WeekOne:IntroductionASHORTINTRODUCTIONTOHARDWARE,SOFTWARE,ANDALGORITHMDEVELOPMENT

Outline• Inthischapteryouwilllearn:• Aboutcomputer hardware,softwareandprogramming• Howtowriteandexecute yourfirstPythonprogram• Howtodiagnose andfixprogramming errors• Howtousepseudocode todescribe analgorithm

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OurFirstDefinitionAlgorithm:

• Analgorithm isastepbystepdescriptionofhowtosolveaproblem

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ComputerPrograms• Acomputerprogramtellsacomputerthesequenceofstepsneededtocompleteaspecifictask• Theprogramconsists ofaverylargenumber ofprimitive (simple)instructions

• Computerscancarryoutawiderangeoftasksbecausetheycanexecutedifferentprograms• Eachprogramisdesigned todirect thecomputertoworkonaspecific task

Programming:

• Theactofdesigning,implementing,andtestingcomputerprograms

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HardwareandSoftwareTHEBUILDINGBLOCKSTHATMAKEUPACOMPUTER

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Hardware• Hardware consistsofthephysicalelementsinacomputersystem.• Someveryvisible examples arethemonitor, themouse,external storage,andthekeyboard.

• Thecentralprocessingunit(CPU)performsprogramcontrolanddataprocessing

• Storagedevicesincludememory(RAM)andsecondarystorage• Harddisk• Flashdrives• CD/DVDdrives

• Input/outputdevicesallowtheusertointeractwiththecomputer• Mouse, keyboard,printer,screen…

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SimpleViewofaComputer’sComponents

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TheCPU• TheCPUhastwocomponents,thecontrolunit andthearithmeticlogicunit

• Thecontrolunitdirectsoperationoftheprocessor.• Allcomputerresources aremanaged bythecontrolunit.• Itcontrolscommunication andco-ordination between input/output devices.• Itreadsandinterprets instructions anddetermines the sequence forprocessing thedata.

• Itprovidestiming andcontrolsignals

• Thearithmeticlogicunit containsthecircuitrytoperformcalculationsanddocomparisons.• Itistheworkhorseportionofthecomputer andits jobistodopreciselywhatthecontrolunittells it todo.

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Storage• Therearetwotypesofstorage:• PrimaryStorage• SecondaryStorage

• Primarystorageiscomposedofmemorychips:electroniccircuitsthatcanstoredataaslongasitisprovidedelectricpower

• Secondarystorageprovidesaslower,lessexpensivestoragethatispersistent:thedatapersistswithoutelectricpower

• Computersstorebothdataandprograms• Thedataandprogramarelocated insecondary storageandloaded intomemorywhentheprogramisexecuted

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Memory• Asimplewaytoenvisionprimarymemoryisatableofcellsallthesamesize,onebyte,andeachcontainingauniqueaddressbeginningwith0.• The“typical” computer hasamainmemoryrangingfrom4gigabytes (GB),to32GB.

• Howbigisagigabyte?• Abyteis8bits.• Akilobyte,KB,is1024bytes,or“about1thousand bytes.”• Amegabyte,MB,is1,048,576bytes,or“about1million bytes.”• Agigabyte,GB,is1,073,741,824bytesor“about1billion bytes.”

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ExecutingaProgram• Programinstructionsanddata(suchastext,numbers,audio,orvideo)arestoredindigitalformat

• Whenaprogramisstarted,itisbroughtintomemory,wheretheCPUcanreadit.

• TheCPUrunstheprogramoneinstructionatatime.• Theprogrammayreacttouser input.

• Theinstructionsanduserinputguidetheprogramexecution• TheCPUreadsdata(including user input),modifies it,andwrites itbacktomemory,thescreen,orsecondary storage.

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Software• Software istypicallyrealizedasanapplicationprogram• MicrosoftWordisanexample ofsoftware• ComputerGames aresoftware• Operatingsystems anddevicedriversarealsosoftware

• Software• Software isasequence ofinstructions anddecisions implemented insomelanguage andtranslated toaformthatcanbeexecuted orrunonthecomputer.

• Computersexecuteverybasicinstructionsinrapidsuccession• Thebasic instructions canbegroupedtogether toperformcomplex tasks

• Programmingistheactofdesigningandimplementingcomputerprograms

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Introduction toAlgorithms• Ifyouwantacomputertoperformatask,youstartbywritinganalgorithm

• AnAlgorithm is:• asequence (theordermattering) ofactions totaketoaccomplish thegiventask

• Analgorithm is likearecipe; itisasetofinstructions written inasequencethatachieves agoal

• Forcomplexproblemssoftwaredeveloperswriteanalgorithmbeforetheyattempttowriteacomputerprogram

• ForthisclasswewillALWAYSwriteanalgorithmforeachproject

• Developingalgorithmsisafundamentalproblemsolvingskill• Ithasuses inmanyfields outside ofComputer Science

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Algorithm:FormalDefinitionAnalgorithmdescribesasequenceofstepsthatis:

1. Unambiguousa. No “assumptions” arerequired toexecute thealgorithmb. Thealgorithm uses precise instructions

2. Executablea. Thealgorithm canbecarriedout inpractice

3. Terminatinga. Thealgorithmwilleventually cometoanend,orhalt

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ProblemSolving:AlgorithmDesign• Algorithmsaresimplyplans• Detailed plansthatdescribe thestepstosolveaspecific problem

• Youalreadyknowquiteafew• Calculate theareaofacircle• Findthe length ofthehypotenuse ofatriangle

• Someproblemsaremorecomplexandrequiremoresteps• Calculate PIto100decimal places• Calculate thetrajectoryofamissile

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ASimpleExample

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• Asimplealgorithmtogetyourselfadrinkoforangejuice• Forsimplicity, thefollowingaretrue:• Youhaveacleanglass inthecabinet• Youhaveorange juiceinyourrefrigerator

• Soonevalidalgorithmis:1. getaglass fromyourcabinet2. gototherefrigeratorandgettheorangejuicecontainer3. opentheorange juicecontainer4. pourtheorange juicefromthecontainer intotheglass5. puttheorangejuicecontainer backintherefrigerator6. drinkyourjuice

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SecondExample:SelectingaCarProblem Statement:

• You have the choice of buying two cars.

• One is more fuel efficient than the other, but also more expensive.

• You know the price and fuel efficiency (in miles per gallon, mpg) of both cars.

• You plan to keep the car for ten years.

• Which car is the better deal?

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Developing theAlgorithmDeterminetheinputsandoutputs

Fromtheproblemstatementweknow:• Car1:Purchaseprice,FuelEfficiency• Car2:Purchaseprice,FuelEfficiency• Pricepergallon=$4.00• Annualmiles driven=15,000• Lengthoftime =10years

Foreachcarweneedtocalculate:• Annualfuelconsumed foreachcar• Annualfuelcostforeachcar• Operatingcostforeachcar• Total costofeachCar

• Thenweselectthecarwiththelowesttotalcost

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Translating theAlgorithmtopseudocode• Breakdowntheproblemintosmallertasks• ‘Calculate totalcost’ foreach car• Tocalculate thetotalcostforeachyearweneed tocalculate theoperating cost• Theoperating costdepends ontheannual fuelcost• Theannualfuelcost isthepricepergallon*theannual fuelconsumed• Theannualfuelconsumed istheannualmiles drive/fuelefficiency

• Describeeachsubtaskaspseudocode• totalcost=purchase price+operating cost

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ThePseudocodeForeachCar,computethetotalcost

Annualfuelconsumed =annualmiles driven/fuelefficiencyAnnualfuelcost=pricepergallon*annual fuelconsumedOperatingcost=Length oftime*annual fuelcostTotal cost=purchaseprice+operatingcost

Iftotalcost1<totalcost2Chose Car1

ElseChoose Car2

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BankAccountExample• ProblemStatement:• Youput$10,000intoabankaccountthatearns 5percent interest peryear.Howmanyyearsdoes ittakefortheaccountbalance tobedouble theoriginal?

• Howwouldyousolveit?• Manualmethod• Makeatable• Addlinesuntildone

• Useaspreadsheet!• Writeaformula• Perline,basedonlineabove

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Developthealgorithmsteps• Youput$10,000intoabankaccountthatearns 5percent interest peryear.Howmanyyearsdoes ittakefortheaccountbalance tobedouble theoriginal?

• Breakitintosteps• Startwithayearvalueof0andabalance of$10,000• Repeat thefollowingwhilethebalance is less than$20,000• Add1totheyearvalue• Multiply thebalance by1.05• (5%increase)

• Reportthefinalyearvalueastheanswer

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Translate topseudocode• Pseudocode• Half-waybetween natural language andaprogramming language

• ModifiedSteps• Settheyearvalueof0• Setthebalance to$10,000• While thebalance is lessthan$20,000• Add1totheyearvalue• Multiply thebalance by1.05

• Reportthefinalyearvalueastheanswer

• ThepseudocodeiseasilytranslatedintoPython

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ThePythonLanguage• Intheearly1990’s,GuidovanRossumdesignedwhatwouldbecomethePythonprogramminglanguage

• VanRossumwasdissatisfiedwiththelanguagesavailable• Theywereoptimized towrite largeprogramsthatexecuted quickly

• Heneededalanguagethatcouldnotonlybeusedtocreateprogramsquicklybutalsomakethemeasytomodify• Itwasdesigned tohaveamuchsimpler andcleaner syntaxthanotherpopular languages suchasJava,CandC++ (making iteasier tolearn)

• Python isinterpreted, making iteasier todevelop andtestshortprograms

• PythonprogramsareexecutedbythePythoninterpreter• The interpreter readsyourprogramandexecutes it

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ProgrammingEnvironments• Thereareseveralwaysofcreatingacomputerprogram• UsinganIntegratedDevelopment Environment (IDE)• Usingatexteditor

• Youshouldusethemethodyouaremostcomfortablewith

• WewillbeusingIDLE inthiscourse

• TheTextbookusestheWingIDE

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Organizeyourwork• Your‘sourcecode’ isstoredin.pyfiles

• Createafolderforthiscourse

• Createonefolderperprograminsidethecoursefolder• Aprogramcanconsist ofseveral .pyfiles

• BesureyouknowwhereyourIDEstoresyourfiles• Youneed tobeable tofindyoufiles

• Backupyourfiles:• ToaUSBflashdrive• Toanetworkdrive

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Python interactivemode• Likeotherlanguagesyoucanwrite/saveacompletePythonprograminafileandlettheinterpreterexecutetheinstructionsallatonce.

• Alternativelyyoucanruninstructionsoneatatimeusinginteractivemode.• Itallowsquick‘testprograms’ tobewritten.• Interactivemodeallows youtowritepythonstatements directly intheconsolewindow

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SourceCodetoaRunningProgram• Thecompilerreadsyourprogramandgeneratesbytecodeinstructions(simpleinstructionsforthePythonVirtualmachine)• ThePythonVirtualmachine isaprogramthat issimilar totheCPUofyourcomputer

• Anynecessary libraries (e.g.fordrawinggraphics)areautomatically locatedandincluded bythevirtualmachine

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BasicPythonSyntax:Print• UsingthePython‘print()’ function.• Afunction isacollection ofprogramming instructions thatcarryoutaparticular task(inthiscasetoprintavalueonscreen).

• It’scodethatsomebody elsewroteforyou!

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Syntax forPythonFunctions• Touse,orcall,afunctioninPythonyouneedtospecify:• Thenameofthefunction thatyouwanttouse(inthepreviousexample thenamewasprint)

• Anyvalues (arguments) needed bythefunction tocarryoutitstask(inthiscase,“HelloWorld!”).

• Arguments areenclosed inparentheses andmultiple arguments areseparatedwithcommas.

• Asequence ofcharactersenclosed inquotations marksarecalled astring

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MoreExamplesoftheprintFunction• Printingnumericalvalues• print(3 +4)• Evaluatestheexpression 3+4anddisplays7

• Passingmultiplevaluestothefunction• print(“the answer is”,6*7)• DisplaysTheansweris42• Eachvaluepassed tothefunction isdisplayed, oneafteranother, withablank

spaceaftereachvalue

• Bydefaulttheprintfunctionstartsanewlineafteritsargumentsareprinted• print(“Hello”)• print(“World!”)• Prints twolinesoftext• Hello• World!

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Errors• TherearetwoCategoriesofErrors:• Compile-timeErrors• akaSyntaxErrors• Spelling,capitalization,punctuation• Orderingofstatements,matchingofparenthesis,quotes…

• Noexecutableprogramiscreatedbythecompiler• Correctfirsterrorlisted,thencompileagain.• Repeatuntilallerrorsarefixed

• Run-timeErrors• akaLogicErrors• Theprogramruns,butproducesunintendedresults• Theprogrammay‘crash’

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SyntaxErrors• Syntaxerrorarecaughtbythecompiler

• Whathappensifyou• Miss-capitalize aword: Print("HelloWorld!")• Leaveoutquotes print(Hello World!)• Mismatch quotes print("Hello World!')• Don’tmatchbrackets print('Hello'

• TypeeachexampleaboveintheWingPythonShellwindow• Whaterrormessages aregenerated?

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LogicErrors• Whathappensifyou• Dividebyzero print(1/0)• Misspell output print("Hello,Word!")• Forgettooutput Remove line2

• Programswillcompileandrun• Theoutputmaynotbeasexpected

• TypeeachexampleaboveinthePythonShellwindow• Whaterrormessages aregenerated?

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Summary:ComputerBasics• Computersrapidlyexecuteverysimpleinstructions

• AProgram isasequenceofinstructionsanddecisions

• Programming istheart(andscience)ofdesigning,implementing,andtestingcomputerprograms

• TheCentralProcessingUnit(CPU)performsprogramcontrolanddataprocessing

• Storagedevicesincludememoryandsecondarystorage(e.g.,aUSBFlashDrive)

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Summary: Python• PythonwasdesignedinawaythatmakesiteasiertolearnthanotherprogramminglanguagessuchasJava,CandC++.

• ThedesignersgoalwastogivePythonsimplerandcleanersyntax.

• Setasidesometimetobecomefamiliarwiththeprogrammingenvironmentthatyouwilluseforyourclasswork.• Itisimportant topracticewiththetoolsoyoucanfocuson learning Python

• Aneditorisaprogramforenteringandmodifyingtext,suchasaPythonprogram.

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Summary: Python• Pythoniscasesensitive.• Youmustbecarefulaboutdistinguishing between upperandlowercaseletters.

• ThePythoncompilertranslatessourcecodeintobytecodeinstructionsthatareexecutedbytheVirtualmachine.

• Afunctioniscalledbyspecifyingthefunction’snameanditsparameters.

• Astringisasequenceofcharactersenclosedinquotationmarks.

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Summary: Errorsandpseudocode• Acompile-timeerrorisaviolationoftheprogramminglanguagerulesthatisdetectedbythecompiler.

• Arun-timeerrorcausesaprogramtotakeanactionthattheprogrammerdidnotintend.

• Pseudocodeisaninformaldescriptionofasequenceofstepsforsolvingaproblem.

• Analgorithmforsolvingaproblemisasequenceofstepsthatisunambiguous,executable,andterminating.

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