Common Loop Algorithms

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Common Loop Algorithms 1.  SumandAverageValue

2.  Coun4ngMatches

3.  Promp4ngun4laMatchIsFound

4. MaximumandMinimum

5.  ComparingAdjacentValues

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Sum Example •  SumofValues

•  Ini4alizetotalto0•  Usewhileloopwithsen4nel

total=0.0inputStr=input("Entervalue:")whileinputStr!="":value=float(inputStr)total=total+valueinputStr=input("Entervalue:")

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Average Example total=0.0count=0inputStr=input("Entervalue:")whileinputStr!="":value=float(inputStr)total=total+valuecount=count+1inputStr=input("Entervalue:")ifcount>0:average=total/countelse:average=0.0

AverageofValues

•  Firsttotalthevalues•  Ini4alizecountto0

•  Incrementperinput

•  Checkforcount0

•  Beforedivide!

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Counting Matches (e.g., Negative Numbers)

negatives=0inputStr=input("Entervalue:")whileinputStr!="“:value=int(inputStr)ifvalue<0:negatives=negatives+1inputStr=input("Entervalue:")print("Therewere",negatives,"negativevalues.")

•  Coun4ngMatches

•  Ini4alizenega4vesto0•  Useawhileloop•  Addtonega4vespermatch

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Prompt Until a Match is Found •  Ini4alizebooleanflagtoFalse•  Testsen4nelinwhileloop

•  Getinput,andcomparetorange•  Ifinputisinrange,changeflagtoTrue•  Loopwillstopexecu4ng

valid=Falsewhilenotvalid:value=int(input("Pleaseenterapositivevalue<100:"))ifvalue>0andvalue<100:valid=Trueelse:print("Invalidinput.")

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Thisisanexcellentwaytovalidateuseprovidedinputs

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Maximum •  Getfirstinputvalue

•  Bydefini4on,thisisthelargestthatyouhaveseensofar

•  Loopwhileyouhaveavalidnumber(non-sen4nel)•  Getanotherinputvalue•  Comparenewinputtolargest(orsmallest)•  Updatelargestifnecessary

largest=int(input("Enteravalue:"))inputStr=input("Enteravalue:")whileinputStr!=””:value=int(inputStr)ifvalue>largest:largest=valueinputStr=input("Enteravalue:")

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Minimum •  Getfirstinputvalue

•  Thisisthesmallestthatyouhaveseensofar!

•  Loopwhileyouhaveavalidnumber(non-sen4nel)•  Getanotherinputvalue•  Comparenewinputtolargest(orsmallest)•  Updatesmallestifnecessary

smallest=int(input("Enteravalue:"))inputStr=input("Enteravalue:")whileinputStr!="“:value=int(inputStr)ifvalue<smallest:smallest=valueinputStr=input("Enteravalue:")

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Comparing Adjacent Values •  Getfirstinputvalue•  Usewhiletodetermineiftherearemoretocheck

•  Copyinputtopreviousvariable•  Getnextvalueintoinputvariable•  Compareinputtoprevious,andoutputifsame

value=int(input("Enteravalue:"))inputStr=input("Enteravalue:")whileinputStr!="“:previous=valuevalue=int(inputStr)ifvalue==previous:print("Duplicateinput")inputStr=input("Enteravalue:")

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Grades Example •  Openthefile:

•  Grades.py

•  Lookcarefullyatthesourcecode.•  Themaximumpossiblescoreisreadasuserinput

•  Thereisalooptovalidatetheinput

•  Thepassinggradeiscomputedas60%oftheavailablepoints

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The for Loop

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The for Loop •  Usesofaforloop:

•  Theforloopcanbeusedtoiterateoverthecontentsofanycontainer.

•  Acontainerisanobject(Likeastring)thatcontainsorstoresacollec4onofelements

•  Astringisacontainerthatstoresthecollec4onofcharactersinthestring

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while loop -> for loop

stateName="Virginia"i=0whilei<len(stateName):letter=stateName[i]print(letter)i=i+1

whileversion

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while loop -> for loop

stateName="Virginia"i=0whilei<len(stateName):letter=stateName[i]print(letter)i=i+1

whileversion

stateName="Virginia"forletterinstateName:print(letter)

forversion

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while loop -> for loop

stateName="Virginia"i=0whilei<len(stateName):letter=stateName[i]print(letter)i=i+1

whileversion

stateName="Virginia"forletterinstateName:print(letter)

forversion

•  Noteanimportantdifferencebetweenthewhileloopandtheforloop.•  Inthewhileloop,theindexvariableiisassigned0,1,andsoon.•  Intheforloop,theelementvariableisassignedstateName[0],stateName[1],andsoon.

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The for Loop (2) •  Usesofaforloop:

•  Aforloopcanalsobeusedasacount-controlledloopthatiteratesoverarangeofintegervalues.

i=1whilei<10:print(i)i=i+1

foriinrange(1,10):print(i)

while version

for version

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Syntax of a for Statement (Container) •  Usingaforlooptoiterateoverthecontentsofacontainer,anelementata4me.

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Syntax of a for Statement (Range) •  Youcanuseaforloopasacount-controlledlooptoiterateoverarangeofintegervalues

•  Weusetherangefunc4onforgenera4ngasequenceofintegersthatlessthantheargumentthatcanbeusedwiththeforloop

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Planning a for Loop •  Printthebalanceattheendofeachyearforanumberofyears

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Good Examples of for Loops •  Keeptheloopssimple!

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Investment Example

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Programming Tip •  Findingthecorrectlowerandupperboundsforaloopcanbeconfusing.•  Shouldyoustartat0orat1?•  Shouldyouuse<=bor<basatermina4oncondi4on?

•  Coun4ngiseasierforloopswithasymmetricbounds.•  Thefollowingloopsareexecutedb-a4mes.

inti=awhilei<b:...i=i+1

foriinrange(a,b):...

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Programming Tip •  Theloopwithsymmetricbounds(“<=”,isexecutedb-a+14mes.

•  That“+1”isthesourceofmanyprogrammingerrors.

i=awhilei<=b:...i=i+1

#Forthisversionoftheloopthe‘+1’isverynoticeable!foryearinrange(1,numYears+1):

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Summary of the for Loop •  forloopsareverypowerful•  Theforloopcanbeusedtoiterateoverthecontentsofanycontainer,whichisanobjectthatcontainsorstoresacollec4onofelements•  astringisacontainerthatstoresthecollec4onofcharactersinthestring.

•  Aforloopcanalsobeusedasacount-controlledloopthatiteratesoverarangeofintegervalues.

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Steps to Writing a Loop •  Planning:

•  Decidewhatworktodoinsidetheloop•  Specifytheloopcondi4on•  Determinelooptype•  Setupvariablesbeforethefirstloop•  Processresultswhentheloopisfinished•  Tracetheloopwithtypicalexamples

•  Coding:•  ImplementtheloopinPython

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A Special Form of the print Function

•  Pythonprovidesaspecialformoftheprintfunc4onthatdoesnotstartanewlineajertheargumentsaredisplayed

•  Thisisusedwhenwewanttoprintitemsonthesamelineusingmul4pleprintstatements

•  Forexamplethetwostatements:

print(“00”,end=””)print(3+4)

•  Producetheoutput:007

•  Includingend=“”asthelastargumenttotheprintfunc4onprintsanemptystringajerthearguments,insteadonanewline

•  Theoutputofthenextprintfunc4onstartsonthesameline

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Nested Loops

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Loops Inside of Loops •  InChapterThreewelearnedhowtonestifstatementstoallowustomakecomplexdecisions•  Rememberthattonesttheifstatementsweneedtoindentthecodeblock

•  Complexproblemssome4mesrequireanestedloop,oneloopnestedinsideanotherloop•  Thenestedloopwillbeindentedinsidethecodeblockofthefirstloop

•  Agoodexampleofusingnestedloopsiswhenyouareprocessingcellsinatable•  Theouterloopiteratesoveralloftherowsinthetable•  Theinnerloopprocessesthecolumnsinthecurrentrow

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Our Example Problem Statement •  PrintaTableHeaderthatcontainsx1,x2,x3,andx4•  PrintaTablewithfourcolumnsandtenrowsthatcontainthepowersofx1,x2,x3,andx4forx=1to10

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Applying Nested Loops •  Howwouldyouprintatablewithrowsandcolumns?

•  Printtopline(header)•  Useaforloop

•  Printtablebody…•  Howmanyrowsareinthetable?•  Howmanycolumnsinthetable?

•  Loopperrow•  Looppercolumn

•  Inourexamplethereare:•  Fourcolumnsinthetable•  Tenrowsinthetable

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Pseudocode to Print the Table Printthetableheader

forxfrom1to10printanewtablerowprintanewline

•  Howdoweprintatablerow?

Fornfrom1to4printxn

•  Wehavetoplacethisloopinsidetheprecedingloop•  Theinnerloopis“nested”insidetheouterloop

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Pseudocode to Print the Table Printthetableheader:

forxfrom1to10fornfrom1to4printXn

printanewline

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n !

x!

InnerLoop

Flowchart of a Nested Loop x=1

x<=10? n=1

n<=4? Printxn

n=n+1

Printnewline

x=x+1

True

False True

Done

False

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Powertable.py

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Bodyofouterloop,x=1!10

Bodyofinnerloop,n=1!4

The end=“” suppresses the new line, so the numbers are all printed on the same line

The Results

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First Exercise •  Opentheprogram:

•  powertable.py

•  Runtheprogramandreviewtheresults

•  Makethefollowingchanges:•  ChangethevalueofNMAXto6andruntheprogram•  Whatchangesinthetable?•  ChangethevalueofNMAXbackto4•  ChangethevalueofXMAXto4•  Whatchangesinthetable?

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Nested Loop Examples

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Hand Tracing the Loop

•  iwillhavethevalues:•  0,1,2,3–Sowewillhavefourlinesofstars

•  jwillhavethevalues•  0-Sowewillhaveonestar•  0,1-Sowewillhavetwostars•  0,1,2-Sowewillhavethreestars•  0,1,2,3-Sowewillhavefourstars

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Nested Loop Examples (2)

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Second Problem Statement •  Printthefollowingpapernofbrackets:[][][][][][][][][][][][]

•  Thepapernconsistsof:•  Threerows•  Eachrowhasfourpairsofbrackets

•  Whatdoweknow?•  Weneedtwonestedloops

•  Thefirstloop(theouterloop)willprinteachofthethreerows•  Thesecondloop(theinnerloop)willprintthefourpairsofbrackets

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Pseudocode Code, Results Fori=1to3Forj=1to4Print“[]”

Printanewline

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Exam Averages Problem Statement •  Itiscommontorepeatedlyreadandprocessmul4plegroupsofvalues:

•  Writeaprogramthatcancomputetheaverageexamgradeformul4plestudents.

•  Eachstudenthasthesamenumberofexamgrades•  Prompttheuserforthenumberofexams•  Whenyoufinishastudentprompttheusertoseeiftherearemorestudentstoprocess

•  Whatdoweknow?

•  Whatdoweneedtocompute?

•  Whatisouralgorithm/approach?

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Step One: Understand the Problem •  Tocomputetheaveragegradeforastudent,wemustreadandtallyallofthegradesforthatstudent•  Wecanusealooptodothis.(wehaveworkingcodetodothispor8on)

•  Weneedtocomputegradesformul4plestudents•  ThatimpliesasetofnestedLoops

•  Theouterloopprocesseseachstudent•  Theinnerloopprocessthestudent’sgrades

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Step Two •  Computethegradeforonestudent

•  Setupthevariablesandloop

•  Weknowhowmanygradestoprocess,sowecanuseacount-controlledloop

totalscore=0Foriinrange(1,numberofexams+1):ReadthenextexamscoreAddtheexamscoretothetotalscore

ComputetheexamaveragePrinttheexamaverage

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Step Three •  Repeattheprocessforeachstudent•  Sincewedon’tknowhowmanystudentsthereare,wewilluseawhileloopwithasen4nelvalue•  Forsimplicitywewilluse“Y”asthesen4nelvalue

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Step Four: Translate to Python

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Exam Averages Example •  Openthefile:

•  examaverages.py

•  No4cethatthesecondloop(theforloop)isnestedinsidethewhileloop•  InWingyoushouldseealine(theindentguide)connec4ngtheforlooponline17downtothestatementonline21•  Thelineisshowingyouthestatementsthatareincludedintheforloop

•  Ifyoudon’tseetheindentguide:•  Clickontheedittab•  Select“Preferences…”•  UnderEditor,selectInden4on•  Clickthe“ShowIndentGuides”box•  ClicktheApplybupon•  ClicktheOkayBupon

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Turning the Indent Guides On

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Application: Random Numbers and Simulations

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Random Numbers/Simulations •  Gamesojenuserandomnumberstomakethingsinteres4ng

•  RollingDice•  Spinningawheel•  Pickacard

•  Asimula4onusuallyinvolvesloopingthroughasequenceofevents•  Days•  Events

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Generating Random Numbers •  ThePythonlibraryhasarandomnumbergeneratorthatproducesnumbersthatappeartoberandom•  Thenumbersarenotcompletelyrandom.Thenumbersaredrawnfromasequenceofnumbersthatdoesnotrepeatforalong4me

•  random()returnsanumberthatis>=0and<1

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Simulating Die Tosses •  Goal:

•  Togeneratearandomintegerinagivenrangeweusetherandint()func4on

•  Randinthastwoparameters,therange(inclusive)ofnumbersgenerated

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