control flow: loop statementsldillon/cse-ctl/fall2018/meeting... · control flow: loop statements a...

ControlFlow:LoopStatementsAlooprepeatedlyexecutesasuiteofsub-statements,calledtheloopbody.Pythonprovidestwokindsofloopstatements:afor-loopandawhile-loop.ThisexercisegivesyoupracticewithloopsandwiththePythonTurtlemodule.Part(a)[For-loop]:Afor-loophastheform:

for v in collection: suite

wherevisaloopvariable;collectionisasequenceorsetofitems,calledacollection;andsuiteisanindentedsequenceofstatements,calledtheloopbody.Toexecuteafor-loop,theconsoleassignsvsuccessivevaluesfromcollectionandexecutessuitefollowingeachassignment,asshownontheright.Eachexecutionoftheloopbodyiscalledaniterationoftheloop.Theflowdiagramtotherightshowshowtheconsoleexecutesaforloopofthisform.Considertheprogramandtheflowdiagramforitbelow.Discusswithyourpartnerhowtheflowdiagramwasproducedfromtheprogram.

Arethere(more)itemsin

collection?

v=thenextitemincollection

suite

YES

NO

Nextstatement(a+erthefor-loop)

Priorstatement(beforethefor-loop)

Observethatwreferstoavalueoftypestrwhencontrol(execution)reachesline4ofthisprogram.InPython,astrisacollectionofcharacters(orstr’soflength1).Soline4willassigneachcharacterfromwtothevariablec,inorder,andexecuteline5(theloopbody)onsuccessiveiterations.Assumethattheuserenters"ctl"attheprompt(i.e.,thattheusertouchesthec-key,thenthet-key,thenthel-key,andthentheenter/return-key).Withyourpartner,‘execute’theprogramonpaper:Oneofyoukeepstrackofthestatementthatwillbeexecutednext,whiletheotherkeepstrackofthecurrentvalueofeachvariable.Whatoutputwilltheprogramproduceforthisuserinput?AFTERyouhavecompletedtheabove,checkyourunderstandingbysteppingthroughthevisualizationundertheVisualization1linkintheArtifactsectionoftoday’swebsite.Part(b)[Iteratingoverarange]:Pythonprovidesmanyfunctionsthatreturncollectionsforaprogramtoiterateover.Oneofthemostusefulistherangefunction,whichreturnseitheracollectionofintegersor,iftherearenointegersintherange,anemptycollection.Thesimplestformoftherangefunctiontakesasingleargument:1

range(stop):returnstherangeofintegersthatbeginswith0andcountsupto,butdoesnotinclude,stop

Workingwithapartner,downloadtriangles.pyintothefolderyoucreatedforthisweekandopenthefile(useeitherFile => Openoritsshortcut,⌘O).Thisprogramusestheturtlemodule.IfyouperformedtheHourofCodeexerciserecommendedintheCSE231Welcomemessage,youalreadyknowaboutthismodule.Ifyoudidnot,refertothe‘TurtleCheatSheet”asneeded.Lines7—13defineafunctioncalledpick_color,whichiscalledinline18torandomlychangethepencolor.We’lllearnaboutfunctionslater.Fornow,justusepick_colorlikeyouwouldanyotherfunctionthattakesnoarguments.

1Theothertwoformsoftherangefunctiontakeeithertwoorthreearguments:• range(start, stop):returnstherangethatbeginswithstartandcounts

upto,butdoesnotinclude,stop • range(start, stop, step):returnstherangethatbeginswithstart,

counts(upordown)bystep,andgoesto,butdoesnotinclude,stop

Runtheprogram.ItshouldcreateaPythonTurtleGraphicswindowanddrawa(randomly)coloredtriangleinthewindow.Ifyoudon’tseethewindow,lookforitunderotherwindowsonyourdisplay.Intheconsole,presstheenter/return-key,asinstructed.Runtheprograminthismannerseveraltimes.Withapartner,discusshowtheprogramworks,referringtothecheatsheetasnecessary.Checkyourunderstandingbyansweringthefollowing:• Whatlinesofcodemakeupthebodyofthefor-loop?

• Howmanytimesdoestheshellexecutethebodyofthefor-loop?

• Onwhatiterationofthefor-loopdoestheturtledrawthetriangle’sbase?…its

leftside?…itsrightside?

• Whatpurposedoesthelastinputexpression(line22)serve?(Ifyouareuncertain,tryrunningtheprogramwiththatlinecommentedout.)

Modifytheprogramtoprompttheuserforalengthinpixels(apostiveint)andthen:(1) drawafilledequilateraltrianglewithside-lengthequaltotheuser-provided

lengthandwiththebasecenteredontheorigin,and(2) displaywhatsideisbeingdrawnintheconsolejustbeforedrawingeachside.

Forexample(here,theuserenteredthe‘200’afterthefirstprompt):

Part(c)[While-loop]:Awhile-loopismoreversatilethanafor-loop.2Theformofabasicwhile-loopis: while cond: suite wherecondisaloopcondition(Booleanexpression)andsuiteisaloopbody(indentedsequenceofstatements).Toexecuteawhileloop,theconsoleevaluatescondand,ifcondisTrue,itrepeatedlyexecutessuiteuntilcondbecomesFalse,whenitterminatestheloop.3Theflowdiagramtotherightdepictsthecontrolflow.

Acommoninputpatternistorepeatedlyrequesta“legal”input,untiltheuserentersone.Youcanimplementthispatternusingawhileloop,asshownbythepseudo-codetotheright.Modifyyourtrianglesprogramsothatitpromptstheuserforapositivelengththatisnomorethan600,repeatedly,untiltheuserentersalegallength.Itshouldthendrawthetriangleasinpart(b),withthebaseonthex-axisandcenteredonthey-axis.Also,itshouldprintanerrormessageiftheuserentersanillegalinteger.Forexample:

(Youwillfixtheproblemofthetrianglebeing“cutoff”inalaterpartofthisexercise.)

2Everyfor-loopcanbereplacedwithawhile-loop;butsomeproblems—e.g.,theinputprobleminthispart—cannotbesolvedusingafor-loop.3Forsimplicity,wedescribethetypicalcase.Ingeneral,theloopconditioncanhaveanytype;Pythontreatsaloopconditionthatisnon-zeroornon-emptyasTrue,andaloopconditionthatiszerooremptyasFalse.

is

cond == True?

suite

YES

NO

Nextstatement(a+erthewhile-loop)

Priorstatement(beforethewhie-loop)

Part(d)[Problemsolving,nestedloops]:Modifyyourprogramsothatitdrawsa“row”ofasmanytrianglesascanfit,side-by-side,withinthecoordinates(-300,0)to(300,0).Centertherowoftriangleswithrespecttothey-axis.Selectarandomcolorforeachtriangle.Usethelengthenteredbyauser.Inthefollowingexample,youalsoseesomeoutputproducedbymyprogram,whichIusedfordebuggingpurposes:

Beforewritinganycode,workoutseveralexamplesonpaperusingsomeeasyinputlengths,e.g.,150,175.Howcanyoucalculatethemaximumnumberoftrianglesthatwillfit?Knowingthenumberoftriangles,howcanyoucalculatewheretostartdrawingeachtriangleinitsturn?Part(e)[Extraproblemsolvingforexperts]:Modifyyourprogramsothatitdrawsasmanyrowsoftrianglesascanfit,stackedoneontopoftheother,withinasquare600pixelswideand600pixelstallandcenteredattheorigin.Hint:Youcanusesometrigonometrytocalculatetheheightofthetriangles(arow).Then,youcanuseessentiallythesamelogicasinpart(d)tocalculatehowmanyrowswillfitandwheretostartdrawingeachrow.

Part(f)[Forgluttons,someexampracticeproblems]:Tracetheexecutionofthefollowingprogramassumingtheuserentersa319attheprompt.Traceaslongasneededtofigureoutwhatitdisplaysforthisinput.

Whatwillitdisplayiftheuserenters5003012attheprompt?Tracetheexecutionofthefollowingprogramassumingtheuserentersa5attheprompt.Traceaslongasneededtofigureoutwhatitdisplaysforthisinput.