe,andevenre-used.Let'snotgotoocrazyrightnowthough.We'llkeepthekeymapsomewherenearthetop.

localseconds=0

localkey_map={

b=function()

current_color={0,0,1,1}--Blue

g=function()

current_color={0,1,0,1}--Green

r=function()

current_color={1,0,0,1}--Red

w=function()

current_color={1,1,1,1}--White

escape=function()

love.event.quit()

localsquare={100,100,100,200,200,200,200,100}

--Printacounterclock

localclock_display='Seconds:'..math.floor(seconds)

love.graphics.print(clock_display,0,0,0,2,2)

--Checkinthekeymapifthereisafunction

--thatmatchesthispressedkey'sname

ifkey_map[pressed_key]then

key_map[pressed_key]()

--Addupallthedeltatimeaswegetit

seconds=seconds+dt

2.6-Mapping

Ifyoupressakeythatisn'tpartofthemapthenthenewifstatement( ifkey_map[pressed_key]...)willseethatkeydoesn'texistinthemapandnotdoanything. key_map[pressed_key]()isthesameassaying key_map['b'](),key_map['escape']()orwhateverthevalueof pressed_keywasatthetime.

2.6-Mapping

TheworldAworldisaphysicalspacewhereobjectscanbecreated(spawned)andinteract.Shapesandotherthingsdrawnonthescreenarenotimplicitlypartofaworldandwon'tinteractwitheachotherunlesstheyare.Multipleworldscanco-exist,buttheobjectsineachworldwon'tinteract.GoingforwardIwillrefertotheseobjectsasentities.

EntitiesEntitiesaremadeupofdifferentcomponentsthatallowthemtointeract.Thesearethe3fundamentalphysicalcomponents:

shape-somesortofpolygontogiveourentityaphysicalshapethatdeterminestheboundariesoftheentitybody-holdsphysicalpropertiessuchasmassfixture-attachesashapetoabody

Let'swriteanewmain.luafromscratchandseehowtheseareallwiredup.First,aworldneedstobedefined:

localworld=love.physics.newWorld(0,100)

love.physics.newWorldreturnsatable,aninstanceofaworld.Thetableholdsfunctionsthatallowustoapplyattributestotheworld.Italsoholdsalltheentitiesinourworld,whichiscurrentlynoneoninitialization.Accordingtothedocumentationon love.physics.newWorld,our1stand2ndparameterssettheXandYgravityonourworld.Wedon'twantanysidewaysgravity,butwe'llgoaheadandsetanarbitrarynumberfortheverticalgravity.

Whilefocusingontheworld,weshouldallowtheworldtoknowwheneverwegetanupdatetothedeltatime.Aworldwithouttimewouldbefrozen;Bylettingtheworldknowaboutthepassageoftime,itcanknowwhetheritneedstomakeanentityfallanothermeterortwometers…

world.update(world,dt)

Actually,let'sdoonetrickhere.WhencallingafunctioninLuaandthefirstparameterofthefunctionisthetablethefunctionisstoredin,youcanuseashortcutnotation:

world:update(dt)

Asidefrombeingeasiertowrite,you'llseethiswayofinvokingfunctionsusedallovertheplaceintheLÖVEdocumentation.

Finally,we'lladdanentitytothegamein4steps:

1. Createatabletostoreallthepiecesofourentitytogether.Notentirelynecessarybutwe'lllearnlaterwhythisstepmakesthingseasier.

2. Createabody.Thiswillbeaddedtotheentitytableandtheworld.3. Createtheshapewewanttheentitytohave.4. Createafixturetoattachthebodyandshapetogether.

--Triangleisthenameofourfirstentity

localtriangle={}

2.7-Theworld

triangle.body=love.physics.newBody(world,200,200,'dynamic')

--Givethetrianglesomeweight

triangle.body:setMass(32)

triangle.shape=love.physics.newPolygonShape(100,100,200,100,200,200)

triangle.fixture=love.physics.newFixture(triangle.body,triangle.shape)

love.graphics.polygon('line',triangle.body:getWorldPoints(triangle.shape:getPoints()))

Aftercreatingthe bodytableinside triangle,wecalled triangle.body.setMasstosetaweightpropertyonourtrianglesoitcanfall.Noticewewrote triangle.body:setMass(32),whichisthesameassayingtriangle.body.setMass(triangle.body,32)butshorterandmoreconventionaltothewaytheLÖVEdocumentationwrites.

What'sgoingoninside love.drawlooksprettycrazysolet'sbreakthelonglineup.

love.graphics.polygon(

'line',

triangle.body:getWorldPoints(triangle.shape:getPoints())

We'veused love.graphics.polygonpreviouslysoitspurposeshouldalreadybefamiliar.Thefirstparameter 'line'istellingitthatwewantanoutlineofashapedrawn.Thesecondparameterisatablecontainingthepointsthatneedtobeoutlined.Togetthetriangle'spointswecall triangle.shape:getPoints(),butthisonlyreturnstheshapeofthetriangleandtherelativepositionofthepoints.Bythencallingtriangle.body:getWorldPoints(triangle.shape:getPoints())weconvertthoserelativepointstotheirabsolutepositionasthat'swhatthepolygondrawingfunctionneedstoknowsoitcandrawthetriangleexactlywhereitissupposedtobeonthescreen.

Let'sputitalltogetherandaddonemoreentityintothemixsothetwocaninteract:

localtriangle={}

triangle.body.setMass(triangle.body,32)

--Anotherentity

localbar={}

bar.body=love.physics.newBody(world,200,450,'static')

bar.shape=love.physics.newPolygonShape(0,0,0,20,400,20,400,0)

bar.fixture=love.physics.newFixture(bar.body,bar.shape)

localkey_map={

escape=function()

love.event.quit()

love.graphics.polygon('line',bar.body:getWorldPoints(bar.shape:getPoints()))

2.7-Theworld

world:update(dt)

Thisisalottodigestsodon'thesitatetore-readthroughthiscodeseveraltimesifnecessary.Therewerealotofnewfunctionsintroducedinthissection,sointhenextsectionwe'lltakeadeeperlookatthedocumentationandreadmoreaboutthemandtheircomponents.

ExercisesTrychangingthemass( triangle.body:setMass)andgravity( love.physics.newWorld)andseewhathappens.

2.7-Theworld

ReadingdocumentationYoutypicallyrunintotwotypesofdocumentationforsoftware:guidesandAPIdocumentation.Guideswouldbeinformationongettingstarted,tutorials,andbooks.AnAPI(applicationprogramminginterface)isaportionofsoftwarethataprogrammerwritesforhis/herprogramtoallowfellowprogrammerstointeractwiththeirapplication.AsforLÖVE'sprogramminginterface,mostofyourinteractionswiththeframeworkaredonethroughthe loveglobalvariablethattheframeworkpurposelyexposes.APIdocumentationisthemostfundamentalformofsoftwaredocumentationbecausewithoutit,youwouldnotknowwhatallthefunctionsintheprogramdounlessyouwereresourcefulenoughtogoinandstudyallthesourcecodeandfigureeachfunctionoutonyourown.

ThedocumentationforLÖVE(bookmarkthis!)iswritteninawikistylewhereeachtableandfunctionhasanarticlethatdescribeshowtouseit.Fromhereyoushouldseemanymoduleslisted.Clickon love.physics.Again,love.physicsisjustatablewithfunctionsinit.Sowithinthearticleweseeeachofthefunctionsstoredinitincludingthefunctions love.physics.newBodywhichweusedtocreateourentities'bodies, love.physics.newPolygonShapewhichweusedtocreatetheirshapes,and love.physics.newFixturewhichweusedtocreatetheirfixtures.Wealsosee love.physics.newWorldwhichcreatedourworldtable.Let'slookatthefirstfunction'sarticle.

Clickingonthearticlefor love.physics.newBodywegetasynopsisshowinghowthefunctionmightbeused,alongwithadescriptionofitsparametersandwhatthefunctionreturns.Overthecourseofdifferentversionsoftheframework,functionsmaybemodifiedsointhecaseofthisfunctionyoucanseeexamplesofhowtouseitdifferentlyindifferentversionsofLÖVE.Sincethefunctionliststhatitreturnsabody,clickthelinktogoovertothearticleonthebodytable'sdocumentation.

There'salotoffunctionsherethatsetpropertiesonthebodyandgetpropertiesfromthebody.Oneofthemisthebody:setMassfunctionweusedtogiveourentityweight.Wecanseethatittakesoneparameterandthattheparameterismeanttosimulatehowmanykilogramsofmassthebodyhas.Weoriginallytolditthatourtriangleinthelastsectionweighed32kilograms,whichifyouthinktheobjectfelltoofastortooslowthenyoumayneedtoadjustyourworld'sgravitytomatchyourexpectation.

Nowlet'sgobackto love.physicsforamomentandtakealookatoneoftheothercomponentsweaddedtoourpreviouscode,thefixture.Wepreviouslycreatedafixturetablebycalling triangle.fixture=love.physics.newFixture(triangle.body,triangle.shape).However,wehaven'tseenanyofthesefunctionsinthefixturetablethatcouldcomeinhandy.Forinstance,wecouldgiveourtrianglebouncinessbyinvokingfixture:setRestitution.Ourtrianglefixtureisnamed triangle.fixturethough,not fixture.If0isnobounciness(default)and1is100%,trymodifyingthegamecodeandaddingarestitutionof75%:

triangle.fixture:setRestitution(0.75)

Tryrunningthegameandseehowthatworks.Ifyousettherestitutionto 1orhigherthenthetrianglewon'tstopbouncingandwillbounceitselfrightoffthescreen.

CallbacksLet'sbacktracknowtothemainarticleaboutthelovetable.Ifyouscrolldownalittleonthepage,you'llseeasectiontitled"Callbacks"thatcontainssomefunctionswe'vebecomefamiliarwithsuchas love.drawand love.update.Thisisalistofallthefunctionsinthegameloopthatwehaveandhaven'ttalkedaboutyet.A"callback"isafunctionyoucreateandgivetoanAPI(the lovetableinthiscase)thatwilllatergetinvokedasneeded.Creatingfunctionswiththesenamesallowyoutotapintospecificportionsofthegameloopandtriggeryourownevents.

Let'stakealookatthe love.keypressedcallbackforinstance.Inthesynopsisyouseethatithas3parameters(thedocumentationmistakenlycallsparameters"arguments").Weusedthefirstparameter keytoseewhatkeywaspressed.Ifyoueverneedtoknowwhatkeysareavailable,youcanclickonthelinkprovidednexttotheparametername, KeyConstanttoseeawell-definedlistofalltheavailablekeystringspassedintothisparameter.Thesecondparameter scancodewedidn'ttalkabout,butithasawell-defined Scancodearticleexplainingwhatitis.Ifyouarenotfamiliarwithscancodes,takeaminutetoreaditandperhapsyou'lllearnaboutafeatureyoumaywanttouseinyourgame.

Onemorecallbackwe'lllookatwhilewe'rehereis love.focus.Takeamomenttostophereandreadwhatitdoesandwhatparametersittakesbeforecontinuing.Nowitwouldbereallycoolifweweremakingagameandwhentheuserswitchedtoanotherapplicationwindow,thegameautomaticallypausedfortheuser.Sofirstlet'sstartbyimplementingapausefeatureinourearliergamecode:

localworld=love.physics.newWorld(0,9.81*128)

localtriangle={}

--Anotherentity

localbar={}

--Booleantokeeptrackofwhetherourgameispausedornot

localpaused=false

localkey_map={

escape=function()

love.event.quit()

space=function()

paused=notpaused

ifnotpausedthen

world:update(dt)

Noticethe3changes:

Weaddedabooleancalled pausedandsetittofalse

Weaddedanewfunctionto key_mapsothatwhen "space"ispressed,thevalueof pausedissetto notpaused. notisanoperatorforbooleanswepreviouslydidn'tdiscuss.Itsimplysays"theoppositeofthisboolean".Soif pausedis true,thensetting pausedto notpausedwillsetitto false.Lastly,inside love.updatetotoldtheworldtoupdateonlyifweare notpaused.Sothepassageoftimeinthegameworldwillceasewhenpressingthespacekey.

ExercisesNowwiththedocumentationinhand,define love.focusandmakeitsothegamepauseswhentheuserclicksawayfromthegamewindow.Bonus:makethegameprintatextsayingthatthegameispausedwhen pausedis true.Gofindthedocumentationfor love.graphics.printtoseeanexampleondisplayingtext.

ModulesandorganizationEventuallywhenyoustartwritingrealprograms,yourealizeifyoukeepallthecodeinonefilethatthingscangetabitmessy.Puttingyourcodeinseparatefileshelpsyounotonlykeepyourdifferentpiecesofcodeseparatedfromeachother,butithelpsyouvisualizethestructureofyourprogram.

Let'sstartwithasinglemain.luafileandwe'llthensplititintodifferentfiles:

localmy_cool_functon=function()

love.graphics.print('Thisfunctioncamefromfunction-module.lua',100,100,0,2)

localmy_cool_table={}

my_cool_table.print_stuff=function()

love.graphics.print('Thisfunctioncamefromtable-module.lua',100,200,0,2)

print('my_cool_functionis',my_cool_function)

print('my_cool_tableis',my_cool_table)

Whenwegotorunthecode,wegetablankwindowbecausewe'renotdrawinganything.Wedoseeourprintstatementsoutputourfunctionandtabletotheconsolethough:

my_cool_functionisfunction:0x41f2abc0

my_cool_tableistable:0x41f2aa08

Modulesand requireThinkofyourLuafilesasgiantfunctionsthatgetinvokedwheneveryouloadthefile.Justlikeafunction,youcanreturnvaluesfromyourfiles.IfyouloadoneLuafilefromanother,youwillgetwhatevervalueisreturned.Let'smodifyourpreviouscode.Firstdefinethesetwonewfilesinthegamefolder:

function-module.lua

returnfunction()

love.graphics.print('Thisfunctioncamefromfunction-module.lua',100,100,0,2)

table-module.lua

localmy_cool_table={}

my_cool_table.print_stuff=function()

love.graphics.print('Thisfunctioncamefromtable-module.lua',100,200,0,2)

returnmy_cool_table

Thenupdatemain.lua:

localfunction_module=require('function-module')

localtable_module=require('table-module')

print('function_moduleis',function_module)

print('table_moduleis',table_module)

Let'sstartfromthetop.Infunction-module.luawewriteareturnstatementthatreturnsafunctionwithnoname.Wedon'tinvokethefunction,wejustreturnitasavaluethesamewayafunctionmayreturnanumberorstring.Likewiseintable-module.luawedefinedatable(withafunctioninit)andreturnedthetableonthelastlineofthefile.Thefunctionnameandlocalvariablename my_cool_tableisinconsequentialandcan'tbeseenoutsidethetable-module.luafileasmoduleshavetheirownscope.

Inmain.luawearerequiringfunction-moduleusingabuilt-inLuafunction, require. requiretakesoneargument,astringthatequalsthenameofyourfileandittheninvokesthatfileandreturnsbackafunctionwhichweassigntoanewvariable function_module.Wethendothesamethingfortable-module.lua.Werequireit,whichinvokesitandreturnsbackwhateverthatfilereturns.Inthiscaseisatable.Noticethatwhenwepassinthefilenamesasargumentswejustgivethefirstpartofthefilenamewithouttheextension".lua"attheend.ThisfunctionexpectsthatanyfileitisrequiringisaLuafile.

Afterwerequiredthefiles,weprintedthevaluesofthosevariables,soyoushouldseetheresultsoftheprintstatementsappearintheconsolelikebefore:

function_moduleisfunction:0x40479548

table_moduleistable:0x40479bc8

Wepulledinthereturnvaluesfromtheothertwofilesintoourmain.luafileandprintedthevalues,butsincewedidn'tinvokethefunctionsfromthosetwofilesthenwegotablankgamewindowwhenrunningtheprogram.Let'sdefinealove.drawinmain.lualikebeforeandinvokethefunctionswegotbackfrombothmodules:

localfunction_module=require('function-module')

localtable_module=require('table-module')

print('function_moduleis',function_module)

print('table_moduleis',table_module)

function_module()

table_module.print_stuff()

Wewereabletoinvokethefunctionsandusethereturneddataasifitwereallinthesamefile.

OrganizingmodulesItmayhelptoseearealexampleofusingmodulesinagame,solet'stakeourpreviousgamecodefrom2.8-Readingdocumentationandseehowwecanseparateoutfunctionality.Thefirstthingwedidinourgamewasdefineaworld,solet'sstartbyputtingourworld-relatedcodeinadedicatedfilenamedworld.lua:

--world.lua

returnworld

Rememberthatyouneedthereturnstatementattheendofyourfilesorelsethecodewillreturn nilwhenyougotorequireitandthiscouldcauseallkindsofunexepctederrorswhenyourunit.Nextlet'screateafoldernamedentitiesthatwecankeepallourgameentitiesin.Weplanoncreatingmoreentitiessoitwillhelptokeepthemalltogether.Intheentitiesfolder,createafileandnameittriangle.lua.We'llcutallthecodefromtheoriginalmain.luathatrelatedtoourtriangleentityandputithere:

--entities/triangle.lua

localworld=require('world')

localtriangle={}

returntriangle

Noticehowwearerequiringthe worldtablefromworld.lua,becauseweneedtoaccessthattableinthisentity'sfilesowecanaddtheentitytotheworld.Wealsoneedtodothesamethingasabovewiththebarentity:

--entities/bar.lua

localbar={}

returnbar

Nowourmain.luashouldonlycontainourkeymapand lovefunctions:

--main.lua

localbar=require('entities/bar')

localtriangle=require('entities/triangle')

localpaused=false

localkey_map={

escape=function()

love.event.quit()

space=function()

paused=notpaused

love.focus=function(focused)

ifnotfocusedthen

paused=true

world:update(dt)

Ourtwoentitiesandworldgetpulledintomain.luaandeverythingshouldrunexactlyasbefore.Onethingtonoteisthateventhoughwerequireworld.lua3timesinourcode,itisthesameworldandnot3copies.ThisisbecauseLuaknowstoonlyrunamodulethefirsttimeyourequireitandnotinvokeitagain.Onceitrunsthefirsttime,thereturnedresultsarestoredinmemoryforthenexttimeyoutrytorequireit.Wecanprovethisbyaddingaprintstatementtoworld.lua:

--world.lua

print("Thisistheworld")

returnworld

Howmanytimesdoes "Thisistheworld"getprintedtotheconsole?

ExercisesTrycreatingtwonewmodules;Onethatreturnsastringandonethatreturnsanumber.

CollisioncallbacksWhenwritingagamesuchasaplatformeryoumaywantsomethingspecialtohappenwhentwoobjectscollide.Ifit'sapowerup,forinstance,youmaywantthepoweruptodespawn(beremovedfromtheworld)ifaplayertouchesitandthengivetheplayeraspecialability(thinkMarioandmushrooms).Ifaplayerandanenemybumpintoeachother,youmaywanttheplayer'shealthtodecrement.Theworldtablehasamethodthatallowsyoutoprograminfunctionalitylikethisforwhentwoentitiescollide.Itdoesthisbyallowingyoutocreatecallbacksaswelearnedbefore,butthesecallbacksaretriggeredbefore,during,oraftercollision.TakealookatWorld:setCallbacks.

Ifyoulookattheparametersfor World:setCallbacks,youseeitcantakefourfunctions.Thedescriptionoftheseparametershelpsexplainwhenthefunctionswillbecalled. beginContactand endContactshouldbeselfexplanatory;Theyhappenatthepointwherecontactbeginsandendsinacollision,but preSolveand postSolvemaynotbeasobvious.Nonetheless,let'seditthepreviously-createdworld.luafileandwritesomecollisioncallbackstotestthisfunctionality.

--world.lua

localbegin_contact_counter=0

localend_contact_counter=0

localpre_solve_counter=0

localpost_solve_counter=0

localbegin_contact_callback=function()

begin_contact_counter=begin_contact_counter+1

print('beginContactcalled'..begin_contact_counter..'times')

localend_contact_callback=function()

end_contact_counter=end_contact_counter+1

print('endContactcalled'..end_contact_counter..'times')

localpre_solve_callback=function()

pre_solve_counter=pre_solve_counter+1

print('preSolvecalled'..pre_solve_counter..'times')

localpost_solve_callback=function()

post_solve_counter=post_solve_counter+1

print('postSolvecalled'..post_solve_counter..'times')

world:setCallbacks(

begin_contact_callback,

end_contact_callback,

pre_solve_callback,

post_solve_callback

returnworld

Tryitout.Everytimeoneofthecallbacksisinvoked,itwillincrementitsownnumberby1thenprintamessagetotheconsoletellingyouhowmanytimesithasbeeninvoked.It'sclearrightawaythat pre_solve_callbackandpost_solve_callbackgetinvokedmanymoretimesthan begin_contact_callbackand end_contact_callbackinthissituation.

Unlessyou'veeditedthebehaviorofthetriangleentity,itwillbounceabit(becauseofthetriangle'srestitution).Onceitbouncesandneithercornerorsideistouchingthefloorunderneath,thecontactends.Thisprocessisrepeatedeverytimeitbounces.Oncethetrianglesettlesdownitwillslideabit,maybeevenalot...likeanairhockeypuck.Thisisbecauseourtriangleandbarhavenofrictionbetweenthemtopreventthat.Anyways,thisisgoodbecauseitallowsustoseethatwhilethetriangleisslidingitisstillmakingcontact.Whilethetriangleisslidingandstillmakingcontact,the pre_solve_callbackand post_solve_callbackwillcontinuetogetcalledwitheveryframeofmovement.

Pretendourtrianglewasafuturisticracecarmovingacrossaneonstripofroadthatrechargedthevehicle.Youcouldstartincreasingtheracecar'spowermeterinside begin_contact_callbackasthecarmakescontactwiththatsectionofroadandthenstopincreasingpowerwhen end_contact_callbackisinvoked.Thiscouldworkprettywell,butthentheplayermaytryparkingforamomentonthepowerstripandcontinuetogainhealthaslongastheywant.Soanotherapproachcouldbetoonlyincreasethepowermeterastheplayercontinuestomoveandmakecontactwiththeroad,increasinghealthby1pointeverytimethe post_solve_callbackfunctionisinvoked.

Youdon'tnecessarilyneedtouseallofthesecallbacks,soyoucouldjustpassinanemptyfunctionor niltoWorld:setCallbacksfortheargumentsyoudon'tneed.

Withoutknowingwhatentitiesarecolliding,thecollisioncallbacksaren'tveryuseful.Luckily,ourcallbackshaveparametersoftheirownthatwecanaccess.Let'smodifythecodeagainandcheckoutthoseparameters:

--world.lua

--Calledatthebeginningofanycontactintheworld.Parameters:

--{fixture}fixture_a-firstfixtureobjectinthecollision.

--{fixture}fixture_b-secondfixtureobjectinthecollision.

--{contact}contact-worldobjectcreatedonandatthepointof

--contact.Whenslidingalonganobject,theremaybeseveral.

--Seefurther:https://love2d.org/wiki/Contact

localbegin_contact_callback=function(fixture_a,fixture_b,contact)

print(fixture_a,fixture_b,contact,'beginningcontact')

localend_contact_callback=function(fixture_a,fixture_b,contact)

print(fixture_a,fixture_b,contact,'endingcontact')

localpre_solve_callback=function(fixture_a,fixture_b,contact)

print(fixture_a,fixture_b,contact,'abouttoresolveacontact')

localpost_solve_callback=function(fixture_a,fixture_b,contact)

print(fixture_a,fixture_b,contact,'justresolvedacontact')

world:setCallbacks(

pre_solve_callback,

post_solve_callback

returnworld

Thisshouldprintoutsomeinformationintheconsolesimilarto:

Fixture:0x561020bf8570Fixture:0x561020bf7350Contact:0x561020bf7480beginningcollision

Fixture:0x561020bf8570Fixture:0x561020bf7350Contact:0x561020bf7480abouttoresolveacontact

Fixture:0x561020bf8570Fixture:0x561020bf7350Contact:0x561020bf7480justresolvedacontact

Fixture:0x561020bf8570Fixture:0x561020bf7350Contact:0x561020bf7480endingcollision

Fixture:0x561020bf8570isatextrepresentationofourfirstentity'sfixture.The 0x56...isthememoryaddressofthefixturetohelpidentifyit,althoughthisinformationstilldoesn'ttelluswhichentitythisfixturebelongsto.Wealsoprintedoutacontacttable,whichcontainsasetoffunctionsjustliketheentities.Thisinstanceofacontactprovidesinformationsuchaswherethecontacthappenedandhowmuchvelocitywasinvolved.

Let'sworkonmodifyingtheprintstatementssowecancollectmoreusefulinformationonthesecollisions.Thereisapairoffunctionsoneveryfixturethatlet'syousetanyarbitrarydatayouwantonthatfixtureandanotherfunctiontogetthatdatabackoutthefixture.Thesefunctionsarecalled Fixture:setUserDataand Fixture:getUserData.ThesefunctionscanbeusedtosetanameorIDonthefixturetohelpusidentifywhatentityitbelongsto.Wecanaccomplishthisbyfirstmodifyingourentityfilesandpassingsomestringsto Fixture:setUserData:

--entities/bar.lua

localbar={}

bar.fixture:setUserData('bar')

returnbar

--entities/triangle.lua

localtriangle={}

triangle.fixture:setUserData('triangle')

returntriangle

Nowgobacktotheworld'scollisioncallbacksandyoucaneasilyextractthisinformationbackoutofthefixtures:

--world.lua

localname_a=fixture_a:getUserData()

localname_b=fixture_b:getUserData()

print(name_a,name_b,contact,'beginningcontact')

print(name_a,name_b,contact,'endingcontact')

print(name_a,name_b,contact,'abouttoresolveacontact')

print(name_a,name_b,contact,'justresolvedacontact')

world:setCallbacks(

pre_solve_callback,

post_solve_callback

returnworld

Ah,nowwecanseewhichfixtureiscollidingwhich!

bartriangleContact:0x55bf29c07590beginningcontact

bartriangleContact:0x55bf29c07590abouttoresolveacontact

bartriangleContact:0x55bf29c07590justresolvedacontact

bartriangleContact:0x55bf29c07590endingcontact

ExercisesModifytheprintstatementsineachcollisioncallbacktoprintoutthecoordinateswheretheentities'fixturesaremakingcontact.Youcanfindtheinformationyouneedtodothisinthedocumentationforthecontacttablementionedabove.

Breakout(part1):moreentitypracticeLet'sbringalltheseconceptstogetherbymakinganothergame,abreakoutclone.Therequirementsareprettysimple:

TheobjectiveofthegameistodestroyallthebricksonthescreenTheplayercontrolsa"paddle"entitythathitsaballTheballdestroysthebricksTheballneedstostaywithintheboundariesofthescreenIftheballtouchesthebottomofthescreen(belowthepaddle),thegameends

Ifyoustillhavethecodefromtheprevioussections,feelfreetocopythefoldernamingthenewone"breakout"orwhateveryouwantyourbreakoutclonetobecalled.Attheendofthissectiontherewillbealinktoallthesourcecodetouseasareferenceincaseyougetstuck.Thismaybetimeconsuming,butIencourageyoutotypeouteachsectionandstoptounderstandwhatitisyouaretyping.Ifyoucopy,paste,anddon'treadthenitwillbeeasytogetlostinthischunkofthechapterasthingswillmovefast.

Thefirstmodificationwe'llmakeistosetaspecificwindowsizesonomatterwhichversionofLÖVEyou'reonwe'reworkingwiththesamewindowproportionsandentitydimensions.Todothis,openofconf.luaorcreateitifyoudon'thaveitandputinthefollowingcode:

--conf.lua

t.console=true--EnablethedebugconsoleforWindows.

t.window.width=800--Game'sscreenwidth(numberofpixels)

t.window.height=600--Game'sscreenheight(numberofpixels)

Theconf,orconfigurationfileletsyoudefineacallbackinthe lovetablethatmodifiesthegameengine'sconfigurationonload.Youcanreadmoreaboutalltheinterestingthingsyoucandowithitherebutmostofitsfeatureswon'tbenecessaryforoursimplegame.

Thenextmodificationwe'llmakeisdeletingtheentitiesfromthelastsection.Let'screatenewentitiestorepresenttheballandpaddle:

--entities/ball.lua

localentity={}

entity.body=love.physics.newBody(world,200,200,'dynamic')

entity.body:setMass(32)

entity.body:setLinearVelocity(300,300)

entity.shape=love.physics.newCircleShape(0,0,10)

entity.fixture=love.physics.newFixture(entity.body,entity.shape)

entity.fixture:setRestitution(1)

entity.fixture:setUserData(entity)

returnentity

--entities/paddle.lua

localentity={}

entity.body=love.physics.newBody(world,200,560,'static')

entity.shape=love.physics.newRectangleShape(180,20)

returnentity

Beforewetryandrunanything,takealookatafewthingswe'vedonedifferentlyindefiningtheseentitiesthanwe'vepreviouslydone.

Inball.luawearedefiningacircleshapeinsteadofapolygon.Thismeanswehavenosidesorcornerpointswecanreferencewhenspawningortrackingthepositionofthisobject.Circleshavetobetrackedfromtheircenterpointandtheirboundariesbytheirradius.Inthisfilewe'reusing Body:setLinearVelocitytoapplymovementontheballinaspecificdirectionwhentheentityspawns.Inpaddle.luawearedefiningapolygonshape,butinsteadofspecifyingeachpointweareusingthelove.physics.newRectangleShapefunctiontodefinetheshape.Thiswillstillgenerateapolygonasbefore,butinsteadofspecifyingeachpointintheshapewearegivingaheightandwidthandallowingittofigureouttheshapewewantbasedonthosetwoparameters.Thinkofitasashortcutversionofthelove.physics.newPolygonShapefunction.Thepaddlehasastaticbodywhiletheballisdynamic.Whatthisentailsistheballwillbeaffectedbythepaddlebutthepaddlewon'tbeaffectedbytheball.Eventhoughthepaddleisstatic,itcanbemanuallyrepositionedaswe'lldolaterwithbuttons.Inbothentityfiles,wearepassingthefullentitytableasthefixtureuserdatainsteadofjustastringnamelikebefore.Thiswillallowustoeasilyaccesstheentireentityinsidethecollisioncallbackaswe'llseelater.You'llwanttogobackandcomparethatcodefromtheCollisionCallbackssectiontotheseentities,butdon'tworryifitdoesn'tmakecompletesenseyet.

Nowweneedtomodifymain.luatoloadupournewentities:

--main.lua

localpaddle=require('entities/paddle')

localball=require('entities/ball')

localpaused=false

localkey_map={

escape=function()

love.event.quit()

space=function()

paused=notpaused

localball_x,ball_y=ball.body:getWorldCenter()

love.graphics.circle('fill',ball_x,ball_y,ball.shape:getRadius())

love.graphics.polygon(

'line',

paddle.body:getWorldPoints(paddle.shape:getPoints())

ifnotfocusedthen

paused=true

ifnotpausedthen

world:update(dt)

Takenoteofafewthingswe'redoinghere:

Fordrawingthecircle,weneedtoinvoke love.graphics.circle.Fordrawingthepaddle,westillinvoke love.graphics.polygonastherectangleisstillapolygonshape.

Nowlet'sremoveanyprintstatementsinworld.luajusttocleanthingsup.We'llleavethecallbackstheresincewemayusethemlaterbutwe'llleavethememptyfornow.We'llalsosetthegravityto 0becausewewanttheballtobouncefreelylikeintherealBreakoutgameandnotloseanymomentum.

--world.lua

world:setCallbacks(

pre_solve_callback,

post_solve_callback

returnworld

Whathappensifyourunthegamenow?Theballfliesrightoffthescreenwithoutconsequence.Thereareacoupledifferentwaysofpreventingtheballfrommovingoffscreen.Possiblythemostsimpleapproachistoputupsomewalls.Canyouguesswhatthecodetothosewallsmaylooklike?Yup,theywillbeentitiessimilartothepaddleexceptthattheyjustsitattheedgesofthescreen.Let'screatesomeentitiesforthatpurpose:

--entities/boundary-top.lua

localentity={}

returnentity

Takealookatthesenumbersforaminute.Forthelocationofthebodywespecified400pixels.Sostartingfromthetopleftcornerandmovingrightalongthex-axiswe'vespecifiedtheverycenterofan800-pixel-widewindow.Thereasonwe'vedonethisisbecausewewantthetopandbottomwallboundariestostretch800pixelswide,theentirelengthofthewindow,andwhencalling newBodyandspawninganentity'sbodyitwillspawnthecenterpointoftheentityshapeatthatlocation.Notallentityshapesaresquare,orevenpolygonal,soitissimplestforthegameenginetocentertheshapeonthebody'sspawnpointratherthanusinganotherpointofreferenceontheshape,likethetopleftcorneroftheshape(notallshapeshavecorners).Infact,theballandpaddlespawnedcenteredonthelocationwegavefortheirbodies.

Sowemadethewalls800pixelswideandjusttogiveitalittlevisibilitywemadethem10pixelstall.Youwouldthinkwe'dspawnthewallattheverytopofthescreen(0pixelsonthey-axis,)butsinceourwallswillbecenteredtothespawnpointsweshouldmovedownhalftheheightofthewallifwewantitalltoappearonscreen.

Nowtheboundaryonthebottomwillhavethesamedimensions,butitwillbespawnedatthebottomofthescreen(600pixels)minushalftheheightofthewall(5pixels):

--entities/boundary-bottom.lua

localentity={}

returnentity

Theleftandrightboundarieswillfollowthesamepatternexcepttheywillbetheheightofthescreeninsteadofthewidthofthescreen:

--entities/boundary-left.lua

localentity={}

returnentity

--entities/boundary-right.lua

localentity={}

returnentity

Wewon'tseetheseentitiesuntilwerequirethemanddrawthemonthescreen.Somodifymain.luatorequireanddrawthemthesamewaywedotheballandpaddle:

--main.lua

localboundary_bottom=require('entities/boundary-bottom')

localboundary_left=require('entities/boundary-left')

localboundary_right=require('entities/boundary-right')

localboundary_top=require('entities/boundary-top')

localpaused=false

localkey_map={

escape=function()

love.event.quit()

space=function()

paused=notpaused

love.graphics.polygon('line',boundary_bottom.body:getWorldPoints(boundary_bottom.shape:getPoints()))

love.graphics.polygon('line',boundary_left.body:getWorldPoints(boundary_left.shape:getPoints()))

love.graphics.polygon('line',boundary_right.body:getWorldPoints(boundary_right.shape:getPoints()))

love.graphics.polygon('line',boundary_top.body:getWorldPoints(boundary_top.shape:getPoints()))

love.graphics.polygon('line',paddle.body:getWorldPoints(paddle.shape:getPoints()))

ifnotfocusedthen

paused=true

ifnotpausedthen

world:update(dt)

Whenyourunthegame,youshouldseeprettymuchthesamethingasthis:

Ifyoumissedanythingorarehavingissues,here'sacopyofthecompletedsourcecodeforthissection:https://github.com/RVAGameJams/learn2love/tree/master/code/breakout-1

Lookingbackatourlistofminimalrequirementswe'vealreadycompletedonethingonourlist:

Theballneedstostaywithintheboundariesofthescreen

There'sstillquiteabitmoreworktocompletethislistsolet'scontinueinthenextsection.

ExercisesMaybeitwouldbebetteriftheboundarylineswereevenwiththescreensowecouldn'tseethem.Modifytheboundarypositionssoitlooksliketheballisbouncingofftheedgeofthescreen.Whathappensifwe requiretheboundariesbutdon'tdrawthemin love.draw?Doesthegamestillwork?

Breakout(part2):entitymanagement

ReviewIntheprevioussectionwemadeachecklistofrequirementsandaccomplishedoneofthem:

TheobjectiveofthegameistodestroyallthebricksonthescreenTheplayercontrolsa"paddle"entitythathitsaballTheballdestroysthebricks✔TheballneedstostaywithintheboundariesofthescreenIftheballtouchesthebottomofthescreen,thegameends

Inthepreviousexercise,thegoalwastomovetheboundariessotheywerejustoffscreen.Thisgivestheeffectthattheballisbouncingofftheedgesofthegamewindow.

--entities/boundary-left.lua

entity.body=love.physics.newBody(world,-6,300,'static')

--entities/boundary-right.lua

--entities/boundary-top.lua

entity.body=love.physics.newBody(world,400,-6,'static')

Heretheyhavebeenmoved6pixelsoffscreenjusttouseevennumbersandmakecalculationeasier.Previouslywealsoraisedthequestionofwhetherornottheboundarieswouldworkifwestill require'dtheminmain.luabutdidn'tdrawthemin love.draw.Theansweristheystillworkbutwedon'tseethem.Sincetheyareoffscreen,thatdoesn'tmatteranywayandwecansaveourprogramfromdoingextrawork:

--main.lua

EntitylistLet'sthinkabouttheproblemofbrickentitiesforaminute.Wecouldcreateanentityfileforeachbrick,buttheyaremoreorlessthesameexceptthattheyspawnindifferentspots.Imaginemaking50differententityfilesandtheninside love.drawmaking50linestodraweachbrickandsoon.Whatwecaninsteaddoismakeanentityfilefor1brickthenmakealistwith50copiesofit(orhowevermanybrickcopiesweendupfittingonthescreen).Wecanthenloopoverthislisttodrawthebricks.

Let'sfirstcreatethebrickentityfile:

--entities/brick.lua

returnfunction(x_pos,y_pos)

localentity={}

entity.body=love.physics.newBody(world,x_pos,y_pos,'static')

returnentity

Insteadofreturninganentityinthisfile,wereturnedafunctionthattakesanx-positionandy-positionasparameters.Whenthefunctiongetsinvokedwhereveritisrequired,itwillgenerateanewentitywiththosecoordinatesforitsspawnpoint.Here'showwecanuseit:

--main.lua

localbrick=require('entities/brick')

localentities={

brick(100,100),

brick(200,100),

brick(300,100)

localpaused=false

localkey_map={

escape=function()

love.event.quit()

space=function()

paused=notpaused

for_,entityinipairs(entities)do

love.graphics.polygon('fill',entity.body:getWorldPoints(entity.shape:getPoints()))

ifnotfocusedthen

paused=true

ifnotpausedthen

world:update(dt)

Wemadeanentitytablewithalistofbrickentitiesinit,thenin love.drawwemadeaforlooptodraweachentityinthelist.Beforewechangeanythingelsetryrunningthegameandtakingalookthatthebricksappearandthateverythingworks.

RuleofsingleresponsibilityOurgoalfortherestofthissectionwillbetosimplifyentitymanagement.Onestrategywe'llhavefordoingthisistothinkofeachfileinourgameashavingasingleresponsibility.Agoodsignthatwe'redoingthisismain.luaisverysmallandeasytoscanoverwiththeeyesanddigest.

Sowhatistheresponsibilityofmain.lua?

Createthecallbackfunctionsnecessarytorunthegame.

Here'ssomethingsitisdoingthatdon'tfitthatresponsibility:

LoadandstorealltheentitiesFigureouthowtodraweachtypeofentityin love.drawStoreamapofkeypresses

Imagineourgameisanorganizationandeachfileisaroleinthecompany.Ourmainfileislikethesecretarythatknowshowtohandlerequestsfromoutsiders.Ifsomebodycalledaskingthesecretaryaboutbuilding-maintenanceissues,thesecretarywouldn'tgrabplumbingtoolsandtakecareoftheproblembutratherdispatchthepersonwhoseresponsibilityisthatexactkindofproblem.Astheownerofthisorganizationweshouldknoweveryone'srolessoit'seasytoknowwhereeachresponsibilitylies.Itwillmakeiteasierforustogrowthecompanytothesizewedesire.

Oneeasyimprovementistonotwriteoutalltheinstructionsfordrawingeachentitywithinthemainfile,butratherleteachentityfileberesponsibleforeveryfeatureofthatentity,includinghowtodrawthatentity.Wemaywanttogetfancylateranddrawbricksindifferentcolors,forinstance.Thatcouldgetcomplicatedandwedon'twantthemainfiletoretainabunchofcodeaboutbrickcolorsandsuch.

Modifyingtheentitiesisaseasyascreating drawfunctionsintheentitytables:

localentity={}

entity.draw=function(self)

love.graphics.polygon('fill',self.body:getWorldPoints(self.shape:getPoints()))

returnentity

returnfunction(pos_x,pos_y)

localentity={}

entity.body=love.physics.newBody(world,pos_x,pos_y,'static')

love.graphics.polygon('line',self.body:getWorldPoints(self.shape:getPoints()))

returnentity

--entities/ball.lua

localentity={}

entity.body=love.physics.newBody(world,x_pos,y_pos,'dynamic')

entity.body:setMass(32)

localself_x,self_y=self.body:getWorldCenter()

love.graphics.circle('fill',self_x,self_y,self.shape:getRadius())

returnentity

Goaheadandmakealltheentitiesreturnafunctionwith x_posand y_posparametersandwe'lljustaddeverythingtotheentitylistlikethebricks.Don'tforgettochangeoutthenumbersinthe love.physics.newBody(world,200,200,'dynamic')withtheargumentsbeingpassedinbythefunction: love.physics.newBody(world,x_pos,y_pos,'dynamic').Fortheboundariesentityfilesthereisnoneedfor entity.drawfunctions,butstillmakethemreturnfunctionswiththetwoparameters.Nowupdatethe entitieslistinmain.luatoincludealltheentities:

--main.lua

localentities={

boundary_bottom(400,606),

boundary_left(-6,300),

boundary_right(806,300),

boundary_top(400,-6),

paddle(300,500),

ball(200,200),

brick(100,100),

brick(200,100),

brick(300,100)

localpaused=false

localkey_map={

escape=function()

love.event.quit()

space=function()

paused=notpaused

ifentity.drawthenentity:draw()end

ifnotfocusedthen

paused=true

ifnotpausedthen

world:update(dt)

Takealookatour love.drawfunction.Itismuchsimplernowthatitnolongerneedstoknowhowtodraweachentity.Itjustaskstheentityifitknowshowtodrawitselfandifitdoesittellsittodoso.Rememberthatinvokingentity:draw()isjustshorthandforwriting entity.draw(entity)becauseofthe :.

Ok,butputtingtheentitiesinalistdidn'tcleanupthisfile.Nowthisfileisresponsibleforknowingwheretospawntheentitiesandhavingtheminalistjustmakesthisfilebigger.Wellyouseethereasonweputtheminalistisbecausewewanttomakeanewgamefilecalledentities.luathatwillberesponsibleforloading,spawning,andstoringalltheentitieswhenthegamestartsup.Createanewfilethencutalltheentity requirestatementsandtheentitylistandpasteitinthenewfile:

--entities.lua

return{

boundary_left(-6,300),

boundary_right(806,300),

paddle(300,500),

ball(200,200),

brick(100,100),

brick(200,100),

brick(300,100)

Andnowthetopofourmainfileonlyneedstoloadtheentitiesfileanditwillhavethelisttousein love.drawandelsewhereasneeded:

--main.lua

localentities=require('entities')

Whenyourunthegame,youshouldbeseeingsomethingsimilartothis:

Andthat'saboutitforentitymanagement.We'llfigureouthowtohandlekeypressesforthepaddleandeverythingelseinthenextsection.We'llfinishthecleanupinourmainfilewhilewe'reatit.

ExercisesNowthatourentitieshavepassedoffknowledgeonwheretheyspawnovertoentities.lua,ourleftandrightboundariesareidenticalfiles.Replaceboundary-left.luaandboundary-right.luawithasingleboundary-vertical.luafileandspawntwocopiesofthatinentities.lua.Ifyougetstuck,checkouttheentities.luafileinthenextsectionforhowthisisdone.

Breakout(part3):inputs

ReviewIntheprevioussectionwereconstructedourentitiestomakeroomforbricksandadditionalfunctionality.Wehaven'tcompletedanynewitemsonourchecklist:

TheobjectiveofthegameistodestroyallthebricksonthescreenTheplayercontrolsa"paddle"entitythathitsaballTheballdestroysthebricks✔TheballneedstostaywithintheboundariesofthescreenIftheballtouchesthebottomofthescreen,thegameends

Solet'scomeupwithasystemtohandleuserinputandgetthepaddlemoving.

InputserviceInsidemain.luathereissomefunctionalityforthisthatwearegoingtoremoveandrewritestartingwithanewfilethatspecificallyhandlesalltheuserinput.Thiskindoffileistypicallycalledaservicebecauseitabstractsawaytediousfunctionalityintoaneasy-to-useservice.Iencourageyoutowriteouttheserviceinsteadofcopyingandpasting.Readthrougheachfunctionandtrytounderstandwhateachonedoes.

--input.lua

--Thistableistheserviceandwillcontainsomefunctions

--thatcanbeaccessedfromentitiesorthemain.lua.

localinput={}

--Mapspecificuserinputstogameactions

localpress_functions={}

localrelease_functions={}

--Formovingpaddleleft

input.left=false

--Formovingpaddleright

input.right=false

--Keeptrackofwhethergameispause

input.paused=false

--Lookupinthemapforactionsthatcorrespondtospecifickeypresses

input.press=function(pressed_key)

ifpress_functions[pressed_key]then

press_functions[pressed_key]()

--Lookupinthemapforactionsthatcorrespondtospecifickeyreleases

input.release=function(released_key)

ifrelease_functions[released_key]then

release_functions[released_key]()

--Handlewindowfocusing/unfocusing

input.toggle_focus=function(focused)

ifnotfocusedthen

input.paused=true

press_functions.left=function()

input.left=true

press_functions.right=function()

input.right=true

press_functions.escape=function()

love.event.quit()

press_functions.space=function()

input.paused=notinput.paused

release_functions.left=function()

input.left=false

release_functions.right=function()

input.right=false

returninput

Theinputtableiswhatgetsreturned,meaningwhenwe require('input')inanotherfile,wegetbackthattableanditscontents.Insidetheinputtherearethreebooleanpropertiesthatgettoggledbyuserinput: input.left,input.right,and input.paused.Alongwiththesethreeproperties,therearethreefunctionsexposedtoustomakeuseof: input.press, input.release,and input.toggle_focus,allofwhichwewillinvokefromourcallbacksinmain.lua:

--main.lua

localinput=require('input')

input.toggle_focus(focused)

input.press(pressed_key)

love.keyreleased=function(released_key)

input.release(released_key)

ifnotinput.pausedthen

ifentity.updatethenentity:update(dt)end

world:update(dt)

In love.updateweskipupdatesif input.pausedis true.Howeverifthegameisnotpausedthenitwillloopthroughtheentitylist,calling entity.updateiftheentityhasanupdatefunction.Withthisaddedfunctionality,wecanappendan entity.updatefunctionintoourexistingpaddlecode:

entity.update=function(self)

--Don'tmoveifbothkeysarepressed.Justreturn

--insteadofgoingthroughtherestofthefunction.

ifinput.leftandinput.rightthen

return

localself_x,self_y=self.body:getPosition()

ifinput.leftthen

self.body:setPosition(self_x-10,self_y)

elseifinput.rightthen

self.body:setPosition(self_x+10,self_y)

Theleftandrightarrowswillnowmovethepaddle!Thereisn'tmuchelsetosayhereinthewayofinput.Abitunrelatedtotheactualinput,butmoresothepaddlefunctionalityisitmovesoffscreenanddoesn'tadheretotheboundaries?Whyisthat?

Ifyourememberwhenwecreatedthepaddle,itisastaticentity.Itdoesn'thavetheabilitytomoveonitsownorbytheeffectofotherentities.Thiswillcauseussomeproblemslater(andwe'relearningthehardway)!Ratherthanforcingthepaddlewithaninvisiblepush,weforceanewpositionforthepaddlewhenwecall body:setPositioninsidethepaddle's entity.updatefunction.It'slikewe'reteleportingitontopofwhateverspacewewantwithakeystroke,ignoringallphysicsandcollision.Thisissimplertocodeandgetsaroundthefactthepaddle'sstaticbodywon'trespondtoforce.Tofixthis,wecanartificiallysettheboundaryonthepaddlebycheckingifitisoutofboundsbeforemovingit.

return

ifinput.leftthen

localnew_x=math.max(self_x-10,108)

self.body:setPosition(new_x,self_y)

localnew_x=math.min(self_x+10,700)

Calling math.maxmeanswewillsetthenewx-positiontoeither self_x-10or 100,whichevernumberisbigger.Thispreventsusfromgettinganumbersosmallitrunsofftoofartotheleft. math.mindoestheoppositeandtakescareoftherightsideofthescreen.

Oneissueyoumayormaynotnoticeismovementisn'talwaysauniformspeed,anddependingonthespeedofyourcomputerthepaddlemayappeartogofasterorslower.Rememberthearticleondeltatime?Weneedtoscalethedistancetravelledtomatchtheamountoftimethathaspassed.Conveniently,wearegettingthedeltatimefromlove.updatealready.Takeacloserlookatit:

--main.lua

--Deltatimeisbeingpassed

--totheentity.updatefunctionhere

world:update(dt)

Whichmeanswecandothis:

entity.update=function(self,dt)

return

ifinput.leftthen

localnew_x=math.max(self_x-(400*dt),100)

localnew_x=math.min(self_x+(400*dt),700)

Thenumber 400isarbitraryandcanbewhateverspeedyouwantthepaddletomoveat. dtisasmallnumbersoitneedstobemultipliedbyalargenumberlike400tomatchaspeedsimilartowhatwewereseeingbeforewhenwesimplywereaddingandsubtracting 10.

Inthenextsectionwewillworkonthephysicsmoretogivetheballmovementamorerealisticfeel.Wewillalsoimplementtheabilitytodestroybricksusingtheworldcollisioncallbacks.

ExercisesDespitehavingarestitutionof1,theballislosingmomentumasitcollideswithotherobjects.Thisisduetofriction.Howcanthatbefixed?Whenthegameispaused,makeitdisplaytextonthescreensotheplayerknowsthegameisn'tjustfrozen.Hint:you'llneedoneofthedrawfunctionsfromlove.graphicstoprintthetext.

Theanswerstotheseexerciseswillbeinthenextsection'ssourcecode.

Breakout(part4):physics

ReviewInthepreviousexerciseswediscussedissueswiththeballslowingdownduetofriction.Withabitofbrowsingthroughthe love.physicsdocumentationyoumighthaveseenthatfrictionisapropertyofthefixtureandcanbesetto0in fixture:setFriction.

Howaboutcreatingthepausescreentext?Wereyouabletodoitwithouttouchingmain.lua?Takealookatthisentitythatwascreatedjustforthesingleresponsibilityofdisplayingpausetext:

--entities/pause-text.lua

returnfunction()

localwindow_width,window_height=love.window.getMode()

localentity={}

ifinput.pausedthen

love.graphics.print(

{{0.2,1,0.2,1},'PAUSED'},

math.floor(window_width/2)-54,

math.floor(window_height/2),

returnentity

That'sright.Eventhepausescreenisanentity.Thefirstnaturalplacetothinktoputitwouldbethemainfilebutentityfilesareperfectbecausewecancreateasmanyasweneedforeachtaskandaddittoentities.luawhereitwillbehandledbythegameloop.Forcenteringthetextthe love.window.getModefunctionisusedtogetthefullwindowdimensionsthenthosenumbersaredividedinhalf.Thissavesusfrommanuallycodinginnumbersthatwouldneedtobereadjustedifthewindowsizechanged.Additionally, math.floorwasusedforgoodmeasuretomakesurewearereturningawholenumber.Itisrecommendedtorounddecimalsofffromnumberswhenpassingcoordinatestothedrawingfunctions.Otherwiseitmayattempttodrawthatobjectbetweenpixelsonthescreenandcausesomeblurriness.

PhysicsupdatesAnissuewehadwiththegamephysicssincewegotthepaddlemovingisthattheballdoesn'talwaysricochetoffthepaddleasyouwouldexpect.Thisisbecausewemadethepaddlestaticsotheballdoesn'tpushitaround,butthishastheeffectofthepaddlenotinteractingwiththeballcorrectly.Thisiswhere kinematicbodiescomein.Kinematicbodies,likestaticbodiesaren'taffectedbydynamicbodies.Kinematicbodies,unlikestaticbodies,canaffectdynamicbodies.

We'regoingtomake3changestopaddle.lua:

Movetheboundarydimensions,paddledimensions,andpaddlespeedtoeasily-referencedvariablesatthetopofthefile.ChangethebodytypetokinematicOverhaultheupdatecodetomovethebodywithlinearvelocityratherthanmanuallysettinganewlocationonthescreenwitheveryupdate

returnfunction(pos_x,pos_y)

localwindow_width=love.window.getMode()

--Variablestomaketheseeasiertoadjust

localentity_width=120

localentity_height=20

localentity_speed=600

--Thelimitofhowfarleft/righttheentitycanmovetowards

--theedges(withalittlebitofpaddingthrownon).

localleft_boundary=(entity_width/2)+2

localright_boundary=window_width-(entity_width/2)-2

localentity={}

entity.body=love.physics.newBody(world,pos_x,pos_y,'kinematic')

entity.shape=love.physics.newRectangleShape(entity_width,entity_height)

love.graphics.polygon('line',self.body:getWorldPoints(self.shape:getPoints()))

return

localself_x=self.body:getX()

ifinput.leftandself_x>left_boundarythen

self.body:setLinearVelocity(-entity_speed,0)

elseifinput.rightandself_x<right_boundarythen

self.body:setLinearVelocity(entity_speed,0)

self.body:setLinearVelocity(0,0)

returnentity

Itookthelibertyofadjustingthepaddlesize,butwithourniceboundary-sizecalculationsinplacethepaddledimensionscaneasilybeadjustedandtheboundarysizewilltakethosechangesintoaccount.Let'sdrillintotheentity.updatefunction.

Oncetheinputsarecheckedtobetrueorfalsethecurrentx-positionofthepaddleischeckedtoseeifitgoesoutoftheboundaries(calculatednearthetop).Noticethatthecalculationsfortheboundarylocationsaredoneatthetopinsteadofin entity.update.Thismeansthosecalculationsaren'tdoneoneveryupdatesincetheydon'tneedtobe.

Abitmorecomplexthanthepaddlearethecalculationsfortheball:

--entities/ball.lua

localentity_max_speed=880

localentity={}

entity.body=love.physics.newBody(world,x_pos,y_pos,'dynamic')

entity.fixture:setFriction(0)

localself_x,self_y=self.body:getWorldCenter()

love.graphics.circle('fill',self_x,self_y,self.shape:getRadius())

localvel_x,vel_y=self.body:getLinearVelocity()

localspeed=math.abs(vel_x)+math.abs(vel_y)

localvel_x_is_critical=math.abs(vel_x)>entity_max_speed*2

localvel_y_is_critical=math.abs(vel_y)>entity_max_speed*2

--Ballisbouncingtoofasttoreasonablyhit.

--Cutdownitsspeedby75%ifso.

ifvel_x_is_criticalorvel_y_is_criticalthen

self.body:setLinearVelocity(vel_x*.75,vel_y*.75)

ifspeed>entity_max_speedthen

self.body:setLinearDamping(0.1)

self.body:setLinearDamping(0)

returnentity

Inthefirstchunkwegetthecurrentxandyvelocity,whichtellsusthexandydirectionoftheball:

localvel_x,vel_y=self.body:getLinearVelocity()

localspeed=math.abs(vel_x)+math.abs(vel_y)

Anexample vel_x/ vel_ymaybe 212/ -300,whichmeanstheballismovingupandtowardstheright.Thespeediscalculatedbyturningboththesenumbersintoabsolutenumbersandaddingthemtogether(so 512intheexample).

Inthenextchunkthereisasafetychecktomakesuretheballdidn'tricochetwithsomuchforcethatit'sgoingtoofasttopossiblyhit.Ifeitherbooleanvariableistruethenthelinearvelocityismultipliedbyafractionofitselftoquicklyslowitdown:

localvel_x_is_critical=math.abs(vel_x)>entity_max_speed*2

localvel_y_is_critical=math.abs(vel_y)>entity_max_speed*2

--Ballisbouncingtoofasttoreasonablyhit.

--Cutdownitsspeedby75%ifso.

ifvel_x_is_criticalorvel_y_is_criticalthen

self.body:setLinearVelocity(vel_x*.75,vel_y*.75)

Nowthereisjustachecktoeasetheballbackdowntoacomfortablemaximumspeed.Iftheball'sspeedisgreaterthan entity_max_speedadampingisappliedwhichwillreducetheballsspeedbelow880.Oncethetargetspeedisreachedthenthedampingswitchesbackto0:

ifspeed>entity_max_speedthen

self.body:setLinearDamping(0.1)

self.body:setLinearDamping(0)

Tryoutthechangestofeelitinactioncomparedtothepreviousphysicsandhopefullyyouwillfindthatit'sanimprovement.It'snotaperfectreplicaofthearcadegame,butplayingaroundwiththesetricksandfeaturesyoucangetitprettydarnclosetosomethingsatisfactory.Anotherthingtotryoutifwithintheball's entity.update,addalineunderthespeedvariablethatreads print(speed)andwatchthenumberincreaseanddecreaseagainasthedampingkicksin.Prettyneatthatmostoftheheavycalculationsarehandledbythephysicsengineforus.

CollisionThereare4changesinvolvedtomakethebricksdestructible:

Updateworld.luatocheckforcollisionfunctionalityfortheentitieswhentheycollideUpdatebrick.luatoincludeacollisioncallbackAddanewattributeonthebrickentitytoletusknowitscurrentconditionandifitneedstobedestroyed.We'lljustcallit entity.health.Updatemain.luatoremove/destroyanyentitiesthathavenomorehealth

Firsttheworld:

--world.lua

--Calledattheendofanycontactintheworld.Parameters:

--{contact}contact-worldobjectcreatedonandatthepointofcontact

localentity_a=fixture_a:getUserData()

localentity_b=fixture_b:getUserData()

ifentity_a.end_contactthenentity_a:end_contact()end

ifentity_b.end_contactthenentity_b:end_contact()end

world:setCallbacks(nil,end_contact_callback,nil,nil)

returnworld

Theonlycallbackwe'llbeusingforthistutorialistheend-contactcallback,sofor world:setCallbackswearegoingtoreturning nilfortheresttokeepourcodefastandclean.Takealookatwhatishappeninginsideend_contact_callback.Rememberinsideeachentitywhenweinvoked entity.fixture:setUserData(entity)?Withtheentityattachedtoeachfixture,wecangetaccesstothoseentitiesbyinvoking fixture:getUserDatainthecallbackabove.Oncewehaveaccesstoeachentity,wechecktoseeiftheentityhasany end_contactfunctions,codespecifictothatentitythatneedstorunwhenendingthecollision.

Nowwecangotobrick.luaanddefinethatfunctionality:

localentity={}

--Howmanytimesthebrickcanbehitbeforeitisdestroyed

entity.health=2

entity.end_contact=function(self)

self.health=self.health-1

returnentity

Noticethetwonewvaluesinthetable, entity.healthand entity.end_contact.Inside end_contactwearesubtracting1healthwhenthecollisionends.Healthcouldstartatanynumberandthatmeanstheballwillneedtocollidewiththebrickthatmanytimesbeforethehealthreaches0.Lastly,weneedtogointomain.luaandadjustlove.updatesoitdoessomethingwhenitseesanentitywith0health:

--main.lua

localindex=1

whileindex<=#entitiesdo

localentity=entities[index]

--Whenanentityhasnohealth(brickhasbeenhitenoughtimes

--thenweremoveitfromthelistofentities.Don'tincrement

--theindexnumberifdoingthatthoughbecausewehaveshrunk

--thetableandmadealltheitemsshiftdownby1intheindex.

ifentity.health==0then

table.remove(entities,index)

entity.fixture:destroy()

index=index+1

world:update(dt)

Theentityisremovedfrom entitiesaswellashavingitsfixturedestroyedfromtheworld.Thiswillonlyhappentobrickswith0health.Itwon'thappentoentitieswherewedidn'tdefinehealthbecause nilisnotthesamethingas0.Noticethata whileloopwasusedhere.Thisisbecausewemayremoveentitiesfromthelistweareloopingoverandthiswouldthrowofftheindexcountforaregular forloop.

Inthenextsectionwe'llreviewthechecklistandseewhatislefttocover.

ExercisesItwouldbegreatifthecolorsofthebrickschangeddependinghowmuchhealththebrickhas.Updatethebrick'sentity.drawfunctionwithsomecolors.Hint:wecoveredcolorsin2.4-Gameloop.

Addmorebrickstothescreen.What'stheeasiestwaytodothat?

Breakout(part5):gamestate

ReviewWe'vegottenabitdonesolet'slookatthebasicrequirementsagain:

Theobjectiveofthegameistodestroyallthebricksonthescreen✔Theplayercontrolsa"paddle"entitythathitsaball✔Theballdestroysthebricks✔TheballneedstostaywithintheboundariesofthescreenIftheballtouchesthebottomofthescreen,thegameends

Inthepreviousexercisethequestionwasbroughtupwhatwouldbetheeasiestwaytodrawabunchofbricksacrossthescreen.Asimple,butverytediousanswertothatwouldbetopositionthebricksoneatatimeinentities.lualikeso:

brick(40,80),

brick(100,140)

--andsoon...

Ifyouwanttomakeyourbricksintoashapeorsculpturethenthatmightbethebestapproach.Ifyoujustwanttoarrangeyourbricksintoagrid,thentheeasiestwaywouldbetowriteanumericfor-loop.

--entities.lua

localboundary_vertical=require('entities/boundary-vertical')

localpause_text=require('entities/pause-text')

localentities={

boundary_vertical(-6,300),

boundary_vertical(806,300),

paddle(300,500),

pause_text(),

ball(200,200)

localrow_width=love.window.getMode()-20

fornumber=0,38do

localbrick_x=((number*60)%row_width)+40

localbrick_y=(math.floor((number*60)/row_width)*40)+80

entities[#entities+1]=brick(brick_x,brick_y)

returnentities

Okthisadmittedlylooksmorecomplicatedatfirst,butifyourememberthearithmeticandordersofoperationcoveredin1.1-Interactivecodingstatementsareprocessedfromtheinnerparenthesisandworkedoutwards.Sowhythelongcalculation?Let'sstartoffwithasimplercalculation:

localbrick_x=number*60

Startingwiththe number0upto38,therewillbe39loopsandtherefore39bricksdrawn.Onthefirstloop, numberis0.Sincethebricksare50pixelswidethiswoulddrawthebrickswitha10pixelspacebetweeneach.Firstbrickat60,then120,then180...Ok,butthenafteronlyadozenbrickswewouldstartrunningoffthescreen.Thisiswherethemoduluscomesinhandy:

localbrick_x=(number*60)%row_width

row_widthishowwidewewantarowofbrickstobebe.Inthiscase row_widthisthescreenwidth,800pixels,subtract20pixelsforpadding.Sodrawthebricksevery60pixels,butthenwhenyougetto780pixels,startbackat0pixelsandbegindrawinganewrow.Thanksmodulus!Nowjusttogivethebrickssomespacingontheleftsideawayfromthewall,wecangoaheadandadd40pixelstothefinalresultforthex-position:

Thebrick'sy-positioniscalculatedalittlebitdifferently.Whatweneedtofindoutiswhichrowwe'reonsoweknowwhereonthey-axistodraw.Ifwetakethe numberandmultiplyitby60thendoamodulusweknowthatgivesusthex-position.Solet'stakethatchunkofcodefromaboveandmakethatthebasisofoury-positioncalculation:

localbrick_y=(number*60)%row_width

Ratherthanusingmodulus,ifweuseregulardivisionwegetasmallremaindereverytime (number*60)exceedstherowwidth:

localbrick_y=(number*60)/row_width

Thiswillgiveusanumberwithdecimalssotokeepthingsroundedwecanuse math.floortosnapthey-positiondowntothenearestwholenumber:

localbrick_y=math.floor((number*60)/row_width)

Great!Noweverytimethex-positionexceedstherowwidth,wegetbackthenumberoftherowwe'reon...0forthefirst,1forthesecond,2andsoon.Withthisnumberwecannowspaceouteachrowby40pixels:

localbrick_y=math.floor((number*60)/row_width)*40

Thenfinallyjusttoshiftthebricksalittlefurtherdownthescreenwegiveitapaddingthatlooksright,say80:

Andthereyougo.Theentitycanjustbeaddedtotheendoftheentitieslistsoitdoesn'tgetlost:

Inthepreviousexerciseswealsotalkedaboutdrawingthebricksdifferentcolorstoindicatetheirintegrity/healthleftbeforetheywillbedestroyed.Ratherthanreviewthatnow,let'sdiveintostatemanagementandwe'llwrapcoloringupalongtheway.

Statemanagement

Youraverage,every-dayprogramhasalotofinformationitneedstostoryinmemory.Forourgametofunctionwithjustthebasicfeatures,weneedtostoreinformationabouteachentity,whetherornotthegameiscurrentlypaused,orifthegameiswonorlost.Thisinformationiscalledthestate.Thestateisdatathatmaychangeduringthelifetimeoftheapplication.Thinkofthestateofyourlightsinyourroom.Aretheycurrentlyinan"on"or"off"state?Thestatecancausedifferenteffectsontheapplication,likeifthe"pause"stateofthegameis"true"thentheworldwillnolongerreceiveupdates.

Onethingwemustthinkofishowtoorganizethestateofourapplication.Thisissomethingwetakeforgrantedoftenintherealworld;Wedon'thavetofigureoutwheretostorethestateofourlights.It'sapieceofinformationintrinsictothelamp'sdesign.

Sowhydowehavetocaresomuchaboutourgame'sstate?Tobefair,ourgameissmallsoweprobablydon'tneedto.However,itiscrucialtoreconcilesuchthingswhileapplicationsaresmallbecauseitwillbeverydifficulttogobackandfixabunchofcodeoncetheapplicationisbig.Thewayyoushouldorganizethestateofyourapplicationshouldaccomplishafewthings:

Itshouldbeeasytofindandaccessthenecessarydatathatmakesupthestate.Forinstance,howeasyisitforourmainfiletoaccesstheentitiesandloopovertheminthe love.updatefunction?Thereshouldonlybeonecopyofthestate.Ifwewanttoaccessthe"paused"stateofourgameinmultipleplacesthatisfine,butweshouldn'thavemultiple"paused"variablesfloatingaroundourgame.Ifwehada"paused"variableinsideanentityfileandanotherinsidetheinputserviceupdatingindependentlythentheycouldgetoutofsyncandourgamewouldgetconfusedonwhenitshouldbepaused.Thestateshouldonlybeaccessedwhereitisneeded.Ifyouwereaccessingorstoringthe"paused"stateinsidetheballentity,thenifthatballwasdestroyedthensomethingbadwillhappenthenexttimethegamecheckstoseeifitispaused.

Whatfilescontainthestateofourgame?

entities.lua-Eachentitytableisresponsibleforitsownstate.Forinstance,eachbrickstoresthestateofitsownhealth.Alltheentitiestablesaregeneratedandstoredhere.Theentitiesarenotstoredintheentitiesfolder.Thosearejustfunctionsusedtogeneratetheentities.Theblueprints.input.lua-Thisfileisresponsibleforcapturinguserinput,butalsostoringthestateofwhatkeysarecurrentlybeingpressed.world.lua-Thisfileisnotonlytheblueprintsforthegameworld,butitalsostorestheworldinstancethatisgeneratedwhenthegamestarts.Wemadetheworldinstanceeasilyaccessibletotherestoftheapplicationbywriting returnworldattheend.Therewouldbenogameifthiswasn'teasilyaccessible.

Afewpiecesofgamestateweneedtoaddareabooleanofwhetherthegameisover,anotherforifthestageiscleared,andalsoalistofcolorstouseinourgamewhichwe'llrefertoasourpalette.Thisinformationwouldn'treallyfitinanyoftheplaceswelistedabove,andwedon'twanttoaddittomain.luabecauseofourfirstrulethatthegamestateshouldbeeasytoaccesswhereitisneeded.Besides,that'snotthemainfile'sresponsibility.We'llgoaheadandjustmakeanewfilecalledstate.luaandstoretheoverallgamestateinthisfile.Thisisalsoalittlematterofopinionbutthe"paused"andbuttonstateswe'llalsomoveinheresincetheyaffecttheoverallgame'sstate.Thiswillalsomakeitsothatinput.lua'sonlyresponsibilityistocaptureandtranslatetheuserinput,nottohandleanystatewhatsoever.

--state.lua

--Thestateofthegame.Thiswayourdataisseparatefromourfunctionality.

return{

button_left=false,

button_right=false,

game_over=false,

palette={

{1.0,0.0,0.0,1.0},--red

{0.0,1.0,0.0,1.0},--green

{0.4,0.4,1.0,1.0},--blue

{0.9,1.0,0.2,1.0},--yellow

{1.0,1.0,1.0,1.0}--white

paused=false,

stage_cleared=false

It'skindofanicefeelingtokeepallthestatetogether.Wecouldevenmovetheentitieslistintostate.luaandgetridofentities.lua,butthisdoesn'tseemnecessary.Nowwiththisshiftindataweneedtoupdateinput.luaandmain.luatoreferencethenewfile:

--input.lua

localstate=require('state')

--Mapspecificuserinputstogamestates

localpress_functions={

left=function()

state.button_left=true

right=function()

state.button_right=true

escape=function()

love.event.quit()

space=function()

ifstate.game_overorstate.stage_clearedthen

return

state.paused=notstate.paused

localrelease_functions={

left=function()

state.button_left=false

right=function()

state.button_right=false

--Thistableistheserviceandwillcontainsomefunctions

--thatcanbeaccessedfromentitiesorthemain.lua.

return{

--Lookupinthemapforactionsthatcorrespondtospecifickeypresses

press=function(pressed_key)

ifpress_functions[pressed_key]then

press_functions[pressed_key]()

--Lookupinthemapforactionsthatcorrespondtospecifickeyreleases

release=function(released_key)

ifrelease_functions[released_key]then

release_functions[released_key]()

--Handlewindowfocusing/unfocusing

toggle_focus=function(focused)

ifnotfocusedthen

state.paused=true

--main.lua

input.toggle_focus(focused)

input.press(pressed_key)

input.release(released_key)

ifstate.game_overorstate.pausedorstate.stage_clearedthen

return

localindex=1

ifentity.healthandentity.health<1then

index=index+1

world:update(dt)

Noticethechangeto love.update.Wecheckif state.game_over, state.pausedor state.stage_clearedistrueandifso,wereturnfrom love.updatewithoutdoinganyoftheupdatesasthesekindofgamestatesmeritfreezingthescreen.

Nextup,updatepaddle.luatorequire stateinsteadof input.The entity.updatefunctionnowneedstoreferencestate.button_leftand state.button_righttotelliftheplayerhaspressedanybuttons.Tryupdatingitonyourown.Ifyoudogetstuck,thesourcecodewillbeinthelinkatthebottomwaitingforyou.

Ok,nowthatwehaveastatewherewestoredthecolorsitisprobablyagoodtimetotryandupdatebrick.lua.Firstlet'slookatthosecolorsstoredinstate.lua:

palette={

{1.0,0.0,0.0,1.0},--red

{0.0,1.0,0.0,1.0},--green

{0.4,0.4,1.0,1.0},--blue

{0.9,1.0,0.2,1.0},--yellow

{1.0,1.0,1.0,1.0}--white

The palettetableisalistofmoretables.Eachtableinthelistrepresentscolorswherethefirstnumberistheamountofred,2ndtheamountofgreen,3rdtheamountofblue,and4thnumbertheamountofopacity.Settingthelastnumberto 0meansthecoloris100%transparentand 1meansitiscompletelyopaque.Allofthesevaluesmixtogethertoformasinglecolor.Inthecaseofthefirstcolor,wehavetheredvaluesettomaximumopaqueredwithnoothercolorsmixedin.Iwouldencourageyoutogobackandeditthecolorsinthispaletteaftereverythingisworking.Now,insidebrick.lualet'supdate entity.draw:

localentity={}

--Howmanytimesthebrickcanbehitbeforeitisdestroyed

entity.health=2

--Usedtocheckduringupdateifthisentityisabrick

--Ifnobricksarefoundthenthelevelwascleared

entity.type='brick'

--Drawthebrickinadifferentcolordependingonhealth

love.graphics.setColor(state.palette[self.health]orstate.palette[5])

--Resetgraphicsdrawerbacktothedefaultcolor(white)

love.graphics.setColor(state.palette[5])

self.health=self.health-1

returnentity

Beforedrawingthebrick'spolygon,wesetthegraphicsrenderertouseoneofthecolorsfrom state.palette.Thecolortousedependsonwhatthebrick'shealthis.Soifthebrickhas2healththen state.palette[self.health]willbecome state.palette[2]whichwillgrabthe2ndcolorinthelist...green.Ifthebrick'shealthwas1,thenthefirstcolorfromthepalettewouldbeselected...red.Afterthecoloredpolygonisdrawn, entity.drawfinishesupbysettingtherenderercolorbacktowhite.Ifwedidn'tdothisstep,theballandpaddlewouldgetdrawnthesamecolorasthebricks.

Onelastthingweneedtodotogetthegameworkingisupdatepause-text.luaasitisincorrectlylookingforthe"pause"stateininput.luainsteadofthenewstate.lualocation:

--entities/pause-text.lua

returnfunction()

localentity={}

ifstate.pausedthen

{state.palette[3],'PAUSED'},

returnentity

FinaltouchesWeneedthegametoendwhentheplayerdestroysallthebricksorlosestheball.Justlikethepause-textentity,displaysomemessagesbasedonthegamestate.

--entities/game-over-text.lua

returnfunction()

localentity={}

ifstate.game_overthen

{state.palette[5],'GAMEOVER'},

returnentity

--entities/stage-clear-text.lua

returnfunction()

localentity={}

ifstate.stage_clearedthen

{state.palette[4],'STAGECLEARED'},

returnentity

Totriggerthe"GAMEOVER"textiseasyenough.Weneedtoaddacollisioncallbacktoboundary-bottom.luatosetthegame's state.game_overtotrueonanycollision:

localentity={}

state.game_over=true

returnentity

Don'tforgetweneedtoupdateentities.luatoaddourtwonewentities:

--entities.lua

localboundary_vertical=require('entities/boundary-vertical')

localgame_over_text=require('entities/game-over-text')

localpause_text=require('entities/pause-text')

localstage_clear_text=require('entities/stage-clear-text')

localentities={

boundary_vertical(-6,300),

boundary_vertical(806,300),

paddle(300,500),

game_over_text(),

pause_text(),

stage_clear_text(),

ball(200,200)

localrow_width=love.window.getMode()-20

fornumber=0,38do

returnentities

Ok,testthatoutandcheckthatthe"GAMEOVER"textworks.Ifitdoes,thenlet'scontinueonandaddtheconditionsforhowtowinthegame.Thisinvolvescheckingthroughalltheentitiesin love.updatetomakesurewestillhavebricks.Ifwedon'thaveanybricksleft,thentheplayerdestroyedthemallandthestageiscleared.

--main.lua

ifstate.game_overorstate.pausedorstate.stage_clearedthen

return

--Switchtotrueifwehavebricksleft

localhave_bricks=false

localindex=1

ifentity.type=='brick'thenhave_bricks=trueend

ifentity.healthandentity.health<1then

index=index+1

--Flagthestageclearediftherearenomorebricks

state.stage_cleared=nothave_bricks

world:update(dt)

Everytime love.updateisran,wesetavariable have_brickstofalse.Ifthisbooleanstays falseallthewaytothebottomofthefunctionthen state.stage_clearedgetsswitchedtotrueandthegameiswon.Insidethe whileloop,however,wecheckeveryentitytoseeifwefindan entity.typeof 'bricks'andifso, have_bricksgetsflippedtotruetostopthegamefrombeingwonyet.

Sothataboutdoesitforcompletingourchecklist.Thegamemaynotbeasfeature-completeasatruebreakoutgame,butthatroomforimprovementleavesopportunityforyoutomodifythegametoworkhowyouwantitto.It'sreallyuptoyourimagination.Tryoutafewexercisesifyoucan'tthinkupanynewfeatures.Ifyouarehavingtroublerunningthegame,besuretocheckoutthesourcecode:

https://github.com/RVAGameJams/learn2love/tree/master/code/breakout-5

ExercisesInsteadofgettingagameoverassoonastheballtouchesthegroundonce,addanewpropertyinstate.luanamed livesandsetittoasmanylivesasyouwanttheplayertohave.Makeissothe state.livesdecreaseswhentheballhitsthegroundandmakethe game_overnottriggerunless state.lives<1.TrysettingthepaddletodifferentshapetomakethegameplaydifferentlyComeupwithnewfeaturestomakethegameplaybetterandfeelmorepolished

ChangetheballandpaddlecolorsAddabackgroundcolor

FigureouthowtoplayasoundeffectwhentheballcollideswiththingsCreatesomekindofpower-upentity

BinaryandbitmasksIn2.10-Collisioncallbackswesawhowtoreacttoentitiescolliding.Inthissectionwe'regoingtodiscusshowwecanbettercontrolwhencollisionshappen.Asthetitlesuggests,thiswillinvolveunderstandingsomebinary.

Let'ssaywehaveabeat-em-upgamewheretwoplayersarefightingbadguysandwedon'twantplayerstocollidewitheachotherandinsteadonlycollidewithenemies.Thecollisioncallbackcouldlooksomethinglikethis:

localbegin_contact_callback=function(fixture_a,fixture_b)

localentity_a_type=fixture_a:getUserData()

localentity_b_type=fixture_b:getUserData()

--Checkthesearen'tthesametypeofentity

ifentity_a_type~=entity_b_typethen

--Somecodetohandlethecollisiongoeshere...

Butwhatifyouhadpower-upsandyouwantplayerstocollidewiththepower-upsbutyoudon'twantenemiestouchingthepower-ups?Thingscangetcomplicatedprettyquickly:

localbegin_contact_callback=function(fixture_a,fixture_b)

locala=fixture_a:getUserData()

localb=fixture_b:getUserData()

if(a=='powerup'andb=='player')or(a=='player'andb=='powerup')then

--Somepower-upcode...

--Don'tletpower-upscollidewithotherentitytypeslikebadguys

elseifa~=banda~='powerup'andb~='powerup'then

--Codetohandletherestofthecollisions...

Let'sfindabetterway!

BinaryoperationsBackin1.0-Programmingbasicswediscussedoperations–howtooperateonstringswithequality( ==)checks,howtooperateonnumberswitharithmeticoperations,andevenhowtoperformbooleanoperationslike andandor.Binarynumbershavetheirownoperations,oftencalledbitwiseoperations.Toperformbinaryoperations,let'sfirstlookathowtorepresentbinarynumbers.Typinganumberlike 101,Luawillinterpretitasadecimalnumber(literallyone-hundredone)soweneedtorepresentitasastringandconvertittoanumber.Toconvertastringtoanumber,youpassinthenumberandthebase(base-2inthiscase)likeso:

print(tonumber('101',2))

Whichconvertsthebinarynumber 101todecimalwhenitprintsout:

Forcountinginbinaryandlearninghowtoreadandconvertbetweenbinaryanddecimal,therearemanyresourcesthatalreadyexplainitinmuchbetter.Learninghowtodotheconversionsisn'tnecessarytolearningthesebasicbinaryoperations,butisanessentialskilltohaveinthefieldofcomputerscience.

Movingon,let'stakealookatsomeofthebasicoperations.

Binarynumbersaresimilartobooleansinthatbinaryonlyhas1'sand0's.TheANDoperatoralsoworkssimilarlytotheboolean and.Yougiveittwodigitsandbothmustbe 1( true)fortheoutputtobe 1.

UnfortunatelyatthetimeofwritingthistheonlineREPLhasanoutdatedversionofLuathatdoesn'tsupportbinaryoperations.Noworries,let'screateamain.luafileandtryitoutusingLÖVE.Toperformbinaryoperations,theincluded 'bit'librarymustbeloaded.Whenrequired,itwillreturnatablewithmanyfunctionsinitrelatedtobinaryoperations.Thefirstfunctionwe'lltry, bit.band()performsabinaryANDoperation.

localbit=require('bit')

print(bit.band(0,0))

Thiswillprinttothedebugconsole:

Youcanpassitthedecimals 1and 0asthosenumbersarethesameinbinaryanddecimal.

Theoperationisnotlimitedtotwoinputs:

print(bit.band(1,0,1))

Youcanalsopassitmulti-digitnumbers:

print(bit.band(

tonumber('111',2),

tonumber('101',2)

Notethatyouneedtoalwaysuse tonumber()toconvertyourbinarystringtoanumberasthefunctionalwaysexpectsadecimalnumber.Likewisetheoutputwillalwaysbeadecimalnumber:

Layitoutlikeanarithmetictableandyoucansolveitjustaseasily:

101-->5

11011010

10111100

--------

10011000-->152

ORLikewiththeboolean or,thebinaryORoutputwillbe 1ifeitherthefirstorthesecondnumberis1.Youcouldsayitistheleastpickyoperatorinthatitdoesn'tcareaslongasitgetsa1somewhereatleastonce.

--main.lua

localbit=require('bit')

print(bit.bor(1,1))

print(bit.bor(1,0))

print(bit.bor(0,0))

print(bit.bor(0,0,0,1,0))

Xor(exclusiveor),returns1onlywhenitgetsone1.Let'scompareXORinatabletotheothers:

inputA inputB output

Binaryoperationsaresomeofthemostfundamentalcomputeroperationsandcanbephysicallybuiltwithafewtransistors.Giventhesimplicityoftheseoperations,italsomakesforafastmethodofcalculatingcollisions.

Bitmasks

Let'stakealookatthissceneforamomentandidentifyfromthecrudelydrawnshapessomepotentialentities:

Alltheseentitiesfallintouniquecategoriesinthatwewanteachofthemtocollidewithcertainotherentities.Ifthiswereagame,we'ddefineeachcategorywithauniquebinarydigit,orbit,solet'sfirstdothat:

entity category

sun 0000

player 0001

powerup 0010

enemy 0100

ground 1000

Let'ssetsomerulesforeachoftheseentities.Forinstance,wewanttheplayertocollidewiththepowerup(0010),enemy(0100),andofcoursetheground(1000).Totellthegameenginethis,wecreateabitmaskforthefixture.Thisisabinarynumberwithallthebitsswitchedonthatwewanttheentitytocollidewith.Inotherwords,theplayer'sbitmaskwouldbe(1110).Weleftthefirstbitblanksothattheplayercan'tcollidewithotherpotentialplayers(player2).Let'supdatethetablewiththebitmaskwewanteachentitytohave:

entity category bitmask

sun 0000 0000

player 0001 1110

powerup 0010 1001

enemy 0100 1001

ground 1000 1111

Sohowdoesitallcometogetherandwork?Whentwoentity'sfixturescontact,abinaryANDoperationisperformedagaintheentity'sbitmaskandtheotherentity'scategory.Iftheresultingnumberisn't0000thenwehaveacollision.Taketheplayerandenemyforinstance:

0001player'scategory

1001enemy'sbitmask

0001wehaveacollision

Andhowabouttheenemyandthepowerup:

0100enemy'scategory

1001powerup'sbitmask

0000wehaveNOcollision

Armedwiththisknowledge,wecanassertthefollowinginformationfromthetableabove:

Thesuncollideswithnothing(anddoesn'tevengetacategory).It'sjustinthebackgroundandnon-interactive.Theplayercollideswitheverythingexceptotherplayers(andofcoursethesun).Thepowerupcollidesonlywiththegroundandplayers.Theenemycollidesonlywiththegroundandplayers.Thegroundcollideswitheverything.

Copyordownloadthe"collision"gamefromtheexamplecodeandrunit:

https://github.com/RVAGameJams/learn2love/tree/master/code/collision

Dotheentitiesinteractasexpected?Takealookinsidetheentitiesfoldertoseetheparticularfunctionbeingcalledtoaccomplishapplythecategoriesandbitmaskstoeachentity–Fixture:setFilterData

--square.lua

square.category=tonumber('0001',2)

square.mask=tonumber('1110',2)

square.group=0

square.fixture:setFilterData(square.category,square.mask,square.group)

Theexamplesaboveonlyuse4bitsforthecategoryandmaskbceausethat'sallweneeded,howeverLÖVEsupportsupto16bitsforthecategoryandbitmask( 0000000000000000).Thegrouppropertyisn'tusedandshouldbesetto0whenitisn't.Wehaven'tmentionedgroupsbeforebecauseifyouknowhowtousecategoriesandbitmasksthenyoudon'tneedtousegroupsascategoriesandbitmasksofferamorepowerfulwayofdoingthesamething.Thatbeingsaid,collisiongroupsshouldberelativelystraight-forwardtolearnaboutsoitwillbeleftupasanexercisetoreadandstudy.

ExercisesPlaywiththebitmasks.Canyoumaketheenemycollidewiththepowerupinsteadoftheplayer?TakealookathowgroupsworkasdescribedinFixture:setGroupIndex.Thisisasimpler,butmorelimitedmethodofdetectingcollision.Canitbeusedtoimitatethecollisionrulesabove?

Networking(part1)Whencreatingaprogramsuchasagame,oneofthefirstthingstoconsidershouldbewhetheritisanetworkedapplication.Suchachoicewillradicallychangethestructureandcomplexityoftheapplication.Tobuildanetworked("online")multiplayergame,wemustunderstandsomenetworkingbasics.Someofthisinformationisoversimplified,butlet'sestablishabaselineofknowledge.

Internetprotocol(IP)Networksarepossiblebecausecomputersagreeonawaytocommunicatewitheachother.Likeogres,messagessentacrosstheinternethavemanylayers.Eachlayerrepresentsadifferentprotocolthatinterpretshowthemessageshouldbehandled.Theinternetprotocol(IP)tellscomputershowtorelaymessagestotheirintendeddestination.Therearetwothingsweneedtoknowaboutthisprotocol:IPaddressesandports.

EverydeviceconnectedtotheinternethasanIPaddressassignedtoitwhenitconnects.MessagessentoutfromyourdevicearesentwithadestinationIPaddressattachedsoitknowswheretogo.Messagesarerelayedfromonemachinetoanotheruntilitreachesthedestinationmachine'saddress.

Ifyouopenyourterminalorcommandpromptandtype pinggoogle.comyouwillgetaresponsebackthattellsyouthedestinationIPaddress;TheIPaddressoftheserverrunningthegoogle.comhomepageyousee.YoumayevenbeabletotypethatIPaddressintoyourwebbrowseranditwilldirectyoutothewebsiteinthesamefashiontypinggoogle.comintheaddressbarwould(althoughthiswon'tworkforallwebsitesbecauseofunrelated,complicatedreasons).Let'ssayyouconnectedtogoogle.comthroughtheIPaddress172.217.7.14.You'reactuallyconnectingtothatIPthroughaspecificport.Portsarerepresentedasnumbers,sothatIPlikemosteveryotherwebsiteontheinternetisaccessedthroughport443.

IPportsarelikethemaritimeportsthatharborships.Asingledestinationcanhavemultipleportsfordifferentpurposes.IfIwerebringinginamilitaryvesselImaygotoadifferentportthanacommercialvessel.

DependingonyourintentionsforanetworkconnectionyouwillusedifferentIPports.Forinstance,ifyouaretryingtoviewawebsitelocatedat172.217.7.14youwilluseport443foranHTTPSconnection,port80foranHTTPconnection(ifallowed),andifIamanadministratorofthemachinerunningon172.217.7.14Iwilluseacompletelydifferentporttoestablishabackdoorconnectionsuchasport22.

ForourexampleprogramwewilltryconnectingtoaspecialreservedIPaddress,127.0.0.1.ThisIPaddressisyourmachine'sownIPaddressituseswhenitwantstoconnecttoitself.Sincewe'llbetestingourprogrambyrunningbothcopiesonthesamecomputerwewon'tneedtoworryaboutmultipleIPaddressesfornow.Fortheportyouhavearangefrom0to65535anditdoesn'treallymatterwhichoneyouusesolongasit'snotalreadyinuseorbeingreservedforotherpurposes.We'llpickarandomonethatisunlikelytobeinusebyotherprograms...6789.

TransportlayerThetransportlayerdecideshowyourdatawillbepackagedandstreamed.Youhaveachoiceonafewdifferentprotocolsforthetransportlayer.Understandingthedetailsofeachprotocolinthetransportlayerisn'ttooimportantforthissectionofthebookbutlet'sdiscusswhywemaywanttouseoneortheother.

TCP-Thisprotocolprovidesdifferentfeaturestomakesuredatadoesn'tgetcorrupt.Mostnotably,itwaitsforaconfirmationresponsefromtheotherendtomakesurethemessagewasreceived.Ifaresponseisn'treceivedbyacertaintimeoutthentheconnectionisconsideredafailure.WebsitesuseTCP99%ofthetimebecauseofitsreliabilityandensuringyou'vereceivedthesite'sfullcontent.

UDP-Thisprotocolsendsdatatoaserverandexpectsnoresponseback.Sendingdatawithoutconfirmingitreachesthedestinationcouldleadtolessreliabledatatransportation.However,lessbackandforthcommunicationcouldmeanafasterconnection.Thisprotocolisusedbyapplicationsneedingtosendlotsofdataquickly,likeanaudiostreamoravideogame.Thisistheprotocolwe'lluse.

ImagineyouhavetwoplayersneedingtocommunicatetheirpositionwitheachothersowedecidetouseUDP.Youmaysendmessagesbackandforthseveraltimesasecondtocommunicateyourpositions.Sinceyouaresendingdatasorapidly,ifoneofthosemessagesislostthentheplayerpositioncanbere-synchronizednextmessage.Thisisfastandunlessoneoftheplayershasafaultyinternetconnectionyoutypicallywon'tnoticeasmalljitterorhiccupeverynowandthen.

Nowimagineanotherscenariowherewewanttosendamessagethataplayergainedanextralife.IfwewereusingUDPandthatmessagegotlost,wecouldhavetwoonlineplayerswithout-of-syncinformationthatwouldultimatelyjeopardizegameplay.OnesolutionaroundthiswouldbetouseTCPformission-criticalmessagesandUDPforeverythingelse.AnothersolutionistokeepallmessagesinUDP,buttowriteacallbackinLuaaroundourmission-criticalmessagestocheckthatwegetareply.Yup,youcanhaveyourapplicationsendUDPmessagesandexpectaresponsebuteventhoughUDPdoesn'thavethisfeatureaspartofitsprotocolyoucanstillprograminyourapplicationatimeoutthatexpectsaresponse.Thissoundslikealotofwork,butLuaandmanyotherlanguageshavelibrariesavailableyoucanrequireinyourprojectthatdothisforyou.We'llseehoweasythisislateron.

ApplicationlayerFinallywehavetheprotocolwecreateforeachrunningcopyofagametoknowhowtocommunicateonceaconnectionisestablished.Forinstanceifamessagewiththestring "ping"isbeingreceived,wemaywanttorespond "pong".Themorecomplicatedthegameis,themorecomplicatedtheprotocolwillbe.Let'scheckoutoneofthelibrariesLuaoffersfornetworkingandbuildatestprogramwithabasicapplicationprotocolwheretheserverrespondsto "meow"with "bark"andtheclientrespondsto "bark"with "meow".Asyoucanguessthiswillleadtoaninfiniteback-and-forthconversationbetweenthetwohostsifwearesuccessful.

ENetThereareseveralthird-partylibrariesforLuafornetworking.LÖVEincludestwoofthemostpopular,LuaSocketandlua-enet.LuaSocketisveryflexibleandallowsyoutocreateTCPandUDPconnections.Lua-enetisbuiltontopoftheENetlibrary,asimpleyethighperformancenetworkinglibrary.ItusesUDP,buthandleseverythingaroundthetransportlayerforussowecanfocusonourapplicationlayer.ItevendoesmessageconfirmationoverUDPforuswhenweneedittosowegetthebestofbothworlds.Let'screateaserverandclientprograminLÖVEandwe'llrunthemseparately,connectingthemtoeachother.

OurserverapplicationCreateafoldercalled serverandinitcreateafilenamed server.lua.We'llstartbyrequiring enet:

--server/server.lua

localenet=require('enet')

Thisfilewillreturnatableoffunctionsforstartingandstoppingtheserver.Tostarttheserver,weneedtocreateahostandtellitwhichIPaddressandportitisrunningon.Let'screatea server.startfunctionthatdoesjustthat:

--server/server.lua

localhost

localserver={}

server.start=function()

host=enet.host_create('127.0.0.1:6789')

returnserver

TheIPaddressis 127.0.0.1aswesaidwewoulduse.ThatistellingENetwewanttostarttheserveronourmachine'slocaladdress.TheIPaddressisfollowedbyacolon( :)thentheportnumber( 6789)whichisanarbitraryportthatshouldbefreetouse.Ifwecreateamain.luafilewecanrequireserver.luaandcreateaserverwhenLÖVEstarts.

--server/main.lua

--Ourserverapplication

localserver=require('server')

love.load=function()

server.start()

Ifwetryandrunthis,nothingwillhappen.Let'sdefine love.drawandprintsometexttotelluswhensomeoneconnectstoourserver:

--server/main.lua

--Ourserverapplication

localserver=require('server')

--Keeptextpixelssharpandintactinsteadofblurring

--https://love2d.org/wiki/FilterMode

love.graphics.setDefaultFilter('nearest','nearest')

server.start()

--Scaleupthesizeofthetextbeingprinted

localtransform=love.math.newTransform(0,0,0,3)

ifserver.is_connected()then

love.graphics.print('clientconnectedtous(seeconsole)',transform)

love.graphics.print('serverstarted...awaitingclients',transform)

--It'sconvenienttobeabletopressescapetoclosetheprogram

love.event.quit()

Withthisdone,weneedtofigureouthowtheserverknowssomeoneisconnected.Wecall server.is_connected()inlove.draw,solet'sstartbydefiningthat:

--server/server.lua

localhost

localreceived_data=false

localserver={}

server.is_connected=function()

returnreceived_data

returnserver

Ok,so server.is_connected()willreturnthevalueof received_datawhichdefaultsto false.Nowthepartthatdoesalltheaction:

--server/server.lua

localhost

localpeer

localserver={}

server.is_connected=function()

returnreceived_data

server.update=function()

ifnothostthenreturnend

localevent=host:service()

ifeventthen

received_data=true

peer=event.peer

print('----')

fork,vinpairs(event)do

print(k,v)

event.peer:send('bark')

returnserver

Let'stakeacloselookat server.updatepiecebypiece.Firstthingisan ifstatementtocheckthat hostisdefined.If server.updateiscalledbefore server.startthenitwon'tbesothereisnoserverupdatetobemade.Ifourserverhostwascreatedandwegetpasttheif-statementcheck,wecall host:service().Ifwereadthedocumentationfor host:servicewecanseethepurposeofcallingthisistocheckforanyincomingpackets(messages)andsendoutanywehavequeuedup.Ifwereceiveany,wewillgetbackan eventtable.Ifwedogetaneventtable,we'llchange received_datato true(whichinturnmeans server.is_connected()nowreturns true).Nextwewillcapturethepeer(theclient)thatsentusthisdatawhichwecanusetosendmessagestolater:

peer=event.peer

Whilewehavetheeventtable,let'sjustiterateoveritandprintitscontentstotheconsole:

print(k,v)

Thenfinallywe'llsendtheclientamessagethatsimplyreads"bark".

event.peer:send('bark')

Wecannowcall server.updateinsideourgameloop's love.updatefunction:

love.update=function()

server.update()

Weneedtotestourserver,buttotestourserver,weneedaclient.

OurclientapplicationCreatea"client"folderlikethe"server"foldercreatedabove.Mostofthecodewillbeidenticaltoourserver.Themaindifferenceisthatwhenwecreateahostwewon'tpassitanIPaddressandporttoserveon,butinsteadwilltellittoconnecttotheaddressandporttheserverisrunningon.

--client/main.lua

--Ourclientapplication

localclient=require('client')

client.start()

ifclient.is_connected()then

love.graphics.print('connectedtoserver(seeconsole)',transform)

love.graphics.print('establishingaconnection...',transform)

love.event.quit()

client.update()

--client/client.lua

localclient={}

localhost

localpeer

client.start=function()

host=enet.host_create()

peer=host:connect('127.0.0.1:6789')

client.is_connected=function()

returnreceived_data

client.update=function()

ifhostthen

ifeventthen

received_data=true

print('----')

print(k,v)

event.peer:send('meow')

returnclient

Ifwereceiveamessagefromtheserverwe'll"meow"backatit.

TestingthingsoutIfyouruntheserveryouwillseeamessagesaying"serverstarted...awaitingclients".Sinceweareprintingtotheconsole,ifyouarerunningthiscodeonWindowsrememberthatyouwillneedtoenabletheconsole.Thiscanbedonebycreatingaconf.luafileinboththeclientandserverfolders.

t.console=true--EnablethedebugconsoleforWindows.

t.window.width=800--Game'sscreenwidth(numberofpixels)

t.window.height=600--Game'sscreenheight(numberofpixels)

Iftheserverisupandrunningwiththeconsoleenabled,goaheadandstarttheclientwithitsconsoleenabledtoo.Youshouldimmediatelyseeafloodofeventsprintingoutintheserverandclientconsoles.

Serverconsole:

peer127.0.0.1:58384

channel0

datameow

typereceive

Clientconsole:

peer127.0.0.1:6789

channel0

databark

typereceive

Thiswillgobackandforthuntilyoucloseeitheroneofthem.Ifyoucloseonethough,themessageswillstopanditwilljustsitthere.Ifyouclosetheserverfirst,forinstance,theclientwillsittherethenafterseveralsecondsamessagewillappear:

peer127.0.0.1:6789

typedisconnect

Normallyadisconnectlikethiswouldn'tbedetectedwithUDP,buttheENetlibrarysends"heartbeat"messagesbackandforthtomakesurebothpeersarestillconnectedtoeachother.Thetimeoutisdefinedtobesomewherebetween5and30secondsbeforethepeerrealizesithasbeendisconnnectedfromtheotherone.Justtopolishthingsoffhere,let'smakeENetsendadisconnecteventtotheotherpeerimmediatelywhenweareclosingourapplication.Thelua-enetdocumentationlistsafunctionwecaninvoketodothat, peer:disconnect_now.LÖVEhasa love.quitcallbackthatiscalledwhenourapplicationisclosing.Wecanwritea server.disconnectfunctionandcallitfromlove.quit.

Server:

--server/main.lua

love.quit=function()

server.disconnect()

--server/server.lua

server.disconnect=function()

ifpeerthen

peer:disconnect_now()

peer=nil

host=nil

received_data=false

The client.disconnectcodewouldbeidentical.

ToseethisfullexampleorifyouhaveanyproblemsgettingyourcodetoruncheckoutthecodeonGitHub:https://github.com/RVAGameJams/learn2love/tree/master/code/networking-1

Inthenextpartwewilllookatnetworkarchitectureandaddentitiestothescreentoworkwith.

ExercisesWhathappensifyoutrytoconnectmultipleclientstotheserver?WhataboutrunningmultipleserversonthesameIPaddressandport?Whydoesitbehavelikeitdoes?

Networking(part2)Intheprevioussectionwemadetwoapplicationsthatcouldtalktoeachother.Oneapplicationwastheserverandthesecondoneconnectingtoitwastheclient.Ingamedesignthisstyleofnetworkingcanbedescribedasadirectconnection.

Directconnection

Inadirectconnectiononeoftheplayerstakesontheroleofserver,meaningtheirgameworldistheultimateauthorityifthereareanydiscrepanciesorout-of-synccommunicationbetweenthetwo.Thisalsomeanstheserverplayercanfindwaystocheatandexploitthegame.

Oneoftheadvantagestothissetupissinceyouaredirectlyconnectedtoeachother,yougetasminimallagaspossible.Thisadvantagedoesn'tholdtrueiftherearemorethan2players.Ifplayer1istheserverandplayer2and3areconnectedtoplayer1,thenplayer2and3havetorelayupdatestoeachotherthroughplayer1insteadofdirectlytoeachother.Outsideof2-playergames,thissetupisn'taspopularashavingadedicatedserver.

Dedicatedserver

Dedicatedserversareexactlywhattheysoundlike.Theyarehostsdedicatedtoservingplayers.Thedifferencehereisallplayersareclientsandtheserverisaneutralgroundwhereplayerscanconnectandcommunicateindirectlywitheachotherthroughit.Serverstypicallyrunamodifiedversionofthegamecodethathasnouserinterfaceandthereforecanrunonalessexpensivecomputer.Ifoneoftheplayersisdetectedcheatingtheservercandetectthatsomethingiswrongandkickthemfromthegame.Theserveristheultimateauthorityoverthestateofthegameworld.

Ournetworksetupwillsortofbeamixbetweenthetwostylesofnetworking.We'llhaveadedicatedserverthatdoesn'tparticipateinthegameplay,buttheserverwillhaveagraphicalinterfacesowecanviewwhatisgoingonduringourtesting.

ConsolidatingourcodeRatherthanmanagingtwofoldersofcodelikeintheprevioussection,we'llcombinethecodeanduseamenusystemtoselectbetweenbeingaserverandbeingaclient.Themenucodeisn'timportanttothistutorialsotrytofocusontheclientandservercodeasbefore.Therefactoredcodecanbefoundinthecoderepositoryhere.

Giventheamountoffilesitiseasiesttodownloadthezipofthewholeprojectwhereyouwillfindtherelevantfilesinside code/networking-2:https://github.com/RVAGameJams/learn2love/archive/master.zip

Oncedownloaded,whenyouruntheprogramyoushouldbegreetedwithamenuscreenlikeso:

Testitoutandconfirmyoucanconnectaserverandclientinstancewiththenewcode.

Ok.Themodificationsto main.luashouldbeeasyenoughtounderstand.Let'stakealookatthatandthenew"net"servicefirstbeforewebeginmakinganymodifications.Atthetopofthefilewe'reloadingthenetandmenuservicesthentellingthemenuservicewhichmenutoloadonstartup:

--main.lua

localmenu_service=require('services/menu')

localnet_service=require('services/net')

menu_service.load('main-menu')

Next,ifakeyispresseditwillpassthatpressed-keyeventtothemenuservice.Ifweareinthegameandnomenuisloaded,themenuservicewilldonothingwiththeevent.

menu_service.handle_keypress(pressed_key)

Inside love.drawwehaveasimilarstory.Ifwehaveanactivemenuthen menu_service.draw()willdrawitOtherwiseitwon'tdoanything.(Ifyouopen services/menu.luayouwillseethedrawfunctionwherethisallhappens.)

menu_service.draw()

ifnet_service.is_connected()then

love.graphics.print('peerconnected(seeconsole)',transform)

Anotherthinginside love.drawisachecktoseeifwe'vemadeaconnectioneitherasserverorclient(usingnet_service.is_connected())thendrawthe"peerconnected"textonthescreenasbefore.Weusetheword"peer"asagenerictermtorefertoeithertheclientwe'reconnectedto(ifwe'retheserver)ortheserver(ifwe'retheclient).

Inside love.updateand love.quitwehavecombinedthecodewehadbeforeandaddeda menu.update()call.Ifthereisamenu,updateit.Ifeitheraserverhostorclienthostisrunning, net.update()willupdateit.

menu_service.update()

net_service.update()

love.quit=function()

net_service.disconnect()

Soeverywherewewerecalling"server"or"client"wejustcall net_serviceanditwilldoit'sthingnomatterthetypeofconnection.Let'sopenup net.luaandwe'llseesomethingveryclosetotheoriginalcode:

--net.lua

--Populateoneortheotherdependingifwestartaserverorclienthost

localclient_host

localserver_host

--Asaserver,wewanttokeeptrackofalltheconnectedclients

localpeers={}

--Theservicewewillbereturning

localnet={}

Atthetopofthefilewecreatesomeemptylocalvariables.The nettableisfulloffunctionsthatarebeingusedinmain.luaandelsewhere.

net.start_server=function()

server_host=enet.host_create('localhost:6789')

net.start_client=function()

client_host=enet.host_create()

server_host=client_host:connect('localhost:6789')

Onthemenuwhenyouselect"Host"or"Join",the net.start_serverand net.start_clientfunctionsarebeingcalledrespectively.

Belowthataresomefunctionstocheckwhatkindofconnectionwehave:

net.is_connected=function()

returnreceived_data

net.is_client=function()

returnclient_hostandtrueorfalse

net.is_server=function()

returnserver_hostandnotclient_host

Thenfinallywehavethe updateand disconnectcodelikeweoriginallyhadintheserverandclientservices,butcombined.Oneadditiontothedisconnectfunctionisweareloopingoverthe peerslist.The peerslistexistsbecauseweareexpectingtohavemultipleplayersconnecttotheserverandiftheserverisrunning,itwillwanttodisconnectfromthemallwhenthegamequits.

CommunicationlayerBeforeweupdatethecode,let'sdiscussthefunctionalityanddrawoutthenetworkcommunicationforthatfunctionality.

WhenconnectingtotheserveryoushouldhaveacontrollableplayerspawnonscreenWhenyoumoveyourplayer,yourpeersshouldbeabletoseeyourplayermoveWhenotherpeersmovetheirplayers,youshouldbeabletoseethemmoveEachplayershouldlookdifferentsoyouknowwhichoneisyou

Client Server Description

(Createahost) (Createahost)Bothclientandserver'snetservicesarebooted.Nocommunicationhasbeenmadebetweenthetwoatthispoint.

Send"connect"messagetoserver

Clientsendsaconnectmessageautomatically.Thiswillbeinterpretedasarequesttojointhegame.

(Spawnentity)Servergeneratesandstoresalltheentitiesinthegame.Whenspawninganentity,associatetheclient'sconnectionIDwithit.

Respond"your-id|4177457821|100|500"

Serverrespondsbypassingtheclienta "your-id"message.Allmessagessentmustbestringssointhiscasewheremultiplevaluesneedtobeembeddedinthemessage,eachvalueisseparatedbyapipe("|")charactertohelpusre-separatethevalueswhenreceivingthemessageclientside.ThefirstvalueistheuniqueplayerIDthattheserverhasassignedtheclient,followedbytheXpositionthenYposition.

Send"peer-id|233142890|326|177"

Serversendsthenewclientotherpeersthatneedtobespawnedonscreen.IncludedaretheID,XandYpositionofthatplayer.

Send"move|233142890|327|177"

ServerislettingtheclientknowtheplayerwiththeID"233142890"haschangedposition.NoticetheupdatedXposition.

Send"move|233142890|328|177" Anothermoveupdate.

Send"move|100|502" Clientislettingtheserverknowitismovingitsplayertoo.

Send"peer-id|81850530|500|500" Anotherplayerhasjoinedtheserver.

AddingentitiesOnconnectionfromaclient,theserverneedstospawnentitiessolet'screateanentityserviceforhandlingallourentity-relatedneeds:

--entity.lua

localentity_service={}

--Allplayerentities

entity_service.entities={}

entity_service.spawn=function(player_id,x_pos,y_pos)

return{

--TODO:We'lladdacolorrandomizerlater

color={1,1,1,1},

id=player_id,

--TODO:We'lladdashaperandomizerlatertoo

shape=love.physics.newPolygonShape(0,0,50,0,50,50,0,50),

x_pos=x_pos,

y_pos=y_pos

entity_service.draw=function(entity)

love.graphics.setColor(entity.color)

localpoints={entity.shape:getPoints()}

foridx,pointinipairs(points)do

ifidx%2==1then

points[idx]=point+entity.x_pos

points[idx]=point+entity.y_pos

love.graphics.polygon('line',points)

returnentity_service

Theseentitieswilljustbebasicshapes.Noworld,body,orfixturestoworryaboutasdealingwithphysicsisabitoutofthescopeofthissection.

Nowwe'lldosomeheavyupgradestothenetservice.Somewherenearthetopofthefilewe'lldefinetwotableswithcallbacksthatwillgetinvokedwhenaneventcomesin.They'llbeemptyfunctionsfornowandwe'llfillthemoutaswego.

--Callbackstoinvokewhencertaineventsarereceivedfromapeer

--Defineacallbacktohandleeverytypeofmessageinourapplicationprotocol

localmessage_handlers={

['your-id']=function(message,event,is_server)

['peer-id']=function(message,event,is_server)

['move']=function(message,event,is_server)

--TheseeventtypesaredefinedbyLua-enet.A"receive"typeofevent

--isagenericeventthatcarriesanyofthemessagesabove.

localevent_handlers={

connect=function(event,is_server)

disconnect=function(event,is_server)

receive=function(event,is_server)

Thenwe'llmodifythe net.updatefunctionsoitcancalloneofthethreecallbacksinsidethe event_handlerstable:

net.update=function()

localhost=client_hostorserver_host

ifnothostthenreturnend

ifeventthen

received_data=true

--event.typewillbeeither"connect","disconnect",or"receive"

event_handlers[event.type](event,net.is_server())

--Printouttheeventtablefordebugpurposes

print('----')

print(k,v)

Soyousee, event_handlers.connectwillbecalledwhena"connect"typeeventcomesinandwe'llpassittheeventand net.is_server()booleanasitstwoparameters.Nowwecangobackandfilloutthe"connect"eventhandler.Readeachcodecommentasthereisalotgoingonhereinjustafewlinesofcode.

--Onlytheserverneedstodostuffhereonconnect

ifis_serverthen

--event.peer:connect_id()providesuswithauniquenumber.

--We'llconvertthatnumbertoastringanduseitastheplayerID.

localplayer=entity_service.spawn(tostring(event.peer:connect_id()),100,100)

--StorethisplayerintheplayertablewiththeplayerIDasthekey.

entity_service.entities[player.id]=player

--Sendtheinitial"your-id"messagebacktotheconnectingclientsotheycanspawnthisentitytoo.

event.peer:send('your-id|'..player.id..'|'..player.x_pos..'|'..player.y_pos)

--TODO:Addcodetoremoveentitieswhenaclientdisconnects

--TODO:Addcodetoparse"receive"eventsandcalltheappropriatemessagehandlercallback

Sincewe'reusingtheentityserviceinnet.luawe'llneedtorequireitattheverytop:

localentity_service=require('services/entity')

Nowthattheplayerreceivesthemessagetospawnanentity,wecanfilloutthe"receive"eventhandler.

ifis_serverthen

--TODO:Addcodetoremoveentitieswhenaclientdisconnects

--Extractthemessageoutfromtheeventandcalltheappropriatecallbackabove

localmessage={}

formatchin(event.data..'|'):gmatch('(.-)|')do

table.insert(message,match)

message_handlers[message[1]](message,event,is_server)

Don'tletthiscodefeelintimidatingasit'sonlytakingthestringmessagefromtheevent( "your-id:87335:500:500")andsplittingitintoalisttable( {"your-id","87335","500","500"})sowecanworkwiththedata.Oncewehavethemessagefragments,wecall message_handlers[message[1]](),where message[1]willbeoneofthekeysinthemessage_handlerstable: "your-id", "peer-id",or "move".Let'sfilloutthe"your-id"handlerfirst:

--Callbackstoinvokewhencertaineventsarereceivedfromapeer

--Defineacallbacktohandleeverytypeofmessageinourapplicationprotocol

localplayer_id=message[2]

localx_pos=message[3]

localy_pos=message[4]

entity_service.player_id=player_id

entity_service.entities[player_id]=entity_service.spawn(player_id,x_pos,y_pos)

['peer-id']=function(message,event_is_server)

Nowwhentheclientgetsthe"your-id"message,itcanspawnanentitytoo.Let'saddtheappropriatecodetomain.luafordrawingentitiessowecanseeifourprotocolisworkingsofar:

--main.lua

localentity_service=require('services/entity')

menu_service.draw()

ifnet_service.is_connected()andnet_service.is_server()then

love.graphics.setColor({1,1,1,1})

love.graphics.print('Serverpreview.Seeconsolefordetails.',transform)

for_,entityinpairs(entity_service.entities)do

entity_service.draw(entity)

Ifyou'veupdatedeverythingcorrectly,you'llseethishappenwhenaserverandclientrunside-by-side:

Ifyougetanerror,remembertocheckthelinenumberandfilenamewheretheerroroccurredandmakesurethecodelookssimilartohowitisabove.Ifyougetstuck,therewillbeafullexampleavailableatthebottomofthissection.

Ifeverythingisworkingforyouthenfantastic.Thismeanstheserverandclientaresuccessfullysyncinganentitystatewitheachother.

Nextlet'saddmovementsowecanseeliveupdateshappeningbetweenthetwogamewindows.Insidetheservicesfolder,we'llcreateaninput.luafilethatreturnsanemptytable:

--input.lua

return{}

Whyarewereturninganemptytable?Well,it'sonlyemptyrightnow,butaskeysarepressedandreleasedourtablewillgetupdated.Let'supdate love.keypressedandadda love.keyreleasedfunctiontomain.luatoseehowthatworks:

--main.lua

localinput_service=require('services/input')

menu_service.handle_keypress(pressed_key)

input_service[pressed_key]=true

input_service[released_key]=false

Nowwhenwepresssomearrowkeysforinstance,ourtableininput.luawilllookmorelikethisinmemory:

left=true,

right=false,

up=true,

down=false

Nowinsideentity.luawe'lladdan entity_service.movefunctionthatchecksforinputchangesandmovestheentityifanyofthearrowkeysarepressed:

--entity.lua

localinput_service=require('services/input')

entity_service.move=function()

localplayer=entity_service.entities[entity_service.player_id]

--Don'tlettheplayerpressupanddownatthesametime

ifinput_service.upandnotinput_service.downthen

player.y_pos=player.y_pos-2

elseifinput_service.downandnotinput_service.upthen

player.y_pos=player.y_pos+2

--Don'tlettheplayerpressleftandrightatthesametime

ifinput_service.leftandnotinput_service.rightthen

player.x_pos=player.x_pos-2

elseifinput_service.rightandnotinput_service.leftthen

player.x_pos=player.x_pos+2

Thiswillcauseourentitytomoveacrosstheclient'sscreen,buttheserverwon'tgettheseupdatesunlesstheclientsendsthemover.Weneedtogobacktomain.luaandsenda"move"message.Inside love.updatechecktoseeiftheplayerpositionhaschangedandsendamovemessagetotheserverifso:

menu_service.update()

--Checktoseeifaplayerhasspawnedandupdateitsmovementifanydirectionkeysarebeingpressed

ifentity_service.player_idthen

localplayer=entity_service.entities[entity_service.player_id]

localold_x=player.x_pos

localold_y=player.y_pos

entity_service.move()

ifplayer.x_pos~=old_xorplayer.y_pos~=old_ythen

net_service.send('move|'..player.id..'|'..player.x_pos..'|'..player.y_pos)

net_service.update()

The net_service.sendfunctionwehaven'tdefinedthatyet.Jumpbackovertonet.luaandwe'lldefinethatforsendingeitherclientorservermessagesifneeded:

net.send=function(message)

ifnet.is_client()then

server_host:send(message)

server_host:broadcast(message)

Nowwearesendingamessagetotheserverwhenwemove,butweneedtohavetheserverreadthemessageandupdatetheentitypositiononitsendtoo.Let'sfilloutthe"move"messagehandlerinnet.luatoaccomplishthis:

entity_service.player_id=player_id

--TODO:handlepeer-idmessagesforwhenmoreplayersjointheserver

entity_service.entities[player_id].x_pos=x_pos

entity_service.entities[player_id].y_pos=y_pos

ifis_serverthen

--Relaythismessagetotheotherplayers

forid,peerinpairs(peers)do

ifid~=player_idthen

peer:send(event.data)

Noticetheextraservercheck.Iftheserverreceivedthe"move"commanditshouldrelayitovertoanyotherconnectedplayersotheycanseeyoumovingtoo.The peerstableneedstobedefinedinsidenet.luaabovethemessagehandlersoryoumaygetanerrorwhenittriestoloopoverthetableandseesanilvaluethathasn'tbeendefinedyet.

Tryitoutagainandweshouldseethesquaremovingonbothscreensnow.

Allthathardworkisstartingtopayoff!Wehaveafewmorechangestomakeforthistobefullyfunctionalthough.Ifyoutryandrunthegamenowwithmultipleclients,theplayerswon'tseeeachother.Inourapplicationprotocolwedefineda peer-idmessagesothatwhennewplayersconnectwereceiveinformationtospawnthem.

Insidenet.lua,goaheadandfilloutthe"peer-id"messagehandlersothatitspawnsanentitywheninvoked:

Nowtheserverneedstosendallthepeerstoanewplayerconnecting,butitalsoneedstosendnewplayerstopre-existingpeersduringaconnectevent.We'llupdatethe"connect"eventhandlertodothat:

ifis_serverthen

--event.peer:connect_id()providesuswithauniquenumber.

--Letalltheotherpeersknowaboutthisplayer

for_,peerinpairs(peers)do

localpeer_player=entity_service.entities[tostring(peer:connect_id())]

peer:send('peer-id|'..player.id..'|'..player.x_pos..'|'..player.y_pos)

event.peer:send('peer-id|'..peer_player.id..'|'..peer_player.x_pos..'|'..peer_player.y_pos)

--Addthispeertothepeerlist

peers[tostring(event.peer:connect_id())]=event.peer

Aftersendingthenewclienttheiridas"your-id",wesendthatidtoeveryotherclientas"peer-id"intheforloop.Noticeweaddthenewconnectingclienttothepeertableattheveryend.Wedothisafterloopingoverthepeerlistsowedon'tsendthatclienta"peer-id"messageofthemselves.Again,whenregisteringpeersandentitieswecalltostring()onthe connect_id()toensurewearestoringIDsasstringsratherthannumbers.Storingdataintablesitmatterswhetheryoustorethemaskeysornumbers.See1.14-Tables(part2)toseewhatImean.

Anyways,tryrunningthegamenowwithmultipleclientsconnectingtotheserverandyouwillseeeachentitycanmoveseparatelyandthechangeswillbesynchronizedacrossallclients.Onechangetomaketheentitieseasiertodistinguishwouldbetorandomizetheircolorandshape.Let'smodify entity_service.spawninsideentity.lua:

entity_service.spawn=function(player_id,x_pos,y_pos)

localcolors={

{1,0,0,1},

{0,1,0,1},

{0,0,1,1},

{0,1,1,1},

{1,0,1,1},

{1,1,0,1},

{1,1,1,1}

localshapes={

love.physics.newPolygonShape(25,0,50,50,0,50),

love.physics.newPolygonShape(0,0,50,0,50,50,0,50),

love.physics.newPolygonShape(12,0,36,0,49,15,49,33,36,49,12,49,0,33,0,15)

return{

--Cyclethroughthelistofcolorsbasedonwhatevertheplayeridis

--Callingtonumber()tomakeplayer_idanumberinsteadofastringsowecandomathonit

color=colors[(tonumber(player_id)%#colors)+1],

id=player_id,

shape=shapes[(tonumber(player_id)%#shapes)+1],

x_pos=x_pos,

y_pos=y_pos

Hereweuseamodulustocyclethroughthelistofcolorsandshapesandassignonebasedonthepseudo-randomplayerIDwereceived.Thisalsomeansaplayerwilllookthesameoneveryotherplayers'client.Tryrunningitagainandyoushouldseesomethingsimilar:

Thefinalnetworkingcodecanbefoundhere.Again,giventheamountoffilesifyouneedthewholefolderthenitiseasiesttodownloadthezipofthewholeproject.Youwillfindtherelevantfilesinside code/networking-3:https://github.com/RVAGameJams/learn2love/archive/master.zip

ConclusionThisisaboutasbasicinfunctionalityasamultiplayergamecanget.Therearemanyfeaturesmissingfromourcode.Justtonameafewmajorones:

Sanitychecksonmessages.Youcancrashtheserverbysendingitaninvalidmessage.The"move"messageexpectsaplayeridwhentheservershouldbeabletofigurethisoutonitsown.Theservercaneasilybefooledintoaccepting"move"messagesfromaclienttomoveanotherclient.Tomakethingssimplerthereisnoworldorphysicsnoranynetcodetohandlecollisionsoranythingofthelike.Theentitiesjuststickaroundwhenyouclose/disconnectaclient.Ifyouloseconnection,thereisnoattempttorestoretheconnection.

ThereisagreatsetofarticlesbyGabrielGambettaontheproblemsfacedbymakingaaction-basedmultiplayergamesandit'sworthareadtogetahigh-leveloverviewofthechallengesandhowtoreasonaboutthem.Linktohisarticles.

ExercisesHavingalltheplayersspawnattheexactsamepointisn'tideal.Insidenet.luainthe"connect"eventhandler,makealistofspawnpointsandmaketheplayersspawnatoneofthepointsbasedontheir peer:connect_id().Hint:thecodeshouldlooksimilartothecolorcyclecodeinside entity.spawninentity.lua.Insidenet.lua,completethe"disconnect"eventhandlerandmakeissowhenaclientquitstheirentityisremovedfromthegameforallpeers.

Chapter3:ProgrammingindepthThegoalofthischapteristotouchonavarietyoftopicsandproblemsfacedwhileprogrammingandtounderstandandsolvethemusingLua.Awidervarietyoftopicswillbecoveredhere.Onetopicwillleadintoanotherwhichwillthenbuildontopofconceptsintroducedintheprevioustopic.

3.0-Programmingin-depth

PrimitivesandreferencesTakealookatthiscode:

localstring1="hello"

localstring2='hello'

print(string1==string2)

localnumber1=14

localnumber2=14

print(number1==number2)

localtable1={}

localtable2={}

print(table1==table2)

localfunction1=function()end

print(function1==function2)

Whatwouldhappenifyouweretorunthis?

Inchapter1welearnedaboutcomparingstringswiththe ==operatorwhenwetalkedaboutbooleans.RunthecodeaboveintheREPLandseewhatitreturns:

Thestringsequalandthenumbersequal,butwhyaren'tthetablesandfunctionsequalsincetheyarebothempty?Tryprintingthetablesandfunctionsandlookwhathappens:

localtable1={}

localtable2={}

print(table1)

print(table2)

print(function1)

print(function2)

table:0x16af270

table:0x16af220

function:0x16ae840

function:0x16aeff0

Attemptingtoprinteachvalueyouaregivenbackahexadecimalnumber,theplaceinmemorywherethosevaluesarelocated.Eachtableandfunctionresidesinadifferentplaceinmemory.Sohowisthisrelevant?

Whencheckingdatalikestringsandnumbers,the ==operatordoesindeedcheckthatthedatamatches.Thesedatatypesaresimpleandtakeverylittleeffortforacomputertocheckthattheyareequal.Booleans,strings,numbers,and nilareallprimitivetypesofdataandbehavethisway.

Whencheckingdatalikefunctionsandtables,however,the ==operatorchecksthememorylocationofthedataonbothsidesoftheoperatorandifthevariablesreferencethesamelocationthentheyareequal.Inotherwords,the==operatorchecksthesedatatypestoseeiftheyhavethesameidentity.Nomatterhowmanyemptytablesorfunctionsyouhave,eachoneiscreatedwithauniqueidentity.

localstring2="hello"

--Anothercopyof"hello"iscreatedinmemory:

localstring3=string2

--Butthesetwocopiesareequal

print(string2==string3)

localtable1={}

localtable2=table1

print(table1==table2)

Whatistheresultof print(table1==table2)?Aha!Boththesevariablesreferencethesamedata.Quick–amagicianwavestwowandsinfrontofyourfaceandasksyoutocounthowmanywandsthereare.Howdoyouknowiftherearereallytwowandsorifthisisjustatrickwithmirrors?Whatdoyoudo?Youtakeoneofthewandsandbreakitofcourse.Iftheotherwandbreaksthentheywerethesamewandtheentiretime.Let'strythatwiththetwoobjects:

localtable1={}

localtable2=table1

table1.rabbit='white'

print(table2.rabbit)--Equals'white'too

Aslongasyourvariablesreferencethesametable,updatingthetablefromonevariableyouwillseetheresultwhencheckingtheothervariable.Thisdoesn'tworkwithprimitivedatabecauseyou'realwaysmakingacopywhenassigningittoanewvariablename:

localstring2=string1

string1='world'

returnstring2

=>hello

Primitiveversusnon-primitivedatatypesWheneverweassignnon-primitivedatatoanewvariable,we'realwaysreferencingtheoriginaldata:

localgrocery_list={

'carrots',

'celery',

'pecans'

localsame_list=grocery_list

grocery_list[1]='grapes'

returnsame_list[1]

Butassigningprimitivedatatoavariable,evenprimitivedatainsidetables,we'realwaysmakingauniquecopy:

localgrocery_list={

'carrots',

'celery',

'pecans'

localitem_copy=grocery_list[1]

print('item_copyis'..item_copy)

grocery_list[1]='grapes'

print('item_copystillis'..item_copy)

Ifyouneedtomakeeachiteminyourtablereference-able,youneedtomakeeachitemanon-primitivedatatype:

localgrocery_list={

{name='carrots',location='produce'},

{name='celery',location='produce'},

{name='pecans',location='baking'}

localitem_reference=grocery_list[1]

print('item_referenceis'..item_reference.name)

grocery_list[1].name='grapes'

print('item_referenceisnow'..item_reference.name)

Soratherthanreplacingthefirstiteminthelist,thefirstitemwasretainedandonlymodified:

item_referenceiscarrots

item_referenceisnowgrapes

Cloningnon-primitivedatatypesAswearefamiliarwithatthispoint,tablesareaspecialdatatypethatcancontainotherdatatypes.Youcanbuildstructurescontainingstrings,variables,andevenothertables.Thatmakesthetableacompositedatatype,inotherwords,adatatypewithdistinguishableparts.Notalllanguageshavecompositedatatypes,butforLuathetableisoneofitsprimaryfeatures.

Onethingaprogrammermaywanttodowithatableisonceconstructed,createacopyofit.Iftherewasatableforamonsterinavideogame,youmaywanttohavemorethanonetable.Ifyoudidthis:

localenemy1={health=10,strength=12,type='orc'}

localenemy2=enemy1

Youwouldstillonlyhaveonetable.Youcouldusealooptocopyallthevaluesoutofatableandintoabrandnewtable.Afunctiontodothatmaylooklikethis,moreorless:

localcopy=function(orig_table)

localnew_table={}

forkey,valueinpairs(orig_table)do

new_table[key]=value

returnnew_table

localenemy1={health=10,strength=12,type='orc'}

localenemy2=copy(enemy1)

Thereisnothingterriblywrongwiththismethod,butamoreefficientwaytodosuchathingwouldbetoconstructeachmonstertableinsideafunctioninsteadofcopyingonefromanother.Thismethodwillbefamiliaralreadyifyoureadandfollowedthroughthebreakoutgame.

localcreate_orc=function(strength)

return{

health=10,

strength=strength,

type='orc'

localenemy1=create_orc(12)

localenemy2=create_orc(12)

Everytimethefunction create_orcisran,itconstructsanewtablefromscratch.Youdefineanorc-styletableonlyonceanddon'tneedtoreadvaluesinfromonetabletoanother.Afunctionthatconstructstablesforyouisacommonparadigminprogrammingknownasafactoryfunction.Youmadeafactorythatbuildsorcs!Ofcoursethisfactoryfunctionparadigmworkswithothernon-primitivetypesofdataaswell:

localcreate_function=function()

returnfunction()return1+1end

localfn1=create_function()

localfn2=create_function()

print(fn1)

print(fn2)

Afunctionthatgeneratesotherfunctions?Thismayseemlikeanoddthingtowanttodo,butthismethodofprogrammingcanbequiteusefulaswe'llseein3.02-Higher-orderfunctionsandlaterfollow-upsections.Onethingthatshouldbementionedthoughisthatfunctionscanalsobeconsideredacompositedatatypeasitcanreturnotherdatatypes,andevenotherfunctions.Compositeinthatyoucancomposehigher-orderfunctionalityinthewaytablescanbeusedtocomposehigher-orderstructures.

ConclusionWhencomparingorreferencingdata,alwayskeepinmindwhetheryouhandlingprimitiveornon-primitivedata.Ifyouaremodifyingdatainoneplace,thinkifthismightbeaffectingyousomewhereelseinyourprogram.Evenwhenwritingouta localsome_module=require('some-module')inyourcode some_moduleisjustatableandlikeeveryothertable,everyreferencetoitcanaffecteachother.Somodifying some_moduleintwodifferentfilescanhaveeitherbeneficialordisastrousconsequencesdependinghowmuchcareandregardyougiveyourcode.

Higher-orderfunctionsIn1.07Makingfunctionswelearnedabout,well,makingfunctions.Sowhatabouthigher-orderfunctions?Whataretheyandhowdowemakethem?Simplyput,higher-orderfunctionsarefunctionsbuiltontopofotherfunctions.Here'sabasicexample:

localrun_twice=function(some_function,some_data)

some_function(some_data)

run_twice(print,'HelloWorld!')

Itcantakeanyfunctionandrunittwiceforyou,inthiscasethe printfunction,butitcouldbeanyfunctionyoupassit.Typicallyhigher-orderfunctionsreturndata.Here'satrickierexamplethatdoesjustthat:

localtwice=function(fn,val)

returnfn(fn(val))

localadd_four=function(num)

returnnum+4

returntwice(add_four,12)

Takealookatthebottomlineforasecond.Wearecallingthefunction twicewithtwoarguments,the add_fourfunctionandthenumber 12.Thepurposeofthe twicefunctionistotakeavalue, 12inthiscase,andrunitthroughthegivenfunction( add_four)twice.Nowtakealookinsidethe twicefunction.Insideitreturnsfn(fn(val)).Givenwhatweknowisbeingpassedtothisfunction,thiscanbereadassayingadd_four(add_four(12)).Theorderofoperationsaystostartfromtheinner-mostparenthesisandworkyourwayout:

add_four(add_four(12))

becomes

add_four(16)

whichbecomes

andthatiswhatisreturnedwhenyourunthecode.Thepowerofthesehigher-orderfunctionsisthattheyarere-usable.Youcangivethe twicefunctionanythingthattakesandreturnsavalue:

returnfn(fn(val))

localdouble=function(number)

returnnumber*2

returntwice(double,3)

...orsimilartoouroriginalexample:

returnfn(fn(val))

localshout=function(message)

print(message..'!!')

returnmessage

returntwice(shout,'hello')

Thereareallexamplesofhigher-orderfunctionsthatacceptafunctionasanargument.Anotherkindofhigher-orderfunctionisonethatreturnsanotherfunction:

localwrapper=function()

returnfunction()

return'Youfoundthetreasure!'

localkinder_surprise=wrapper()

localsecret=kinder_surprise()

returnsecret

Whenweran wrapperitreturnedusanotherfunctionthatwehadtoinvoketogettotheinnermostvalue.Toavoidallthevariablenames,youcansavesometimeandinvokesuchkindsoffunctionslikeso:

localwrapper=function()

returnfunction()

return'Youfoundthetreasure!'

returnwrapper()()

ClosuresWhichnumberwillprintoutbyrunningthefollowingcode?

localnumber=3

localclosure=function()

localnumber=5

returnfunction()

print(number)

localprint_number=closure()

print_number()

Strange?

Ok,solet'stryathissamefunction-returning-a-functionthingbutpassinginsomedata:

localadder=function(a)

returnfunction(b)

returna+b

localadd_three=adder(3)

returnadd_three(1)

The add_threevariableisassignedauniqueandspecialfunction.Itisassignedtheinnerfunctionwithintheadderfunction,butwiththedatawepassedinnowassignedtothe avariable.Eventhoughthefunctionwasreturnedoutsideofthescopeitwasdefinedin,thescope'sdatawasenclosedinsidethereturnedfunctionuntilthefunctionwasdiscardedandtheprogramexited.Thesetypesoffunctionsarecommoninsituationswhereafunctionneedstobegeneratedmultipletimesbutwithdifferentdatasets.

Thedataintheclosurecanalsocontinuetobeupdated,givingyoutheabilitytomakestoragecontainersforyourdata.Trythisout:

localmake_counter=function()

localnumber=0

returnfunction()

number=number+1

returnnumber

localcount=make_counter()

print(count())

InprogramslikeLÖVEtherearecallbacksystemswhereasimilareffecthappens:

localentity=require('entity')

entity:draw()

Asseeninthepreviouschapter,the love.drawcallbackisdefinedinamain.luafileandlaterinvokedsomewherewithinthegameengine.Since love.drawwasdefinedinthescopewheretheentityvariableisdefined,theentityvariablelivesonandcanbeusedinside love.drawlongafterthemain.luafileisdonebeinginvoked.

ConclusionClosurestakesomepracticetounderstandandappreciate,butonceyouseepracticalexamplesofwhereandhowtousethemtheybecomeanindispensableitemonyourprogrammingtoolbelt.Intheprevioussectionweusedthetermcompositedatatocompareprimitiveandnon-primitivedatatypes.Inthissectionwesawhowtogoaboutcomposinghigher-orderfunctions.Inthefollowingpageswewillcoversomehigher-orderfunctionsthatarethebuildingblocksforoldandmodernsoftwarealike.

Exercises

Inthe make_counterexampleabove,trygeneratingmultiplecounters:

--Dothenumbersineachcounterstayin

--syncoraretheytrackedindependently?

localcount_a=make_counter()

localcount_b=make_counter()

Usingthesame make_counterexample,modifyittoreturnatableinsteadofafunction.Withinthistable,definean incrementand decrementfunctionsothatyoucanmakethecounternumbergoupordown.Howwouldyouusesuchafunction?

MapandfilterIntheprevioussectionwepracticedcreatingsomehigherorderfunctions.Inthissectionswe'llcomposetwohigher-orderfunctionscommonlyusedininternetapplicationsfortransforminglists.

We'llstartbytakingalookatourgrocerylisttoseewhatitemsweneedtopickup:

localgrocery_list={

name='grapes',

price='7.20',

location='produce'

name='celery',

price='5.50',

location='produce'

name='walnuts',

price='6.20',

location='baking'

name='sugar',

price='8.00',

location='baking'

name='mayonnaise',

price='3.50',

location='dressings'

name='cream',

price='3.00',

location='dairy'

Thislisthasmoreinformationthanwewanttoseeataquickglance.Ifwewantedtoonlydisplayanumberedlistofitemnames,wecoulddosobywritingafor-loopthatgeneratesanewlistforus:

localnew_grocery_list={}

forkey,valueinipairs(grocery_list)do

new_grocery_list[key]=key..'.'..value.name

for_,valueinipairs(new_grocery_list)do

print(value)

Herewegeneratedalistwithaloopthenloopedoverthelistagaintoprintourresults:

1.grapes

2.celery

3.walnuts

4.sugar

5.mayonnaise

6.cream

3.03-Mapandfilter

Thisworksgreatforsimplecodelikethisexample,butitcangetmessyifyouareworkingwithmanylistsorifyouwanttotransformliststodifferentformats.

MapHere'sourhigherorderfunction, map.Ittakesalistandafunctionasargumentsthenreturnsanewlist.

localmap=function(list,transform_fn)

localnew_list={}

forkey,valueinipairs(list)do

new_list[key]=transform_fn(value,key)

returnnew_list

Anewlistiscreatedbyloopingovereachitemintheoriginallist,applyingyourfunctiontotheitem,thenassigningthetransformeddatatothenewlist.Ourcodecanbere-writtentousethemapfunction:

localmap=function(list,transform_fn)

localnew_list={}

new_list[key]=transform_fn(value,key)

returnnew_list

localgrocery_list={

name='grapes',

price='7.20',

location='produce'

name='celery',

price='5.50',

location='produce'

name='walnuts',

price='6.20',

location='baking'

name='sugar',

price='8.00',

location='baking'

name='mayonnaise',

price='3.50',

name='cream',

price='3.00',

location='dairy'

localnew_grocery_list=map(grocery_list,function(item,index)

returnindex..'.'..item.name

3.03-Mapandfilter

for_,valueinipairs(new_grocery_list)do

print(value)

Calling map(...)wegetbackthenewlistthenweloopoveritagainjusttoprintourresultsout.Noticehowthesecondargumentwepassedintomapisjustafunctionwithnoname.Functionswithnonamesaresometimescalledanonymousfunctions.Insomelanguagesthey'recalledlambdas,especiallywhenusedinsideahigher-orderfunctioninasituationlikethis.Thetransformfunctiontakesintheitemanditsindexandmustreturnbackanewresultformaptoputinsidethenewfunction.

Maybeafewmoreexampleswillhelpout,sowhatifwewanttoreturnanotherlistwithjustthepricessowecanadduphowmuchweneedtospend?

localprice_list=map(grocery_list,function(item)

print(item.price)

returnitem.price

Herethemapfunctionispassedinatransformfunctionwithaprintstatementinsideit.Thatwayitwillprinttheitempricesasitbuildsthelistsoyoucanseewhateachvaluewillbe.

Ifyouhadotherlistsforwhichyouwantedtoprintprices,itcouldbedonequiteeasilywith map:

localtransform_fn=function(item)returnitem.priceend

map(grocery_list,transform_fn)

map(car_parts,transform_fn)

map(card_transactions,transform_fn)

FilterLet'ssaywewantedtoonlyseethethingsonourgrocerylistthatareinthebakingaisle.Wecouldwritealooptodothat:

localfiltered_list={}

for_,valueinipairs(grocery_list)do

ifvalue.location=='baking'then

filtered_list[#filtered_list+1]=value

for_,valueinipairs(filtered_list)do

print(value.name)

Tryrunningthatandonceitmakessense,let'sthinkabouthowtoturnthisintoare-usablehigher-orderfunctionlikemap.We'llmakeafunctioncalled filterthat,like map,takesalistandafunction.Thefunctionwillreturn trueifitwantstoputaniteminthenewlistor falseifitdoesn't.We'llcallitthepredicatefunction.

3.03-Mapandfilter

localfilter=function(list,predicate_fn)

localnew_list={}

--Thepredicate_fnthatwaspassedinshouldreturn

--avaluethatevaluatestoeithertrueorfalse.

ifpredicate_fn(value,key)then

new_list[#new_list+1]=value

returnnew_list

Andwecanusethisfunctiontofilterdowntojustourbakingitemslikethis:

localnew_list={}

ifpredicate_fn(value,key)then

new_list[#new_list+1]=value

returnnew_list

localgrocery_list={

name='grapes',

price='7.20',

location='produce'

name='celery',

price='5.50',

location='produce'

name='walnuts',

price='6.20',

location='baking'

name='sugar',

price='8.00',

location='baking'

name='mayonnaise',

price='3.50',

name='cream',

price='3.00',

location='dairy'

localfiltered_list=filter(grocery_list,function(item)

returnitem.location=='baking'

for_,valueinipairs(filtered_list)do

print(value.name)

3.03-Mapandfilter

walnuts

Noticeourpredicatefunctionwewrote:

function(item)

Theoperationafterthe returnalwaysreturnsaboolean trueor false,so filterknowsexactlywhattodowiththeitembasedonthoseresults.

Youcanimaginethe filterfunctioncouldbeusefulforprocessingasearchquery.Forinstance,ifwewantedtoseeonlymedium-sizedshirtsthatfitaspecificpricerange:

filter(products,function(item)

ifitem.type=='shirt'then

ifitem.size=='M'then

returnitem.price<40

returnfalse

CaveatsThefilterfunctionreturnsanewlist,buttheitemsintheliststillreferencetheoldlistiftheyaren'tprimitives.Forinstanceifwemodifiedthegrocerylist,thefilteredcopywouldbeupdated.

localfiltered_list=filter(grocery_list,function(item)

grocery_list[3].name='peanuts'

print(filtered_list[1].name)

peanuts

Thisbehaviorcanbeadvantageousifit'sexpected,butit'ssomethingthatshouldbeunderstoodabouthowLuaandsimilarprogramminglanguageswork.Thisisexplainedmorein3.1-Primitivesandreferences.

Anotherthingtoconsideriswhetherornottowritethefunctionsyourselfortouseapre-writtenlibraryyoucanrequireintoyourproject.Notallimplementationsarethesameandsomemayperformbetterthanothers,orbehavedifferently.Somelanguageshavebuilt-inversionsofthesefunctionstostandardizethings.UnfortunatelyLuadoesn'tprovidethesefunctionsbuiltinorasastandardlibrary.

Atleastyounowknowhowtowritethemyourselfiftheneedarises.

ExercisesTryfilteringthegrocerylisttoonly"produce"items,thenmappingthoseresultsdowntojustthenames.Using filter,nowcanyoureturnthenumberofitemsinthegrocerylistwithapriceofmorethan5?Hint:youwillneedtousetonumber()toconverttheitempricestonumbersforcomparing.

3.03-Mapandfilter

StackandrecursionWhenrunningaprogram,theinterpreter(Luainthiscase)keepstrackofvariablesdefinedinascopeandwhichfunctionyouarecurrentlyin.Itorganizesthisinformationintoalistinmemorycalledthestack.Thefirstiteminthestackisthestartingpoint-therootofyourapplication.Takethefollowingexample:

localtwo=function()

print('two')

localone=function()

print('one')

Whenstartingtheprogram,thestartofthestackisthetoplevelofthemodule.TheLuastackcallsthisthe"mainchunk".Whenafunctionisinvoked,anotherlayerisaddedtothestack.Everytimeafunctioniscalledfromanotherfunction,thestackcontinuestobuild.Sowiththeexamplecodeabove,Thestackwillfollowtheprogression:

Stackis {"mainchunk"}.Nowstartexecuting one.Stackis {"mainchunk","one"}.Nowstartexecuting twowhilestillin one.Stackis {"mainchunk","one","two"}.twoisdoneexecuting.Stackisnow {"mainchunk","one"}.oneisdoneexecuting.Stackisnow {"mainchunk"}.Programexits.

Thiscanbevisualizedbythrowinganerroratanypointtheprogram.Theinterpreterwillgiveyoubackastacktracethatdetailswhereitwaswhentheproblemoccurred.Luaprovidesahelpful errorfunctionfordebuggingthatwecanusehere:

localthree=function()

error('Thisisanerror.')

localtwo=function()

print('two')

three()

localone=function()

print('one')

UnfortunatelytheREPLdoesn'tprovideuswithstacktraces,butifyouhaveaLuainterpreteronyourcomputer( luacommand, luajit,orLÖVE)youwillseetheerrormessageandastacktracelikethis:

lua:test.lua:2:Thisisanerror.

stacktraceback:

[C]:infunction'error'

test.lua:2:inupvalue'three'

test.lua:7:inupvalue'two'

test.lua:12:inlocal'one'

test.lua:15:inmainchunk

[C]:in?

Fromthe"stacktraceback"youcanseethenewestfromthetopofthestacktotheoldestonthebottom.Incomplexprogramsiscanbeverybeneficialtoseewhichfunctioninvokedanotherfunctiontohelptracedownhowanerrorcameabout.

Understandingthestackisbeneficialformorethanjustreadingerrors.Let'sswitchtheconversationovertosomethingseeminglyunrelatedforabit.

RecursionWhenthinkingofloops,manyprogrammersfirstthinkofthe forlooporthe whileloop.Anothercommonmethodistomakeafunctioncallitself.Similartothe whileloop,youcancreateinfiniteloopslikethisone

localloop

loop=function()

print('hello!')

loop()

Whenafunctioninvokesitself,whetherdirectlyorindirectly,thisiscalledrecursion.Thesamefunctionwillrecuragainandagainuntilaconditionchanges.Orinthecaseabove, loop()willbecalledunconditionally.Withoutacondition,anykindofloopwillruninfinitely(orcrashtrying).Here'saloopthatisalittlesafertorun:

localcount_to_5

count_to_5=function(current_number)

print(current_number)

ifcurrent_number<5then

count_to_5(current_number+1)

count_to_5(1)

Whichprints:

Onequicklittleaside;Noticehowthefunctionwasdefinedinboththesesituations:

localloop

loop=function()

Thevariablewasdefinedbeforethefunctionwascreated.Sincethefunctionneedstoaccessthevariableinsideitself,thevariableneedstoexistatthetimethefunction'sscopeiscreated.Variablescreatedafterthefunctionareunknowntothefunction.Thisisdiscussedin1.17-ScopesandisalimitationofLua'sdesign.Fortunatelythereisshorthandsyntaxforwritingrecursivefunctions:

localfunctioncount_to_5(current_number)

print(current_number)

ifcurrent_number<5then

count_to_5(current_number+1)

count_to_5(1)

isthesameaswriting:

localcount_to_5

count_to_5=function(current_number)

Let'stryanotherrecursiveloop:

localgrocery_list={

'pumpkin',

'pecans',

'butter',

'flour',

'sugar'

localfunctionprint_items(list,index)

index=indexor1

ifindex<=#listthen

print(list[index])

print_items(list,index+1)

print_items(grocery_list)

Whichprintsthegrocerylist.Don'tforgetthe localatthebeginningof localfunctionprint_items,otherwiseyouwillaccidentallygenerateglobalvariablesinyourcodewhentryingtodefinefunctions.

Wecanevenre-implementour mapfunctionfromearliertouserecursioninsteadofa forloop.

localgrocery_list={

'pumpkin',

'pecans',

'butter',

'flour',

'sugar'

localfunctionmap(orig_list,transform_fn,new_list)

new_list=new_listor{}

if#new_list<#orig_listthen

localindex=#new_list+1

new_list[index]=transform_fn(orig_list[index],index)

returnmap(orig_list,transform_fn,new_list)

returnnew_list

localnew_list=map(grocery_list,function(value,index)

returnindex..'.'..value

map(new_list,function(value)

print(value)

returnvalue

Whichprints:

1.pumpkin

2.pecans

3.butter

4.flour

5.sugar

StackoverflowSowhatdoesthestacklooklikeduringrecursionwhenafunctionentersitself?Here'sascripttotest:

localfunctionrecur(n)

--assertislikeerror,buttakesanexpressiontotest.Ifthe

--expressionpassedbecomesfalsethenitthrowstheerrormessage.

assert(n<5,'Thisisaconditionalerror')

print(n)

recur(n+1)

recur(1)

lua:test2.lua:2:Thisisaconditionalerror

stacktraceback:

[C]:infunction'assert'

test2.lua:2:inupvalue'recur'

test2.lua:4:inlocal'recur'

test2.lua:7:inmainchunk

[C]:in?

Everytimethefunctionrecurswegetanotheradditiontothestack.Thiscanbeaproblemifyouareloopingoveralargesetofdatabecausethestackwillconsumemoreandmorememoryasitstacksup.Thiscanbeaccomplishedbycreatingarecursiveloopthatrunsinfinitely.Ifyouhaven'ttriedsoalready,here'saneasyexample:

localfunctionrecur()

recur()

Whenthestackreachesacriticalsize,yougetastackoverflowerror:

lua:test3.lua:2:stackoverflow

stacktraceback:

test3.lua:2:inlocal'recur'

[C]:in?

Withaspecific returnstatementaddedtotheloop,however,wenolongergetastackoverflow:

localfunctionrecur()

returnrecur()

recur()

Thiswillrununtilyoumanuallykilltheapplicationprocess.Killingitreturnsasomewhatmysteriousstacktrack:

lua:test4.lua:2:interrupted!

stacktraceback:

test4.lua:2:infunction<test4.lua:1>

(...tailcalls...)

test4.lua:2:infunction<test4.lua:1>

(...tailcalls...)

[C]:in?

Sohowdidourmodificationsaveusfromoverflowingourstack?

TailcalloptimizationInsideafunctionwhenyoureturnanotherfunctioncall,theinterpreterhastheabilitytore-usethesamelayerofthestackinsteadofcreatinganotherlayer.Thisworkswithdirectrecursion(functioncallingitself)andindirect(mutual)recursionsuchastwofunctionscallingeachother:

localone

localtwo

one=function()

returntwo()

two=function()

returnone()

ProgramminginLuagoesintogreaterdetailonwhenarecursionwillorwon'tbeoptimized,butthesimplethingtorememberisthatthefunction(s)mustreturnthevalueofinvokingafunctionforthistowork.Thefollowingwillbetail-calloptimized:

localone

localtwo

one=function(n)

print(n)

returntwo(n+1)

two=function(n)

print(n)

returnone(n+1)

--Countuntilwerunoutofnumbers

one(1)

Butthefollowingwon't,sinceitreturnsanoperationincludingthefunctioncallinsteadofjustthefunctioncallitself:

localone

localtwo

one=function(n)

print(n)

return1+two(n)

two=function(n)

print(n)

return1+one(n)

--Thiswon'twork!

one(1)

ThecaseforrecursiveloopsSowhywouldwewanttodorecursion?Itseemstrickierthana forloopandperhapsjustaseasytomessupasawhileloop.

It'snotnecessarilyareplacementforthe forloop,butallowsyoutodocertainthingsyoucan'teasilydowithoutrecursion.TakethisexamplefromRosettaCodewhichwillflattenalistoflistsintoasingle,flatlist.Itusesa forloopandarecursiveloopinconjunctionwitheachother:

localfunctionflatten(list)

iftype(list)~="table"thenreturn{list}end

localflat_list={}

for_,eleminipairs(list)do

for_,valinipairs(flatten(elem))do

flat_list[#flat_list+1]=val

returnflat_list

localtest_list={

{{3,4},5},

{{{}}},

{{{6}}},

print(table.concat(flatten(test_list),","))

Whichprints:

1,2,3,4,5,6,7,8

Thisfunctionisn'ttail-calloptimized,butitprobablywon'tbepassedanestedlistdeepenoughtocauseastackoverflow.

Here'sjustafewofthemanysituationswhererecursionisusuallythebesttoolforthejob:

SortingdataSearchingtrees(nesteddata)inadatabaseornestedfoldersinafilesystem.

FindingtheshortestpathbetweentwopointsLoopsthatincrementordecrementinirregularpatternsEvaluatingafinitesetofmovesinagamelikechess

Thepointisn'ttoreplacethe forloop,althoughyoucan.Takethefollowingexample,whichreturnsthefactorialofthegivennumber(5):

localfact=function(n)

localacc=1

foriteration=n,1,-1do

acc=acc*iteration

returnacc

print(fact(5))

Thesamefunctionalitywrittenwitharecursiveloopwouldlookverydifferent:

localfunctionfact(n,acc)

acc=accor1

ifn==0then

returnacc

returnfact(n-1,n*acc)

...butonemethodwouldn'tofferanadvantageovertheotherhere.Dependingonthelanguageyouareworkingin,onemethodmaybeeasiertoreadthantheother.Maybethelanguagesupportsonetypeofloopandnottheother.Thesearethefactorsthatwilloftendothedecidingforyou.

Reduce(fold)Inprevioussectionswediscussedmanymethodsforiteratingoverdataandtransformingit.Inthissectionwe'lldiscussanotherhigherorderfunctionthatisarguablyoneofthemostpowerful.Itisaconceptrecognizedacrossenoughprogramminglanguagestogetitsownwikipediaarticle.Mostpopularlanguagescallitreduce,althoughsomelanguageswillcallitfoldorinject.Here'stheparametersittakes,andalthoughtheorderoftheparametersmaybedifferentinotherlanguagesthefunctionalityandoutputwillbethesame.

reduce(list,fn,starting_value)

Likewith map()and filter(),ittakesalistyouwanttotransformandafunction( fn)todothetransformation.Thetransformationfunctionbehaveslikearecursivelooplikeseeninthelastsection.Here'safunctionthattakesalistofnumbersandgivesyouthetotalsumofthosenumbers.

locallist={23,63,12,48,3}

localsum_fn=function(accumulator,current_number)

returnaccumulator+current_number

localtotal_sum=reduce(list,sum_fn,0)

Wepassreduceastartingnumberof 0.Whathappensis sum_fnisinvokedwiththefirstparameter,theaccumulatorbeingthestartingnumber0and current_numberbeingthefirstnumberinthelist.Whatevervaluethefunctionreturnsbecomesthenewvaluefor accumulatornextlooparound.

Luadoesn'thaveareducefunctionbuiltinsowe'llimplementourownherewithadetaileddescriptionofalltheparameters.Trynottogettoohungupontheactualreducefunction'simplementationatthetop,butratherfocusbelowthatonhowitworks.Therewillbeseveralmoreexamples.Onceyouunderstandhowtouseit,gobacktothetopandlookattheactual reducefunction'simplementation.CopyallthiscodeintothetexteditorwindowontheREPLandrunit:

--Appliesfnontwoargumentscumulativetotheitemsofthearrayt,

--fromlefttoright,soastoreducethearraytoasinglevalue.If

--afirstvalueisspecifiedtheaccumulatorisinitializedtothis,

--otherwisethefirstvalueinthearrayisused.

--@param{table}t-atabletoreduce

--@param{function}fn-thereducerforcomparingthetwovalues

--@param{*}acc-Theaccumulatoraccumulatesthecallback'sreturn

--values;Itistheaccumulatedvaluepreviouslyreturnedinthe

--lastinvocationofthecallback,or`first_value`,ifsupplied.

--@param{*}current_value-Thecurrentelementbeingprocessedinthelist.

--@param{number}current_index-Theindexofthecurrentelement

--beingprocessedinthelist,startingat1.

--@param{*}first_value-Theinitialvalueoftheaccumulation.Ifthearrayis

--empty,thefirst_valuewillalsobethereturnedvalue.Ifthearrayisempty

--andnofirstvalueisspecifiedanerrorisraised.

--@example

----returns'zxy'

--reduce(

--{'x','y'},

--function(a,b)returna+bend,

localfunctionreduce(t,fn,first)

localacc=first

3.05-Reduce

localstarting_value=first~=nil

fori,vinipairs(t)do

--Nostartingvalue,starton

--thefirstelementinthelist

ifstarting_valuethen

acc=fn(acc,v,i,t)

starting_value=true

assert(

starting_value,

'Attemptedtoreduceanemptytablewithnofirstvalue.'

returnacc

locallist={23,63,12,48,3}

print(accumulator)

localtotal_sum=reduce(list,sum_fn,0)

print('Thetotalsumis:',total_sum)

Followingthe printstatementinsideof sum_fn,wecanseethatthe accumulatorstartsoutwiththe0wepassin.Weadd current_numberto accumulatoranditbeginstoaccumulateallthevaluesasitgoes.

Thetotalsumis:149

Ifwedon'tpassinastartingnumber,theaccumulatorwillbeginrightawaywiththefirstnumberinthelist:

print(accumulator)

localtotal_sum=reduce(list,sum_fn)

Thetotalsumis:149

Ifyou'veusedjavascript,youmaybestartingtoseetheuncannyresemblanceitbearstojavascript'sreducefunction.Bothlanguagesareverysimilarsyntactically,andgiventheubiquityofjavascriptthisLuaimplementationfollowsmuchofthesamebehavior.

Let'slookatsomemoreexamplestobetterunderstandhowtoreduceandwhatsituationsdoingsocouldproveuseful.Thereducefunctionisomittedinthefollowingexamples,butyoucancopyandpastethefunctionintheREPLalongsidetheexamplestorunthecodeyourself.

--Concatenatealistofwords

3.05-Reduce

locallist={'this','is','a','sentence'}

localsentence=reduce(list,function(acc,word,index,list)

--Addaperiodifthisisthelastword

ifindex==#listthen

word=word..'.'

--Otherwiseaddaspacebetweenthewords

returnacc..''..word

print(sentence)

thisisasentence.

--Onlykeepoddnumbers

locallist={23,63,12,48,3}

localodd_numbers=reduce(list,function(acc,current_number)

ifcurrent_number%2==0then

returnacc

acc[#acc+1]=current_number

returnacc

end,{})

forkey,valueinipairs(odd_numbers)do

print(value)

Thislookssimilartowhatwemightdowiththe filterfunctionpreviouslycoveredin3.3-Mapandfilter.Infact,wecancompose filterand mapfrom reduce.Takealookatthesamecoderefactoredout:

returnreduce(list,function(acc,val,i,t)

ifpredicate_fn(val,i,t)then

acc[#acc+1]=val

returnacc

end,{})

--Onlykeepoddnumbers

locallist={23,63,12,48,3}

localodd_numbers=filter(list,function(current_number)

returncurrent_number%2~=0

forkey,valueinipairs(odd_numbers)do

print(value)

Anexampleofwrapping reducewithanew mapfunctionwon'tbeexplainedhere,butratherleftuptothereaderasanexerciseattheendofthissection.

3.05-Reduce

Here'sonemoreexamplethatisabitmorecomplex,afunctioncalled composethatcreatesapipelineforpassingdatathrough.Itaccomplishesthisbypassinganyfunctionsyougiveitthroughto reduceasalist:

--Functionthatallowsyoutocomposeotherfunctions

--togethertoformapipeline.Theresultingpipeline

--isafunctionthatyoucanpassyourintendeddatathrough.

localcompose=function(...)

--"..."and"arg"arespecialkeywordsinLua.

--See:https://www.lua.org/pil/5.2.html

localfns=arg

returnfunction(x)

returnreduce(fns,function(acc,v)

returnv(acc)

end,x)

--Someexamplecomposablefunctions

localadd=function(x)

returnfunction(y)

returny+x

localmultiply=function(x)

returnfunction(y)

returny*x

localsubtract=function(x)

returnfunction(y)

returny-x

localnumber_pipeline=compose(add(12),multiply(2),subtract(9))

print(number_pipeline(3))

print(number_pipeline(2))

Alternativereduceimplementations

IteratingtablesLet'sgobacktotheimplementationofreduceforamoment.Takealookattheimplementationofitgivenabove.Noticetheiterationinsideisusing ipairswhichexpectsanarray/list-typetable.Ifwewantedtoreduceanon-listtablewecouldmodify reducetofirstcheckifthetableisanarrayanddoappropriateiterationoverthetablewhetherornotitis.Let'stestthat:

localget_iterator=function(t)

iftype(t)=='table'then

--Ifpropertyof1isemptythen

--iterateasaregularkeyedtable

ift[1]==nilthen

returnpairs(t)

returnipairs(t)

error('Expectedtable,got'..tostring(t))

localacc=first

--Whetherwedoipairsorpairsisconditional

3.05-Reduce

fori,vinget_iterator(t)do

acc=fn(acc,v,i,t)

starting_value=true

assert(

starting_value,

returnacc

locallist={

monday=23,

tuesday=63,

wednesday=12,

thursday=48,

friday=3

localtotal_sum=reduce(list,function(acc,current_number,key)

print(key..':'..current_number)

returnacc+current_number

print('totalsum:'..total_sum)

Thisshouldprintsomethinglikethis:

wednesday:12

friday:3

thursday:48

monday:23

totalsum:149

Notethattheorderthekeysareiteratedinarenotguaranteed.Also"tuesday"wasn'tprintedoutbecauseitwasthestartingnumber,butitwasstillincludedinthetotal.Passinganextraargumentof 0to reducewouldhavecausedallthedaystobepassedthroughourreducerfunctionandprintedout.

Breakearly

Ok,here'sanotherexamplethatseemstrickyatfirstglance;Let'ssayyouimplementedsomesearchfunctionalityontopofreducelikethis:

locallist={23,63,12,48,3}

localfind=function(list,predicate_fn)

returnreduce(list,function(acc,v,i,t)

ifpredicate_fn(v,a,t)then

returnv

returnacc

print(find(list,function(val)

returnval>50

3.05-Reduce

returnval%8==0

Whichprintsouttheexpectedresults:

Butdoyouseewhat'sproblematicaboutthis?Ifwefindtheresultswewant,thereducefunctionwillkeeprunningthroughtheentirelistunnecessarily.Typicallywhendoingasearchyouonlywantthefirstitemyoufindanyway,buttheaboveimplementationwillreturnthelastitemfoundifmorethanonematchismade.Doyourememberhowthereducefunctionpassesinthetableasthelastargumenttothereducerfunction?Wecantakecontrolofiteratorviathetableandkilltheiterationprematurely.Thisinvolvedmutatingthetable:

locallist={23,63,12,48,3}

--Ifaresultwasfound,destroythenextiteminthelist

--topreventtheiterationfromgoinganyfurther.

t[i+1]=nil

returnv

returnacc

returnval>1

Thisreturnsthecorrectresult:

Butifweloopoverthetableafterwardswecanseewe'vemessedwiththeoriginaldatawhichcanleadtounexpectedconsequencesinarealapplication.Ifyourdataiscomingfromanimmutablesource,meaningsomethingisgeneratinganewcopyeachtimeyouuseitthenthiswouldn'tbeaproblem:

localgenerate_list=function()

return{23,63,12,48,3}

reduce(generate_list(),function()

Howeverwecouldfixallofthisifwearewillingtoaddanotherparametertoourreduceimplementation.

localget_iterator=function(t)

iftype(t)=='table'then

--Ifpropertyof1isemptythen

--iterateasaregularkeyedtable

ift[1]==nilthen

returnpairs(t)

returnipairs(t)

3.05-Reduce

error('Expectedtable,got'..tostring(t))

localacc=first

fori,vinget_iterator(t)do

--Exittheloopwhentrue

localshould_break=false

acc,should_break=fn(acc,v,i,t)

ifshould_breakthen

starting_value=true

assert(

starting_value,

returnacc

Nowifwepass trueasasecondreturnparameterthenwewillgetthefirstnumberwearelookingforinsteadofthelast.Loopthroughandprintoutthelistafterwardtomakesurewehaven'tmutateditunexpectedly.

locallist={23,63,12,48,3}

returnv,true

returnacc

end,false)

returnval>1

foridx,valinipairs(list)do

print(idx,val)

reduce_right

Anotherpossiblechangeyouwouldwanttomakeistoreplacetheiteratorwithacustom-madeonetotransformdatainaspecificorderorpattern.Takenfromlua-users.org'sIterationTutorialisthisreverse-ipairs( ripairs)implementationthatallowsyoutoiterateoveratablefromrighttoleft.Thismodifiedversionof reduceistypicallycalled reduce_right.

localfunctionreduce_right(t,fn,first)

localripairs=function(t)

localmax=1

whilet[max]~=nildo

max=max+1

localfunctionripairs_it(t,i)

3.05-Reduce

localv=t[i]

ifv~=nilthen

returni,v

returnnil

returnripairs_it,t,max

localacc=first

fori,vinripairs(t)do

--Exittheloopwhentrue

localshould_break=false

acc,should_break=fn(acc,v,i,t)

ifshould_breakthen

starting_value=true

assert(

starting_value,

returnacc

Thenswapout reducefor reduce_rightintheplacesyouwanttouseit:

locallist={23,63,12,48,3}

returnreduce_right(list,function(acc,v,i,t)

returnv,true

returnacc

end,false)

returnval>1

Recursive

Sincewetalkedaboutrecursioninthelastsection,let'stryarecursiveimplementationof reduce.AlthoughwithLuathere'snopracticalreasontochoosearecursiveimplementationoverafor-looporwhile-loopimplementation,doingrecursionisfun.

localfunctionreduce(t,fn,acc,key)

--Checkforstartingvalue

ifkey==nilandacc==nilthen

key=next(t,key)

acc=t[key]

--Beginnextiteration.NextisaLuabuilt-infunction

--thatfetchesthenextkeyinatableafterthegivenkey.

3.05-Reduce

--See:https://www.lua.org/pil/7.3.html

key=next(t,key)

--Returnaccifwe'veiteratedallkeys

ifkey==nilthen

returnacc

localbreak_early=false

--Collectnewaccumulatorfrompredicatefunction

acc,break_early=fn(acc,t[key],key,t)

--Checktoseeifthepredicatewantstoendearly

ifbreak_earlythen

returnacc

--Recur

returnreduce(t,fn,acc,key,acc)

--Testitbygettingthetotalsumfromatablelikebefore

locallist={

monday=23,

tuesday=63,

wednesday=12,

thursday=48,

friday=3

localtotal_sum=reduce(list,function(acc,current_number,key)

print(key..':'..current_number)

returnacc+current_number

end,0)

print('totalsum:'..total_sum)

Thissupportsbreakingearlylikethetwopreviousimplementations.

ExercisesCreatea countfunctionthatcountsupthenumberofitemsinalistthatmatchthepredicateandreturnsthetotal.Itshouldworklikethis:

localcount=function(list,predicate_fn)

locallist={23,63,12,48,3}

--Printnumberofitemsevenlydivisibleby3(shouldreturn4)

print(count(list,function(v)

returnv%3==0

Gobacktothemapsectionin3.3andseeifyoucanreimplementthe mapfunctionontopof reduce.

3.05-Reduce

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