Chapter 4: Loops

Chapter Goals •  Toimplementwhileandforloops

•  Tohand-tracetheexecu6onofaprogram

•  Tobecomefamiliarwithcommonloopalgorithms

•  Tounderstandnestedloops

•  Toimplementprogramsthatreadandprocessdatasets

•  Touseacomputerforsimula6ons

Inthischapter,youwilllearnaboutloopstatementsinPython,aswellastechniquesforwri8ngprogramsthatsimulateac8vi8esinthereal

world.

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Contents •  Thewhileloop•  ProblemSolving:Hand-Tracing

•  Applica6on:ProcessingSen6nels•  ProblemSolving:Storyboards

•  CommonLoopAlgorithms

•  Theforloop•  Nestedloops

•  ProcessingStrings•  Applica6on:RandomNumbersandSimula6on

•  Graphics:DigitalImageProcessing

•  ProblemSolving:SolveaSimplerProblemFirst

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The while Loop

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The while Loop •  Examplesofloopapplica6ons

•  Calcula6ngcompoundinterest•  Simula6ons,eventdrivenprograms•  Drawing6les…

•  Compoundinterestalgorithm(Chapter1)

Steps

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Planning the while Loop balance=10.0

target=100.0

year=0

rate=0.025

whilebalance<TARGET:

year=year+1

interest=balance*RATE/100

balance=balance+interest

Aloopexecutesinstruc8onsrepeatedlywhileacondi8onisTrue.

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Syntax: while Statement

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Count-Controlled Loops •  Awhileloopthatiscontrolledbyacountercounter=1#Initializethecounter

whilecounter<=10:#Checkthecounterprint(counter)counter=counter+1#Updatetheloopvariable

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Event-Controlled Loops •  Awhileloopthatiscontrolledbyaneventbalance=INITIAL_BALANCE#Initializetheloopvariable

whilebalance<=TARGET:#Checktheloopvariableyear–year+1balance=balance*2#Updatetheloopvariable

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Execution of the Loop

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Execution of the Loop (2)

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Doubleinv.py

Declareandini6alizeavariableoutsideofthelooptocountyear

Incrementtheyearvariableeach6methrough

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while Loop Examples

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while Loop Examples (2)

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Common Error: Incorrect Test Condition

•  Theloopbodywillonlyexecuteifthetestcondi6onisTrue.•  Ifbalisini6alizedaslessthantheTARGETandshouldgrowun6litreachesTARGET•  Whichversionwillexecutetheloopbody?

whilebal<TARGET:year=year+1interest=bal*RATEbal=bal+interest

whilebal>=TARGET:year=year+1interest=bal*RATEbal=bal+interest

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Common Error: InfiniteLoops •  Theloopbodywillexecuteun6lthetestcondi6onbecomesFalse.

•  Whatifyouforgettoupdatethetestvariable?•  balisthetestvariable(TARGETdoesn’tchange)•  Youwillloopforever!(orun6lyoustoptheprogram)

whilebal<TARGET:year=year+1interest=bal*RATEbal=bal+interest

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Common Error: Off-by-OneErrors •  A‘counter’variableisoaenusedinthetestcondi6on•  Yourcountercanstartat0or1,butprogrammersoaenstartacounterat0

•  IfIwanttopaintall5fingersononehand,whenIamdone?•  Ifyoustartat0,use“<“ Ifyoustartat1,use“<=“•  0,1,2,3,4 1,2,3,4,5

finger=0FINGERS=5whilefinger<FINGERS:#paintfingerfinger=finger+1

finger=1FINGERS=5whilefinger<=FINGERS:#paintfingerfinger=finger+1

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Hand Tracing Loops

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Hand-Tracing Loops •  Example:Calculatethesumofdigits(1+7+2+9)

•  Makecolumnsforkeyvariables(n,total,digit)•  Examinethecodeandnumberthesteps•  Setvariablestostatebeforeloopbegins

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Tracing Sum of Digits

•  Startexecu6ngloopbodystatementschangingvariablevaluesonanewline

•  Crossoutvaluesinpreviousline

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Tracing Sum of Digits

•  Con6nueexecu6ngloopstatementschangingvariables

•  1729/10leaves172(noremainder)

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Tracing Sum of Digits •  Testcondi6on.IfTrue,executeloopagain

•  Variablenis172,Is172>0?,True!

•  Makeanewlineforthesecond6methroughandupdatevariables

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Tracing Sum of Digits •  Third6methrough

•  Variablenis17whichiss6llgreaterthan0

•  Executeloopstatementsandupdatevariables

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Tracing Sum of Digits •  Fourthloopitera6on:

•  Variablenis1atstartofloop.1>0?True•  Executesloopandchangesvariablento0(1/10=0)

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Tracing Sum of Digits •  Becausenis0,theexpression(n>0)isFalse•  Loopbodyisnotexecuted

•  Jumpstonextstatementaaertheloopbody

•  Finallyprintsthesum!

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Summary of the while Loop •  whileloopsareverycommon

•  Ini6alizevariablesbeforeyoutest•  Thecondi6onistestedBEFOREtheloopbody

•  Thisiscalledpre-test•  Thecondi6onoaenusesacountervariable

•  Somethinginsidetheloopshouldchangeoneofthevariablesusedinthetest

•  Watchoutforinfiniteloops!

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Investment Example •  Openthefile:

•  Doubleinv.py

•  Runtheprogramwithseveraltestcases•  Theprogramwillpromptyouforarate•  Enteramixofvalidandinvalidrates

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Sentinel Values

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Processing Sentinel Values •  Sen6nelvaluesareoaenused:

•  Whenyoudon’tknowhowmanyitemsareinalist,usea‘special’characterorvaluetosignalthe“last”item

•  Fornumericinputofposi6venumbers,itiscommontousethevalue-1

Asen8nelvaluedenotestheendofadataset,butitisnotpartofthedata.

salary=0.0whilesalary>=0:salary=float(input())ifsalary>=0.0:total=total+salarycount=count+1

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Averaging a Set of Values •  Declareandini6alizea‘total’variableto0•  Declareandini6alizea‘count’variableto0

•  Declareandini6alizea‘salary’variableto0•  Promptuserwithinstruc6ons

•  Loopun6lsen6nelvalueisentered•  Saveenteredvaluetoinputvariable(‘salary’)•  Ifsalaryisnot-1orless(sen6nelvalue)

•  Addsalaryvariabletototalvariable•  Add1tocountvariable

•  Makesureyouhaveatleastoneentrybeforeyoudivide!•  Dividetotalbycountandoutput.•  Done!

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Sentinel.py (1)

Outside the while loop: declare and initialize variables to use

Input new salary and compare to sentinel

Update running total and count (to calculate the average later)

Since salary is initialized to 0, the while loop statements will execute at least once

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Sentinel.py (2)

Prevent divide by 0

Calculate and output the average salary using the totaland count variables

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Sentinel Example •  Openthefile:

•  Sen6nal.py

•  No6cetheuseoftheIF()testinsidethewhileloop•  TheIF()checkstomakesurewearenotprocessingthesen6nelvalue

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Priming Read •  Someprogrammersdon’tlikethe“trick”ofini6alizingtheinputvariablewithavalueotherthanasen6nel.

#Setsalarytoavaluetoensurethattheloop#executesatleastonce.salary=0.0whilesalary>=0:

salary=float(input("Enterasalaryor-1tofinish:"))whilesalary>=0:

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•  Analterna6veistochangethevariablewithareadbeforetheloop.

Modification Read •  Theinputopera6onatthebonomoftheloopisusedtoobtainthenextinput.

#Primingreadsalary=float(input("Enterasalaryor-1tofinish:"))whilesalary>=0.0:total=total+salarycount=count+1#Modificationreadsalary=float(input("Enterasalaryor-1tofinish:"))

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Boolean Variables and Sentinels •  Abooleanvariablecanbeusedtocontrolaloop

•  Some6mescalleda‘flag’variable

done=Falsewhilenotdone:value=float(input("Enterasalaryor-1to

finish:"))ifvalue<0.0:done=Trueelse:#Processvalue

Initialize done so that the loop will execute

Set done ‘flag’ to True if sentinel value is found

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Storyboards

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Storyboards •  Oneusefulproblemsolvingtechniqueistheuseofstoryboardstomodeluserinterac6on.Itcanhelpanswer:•  Whatinforma6ondoestheuserprovide,andinwhichorder?•  Whatinforma6onwillyourprogramdisplay,andinwhichformat?•  Whatshouldhappenwhenthereisanerror?•  Whendoestheprogramquit?

•  Astoryboardconsistsofannotatedsketchesforeachstepinanac8onsequence.

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Storyboard Example •  Goal:Conver6ngasequenceofvalues

•  Willrequirealoopandsomevariables•  Handleoneconversioneach6methroughtheloop

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What Can Go Wrong? Unknownunittypes

•  Whatistheusermisspellscen6metersandinches?•  Whatotherconversionsareavailable?•  Solu6on:

•  Showalistoftheacceptableunittypes

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What Else Can Go Wrong? •  Howdoestheuserquittheprogram?

•  Storyboardshelpyouplanaprogram•  Knowingtheflowhelpsyoustructureyourcode

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