Introduction to Programming I - Hacettepebbm101/lectures/lec3-python... · Lecture Overview •Programming languages (PLs) •Introduction to Python and Programming •Control Flow

Lecture#03– IntroductiontoPythonandProgramming,ControlFlow

Erkut Erdem,AykutErdem&AydınKaya//Fall2017

BBM101IntroductiontoProgrammingI

MontyPythonandtheHolyGrail(1975)

Lasttime… Howtobuildcomputers

2

Memory

Control ALU PC

inst1inst2inst3..instN

A Simple HMMM Program

triangle1.hmmm: Calculate the approximate area of a triangle.

0 read r1 # Get base b

1 read r2 # Get height h

2 mul r1 r1 r2 # b times h into r1

3 setn r2 2

4 div r1 r1 r2 # Divide by 2

5 write r1

6 halt

$ python hmmmAssembler.py -f triangle1.hmmm -o triangle1.b

----------------------

| ASSEMBLY SUCCESSFUL |

----------------------

0 : 0000 0001 0000 0001 0 read r1 # Get base b

1 : 0000 0010 0000 0001 1 read r2 # Get height h

2 : 1000 0001 0001 0010 2 mul r1 r1 r2 # b times h into r1

3 : 0001 0010 0000 0010 3 setn r2 2

4 : 1001 0001 0001 0010 4 div r1 r1 r2 # Divide by 2

5 : 0000 0001 0000 0010 5 write r1

6 : 0000 0000 0000 0000 6 halt

$ python hmmmSimulator.py -f triangle1.b -n

4

5

10

4 / 14

A Simple HMMM Program

triangle1.hmmm: Calculate the approximate area of a triangle.

0 read r1 # Get base b

1 read r2 # Get height h

2 mul r1 r1 r2 # b times h into r1

3 setn r2 2

4 div r1 r1 r2 # Divide by 2

5 write r1

6 halt

$ python hmmmAssembler.py -f triangle1.hmmm -o triangle1.b

----------------------

| ASSEMBLY SUCCESSFUL |

----------------------

0 : 0000 0001 0000 0001 0 read r1 # Get base b

1 : 0000 0010 0000 0001 1 read r2 # Get height h

2 : 1000 0001 0001 0010 2 mul r1 r1 r2 # b times h into r1

3 : 0001 0010 0000 0010 3 setn r2 2

4 : 1001 0001 0001 0010 4 div r1 r1 r2 # Divide by 2

5 : 0000 0001 0000 0010 5 write r1

6 : 0000 0000 0000 0000 6 halt

$ python hmmmSimulator.py -f triangle1.b -n

4

5

10

4 / 14

poweroutput

input

0.0V0.5V

2.8V3.3V

0 1 0

Boolean Algebra and Functions

Any function of boolean variables, no matter how complex, can be expressed in termsof AND, OR, and NOT

Consider the proposition “if you score over 93% in this course, then you will get an A”

The truth values for the above proposition is given by the “implication” function(x =) y) having the following truth table

x y x =) y

0 0 10 1 11 0 01 1 1

The function can be compactly written as NOT x OR x AND y (or x̄+ xy)

10 / 21

TheHarveyMudd MiniatureMachine(HMMM)

LectureOverview• Programminglanguages(PLs)

• IntroductiontoPythonandProgramming

• ControlFlow

3

Disclaimer:Muchofthematerialandslidesforthislecturewereborrowedfrom—E.Grimson,J.Guttag andC.Terman MIT6.0001class—RuthAnderson,MichaelErnstandBillHowe’sCSE140class—SwamiIyer’s Umass BostonCS110class



• ControlFlow

4

ProgrammingLanguages

• Syntaxandsemantics

• DimensionsofaPL

• Programmingparadigms

ProgrammingLanguages• Anartificiallanguagedesignedtoexpresscomputationsthatcanbeperformedbyamachine,particularlyacomputer.

• Canbeusedtocreateprogramsthatcontrolthebehaviorofamachine,toexpressalgorithmsprecisely,orasamodeofhumancommunication.

• e.g.,C,C++,Java,Python,Prolog,Haskell,Scala,etc..

CreatingComputerPrograms• Eachprogramminglanguageprovidesasetofprimitiveoperations.

• Eachprogramminglanguageprovidesmechanismsforcombiningprimitivestoformmorecomplex,butlegal,expressions.

• Eachprogramminglanguageprovidesmechanismsfordeducingmeaningsorvaluesassociatedwithcomputationsorexpressions.

AspectsofLanguages• Primitiveconstructs– Programming language– numbers,strings,simpleoperators

– English– words

• Syntax– whichstringsofcharactersandsymbolsarewell-formed– Programminglanguage–we’llgettospecificsshortly,butfor example3.2+3.2isavalidCexpression

– English– “catdog boy”is notsyntacticallyvalid, asnotinformofacceptablesentence

AspectsofLanguages• Staticsemantics– whichsyntacticallyvalidstringshave a meaning

– English– “Iarebig”hasform<noun><intransitiveverb><noun>,sosyntacticallyvalid,butisnotvalidEnglishbecause“I”issingular,“are”isplural

– Programminglanguage– forexample,<literal><operator><literal>isavalidsyntacticform,but2.3/’abc’isastaticsemanticerror

AspectsofLanguages• Semantics– whatisthemeaningassociatedwithasyntacticallycorrectstringofsymbolswithnostaticsemanticerrors

– English– canbeambiguous• “Theysawthemanwiththetelescope.”

– Programming languages – alwayshasexactlyonemeaning• Butmeaning(orvalue)maynotbewhatprogrammerintended

WhereCanThingsGoWrong?• Syntacticerrors

– Commonbuteasilycaughtbycomputer

• Staticsemanticerrors– Somelanguagescheckcarefullybeforerunning,otherscheck

whileinterpretingtheprogram– Ifnotcaught,behaviorofprogramisunpredictable

• Programsdon’thavesemanticerrors,butmeaningmaynotbewhatwasintended– Crashes(stopsrunning)– Runsforever– Producesananswer,butnotprogrammer’s intent

OurGoal• Learnthesyntaxandsemanticsofaprogramminglanguage

• Learnhowtousethoseelementstotranslate“recipes”forsolvingaproblemintoaform thatthecomputercanusetodo theworkforus

• Computationalmodesofthoughtenableustouseasuiteofmethodstosolveproblems

DimensionsofaProgrammingLanguageLow-levelvs.High-level

• Distinctionaccordingtothelevelofabstraction

• Inlow-levelprogramminglanguages(e.g.Assembly),thesetofinstructionsusedincomputationsareverysimple(nearlyatmachinelevel)

• Ahigh-levelprogramminglanguage(e.g.Python,C,Java)hasamuchricherandmorecomplexsetofprimitives.

• Distinctionaccordingtotherangeofapplications

• Inageneralprogramminglanguage,thesetofprimitivessupportabroadrangeofapplications.

• Atargetedprogramminglanguage aimsataveryspecificsetofapplications.– e.g.,MATLAB(matrixlaboratory)isaprogramminglanguagespecificallydesignedfornumericalcomputing(matrixandvectoroperations)

DimensionsofaProgrammingLanguageGeneralvs.Targeted

• Distinctionaccordingtohowthesourcecodeisexecuted

• Ininterpretedlanguages(e.g.LISP),thesourcecodeisexecuteddirectlyatruntime(bytheinterpreter).– Interpretercontroltheflowoftheprogrambygoingthrougheachoneoftheinstructions.

• Incompiledlanguages(e.g.C),thesourcecodefirstneedstobetranslatedintoanobjectcode(bythecompiler)beforetheexecution.

DimensionsofaProgrammingLanguageInterpretedvs.Compiled

ProgrammingLanguageParadigms• Functional

• Treatscomputationastheevaluationofmathematicalfunctions(e.g.Lisp,Scheme,Haskell,etc.)

• Imperative• Describescomputationintermsofstatementsthatchangeaprogramstate(e.g.FORTRAN,BASIC,Pascal,C,etc.)

• Logical(declarative)• Expressesthelogicofacomputationwithoutdescribingitscontrolflow(e.g.Prolog)

• Objectoriented• Uses"objects"– datastructuresconsistingofdatafieldsandmethodstogetherwiththeirinteractions– todesignapplicationsandcomputerprograms(e.g.C++,Java,C#,Python,etc.)

ProgrammingLanguageParadigms• Functional

• Treatscomputationastheevaluationofmathematicalfunctions(e.g.Lisp,Scheme,Haskell,etc.)

• Imperative• Describescomputationintermsofstatementsthatchangeaprogramstate(e.g.FORTRAN,BASIC,Pascal,C,etc.)

• Logical(declarative)• Expressesthelogicofacomputationwithoutdescribingitscontrolflow(e.g.Prolog)

• Objectoriented• Uses"objects"– datastructuresconsistingofdatafieldsandmethodstogetherwiththeirinteractions– todesignapplicationsandcomputerprograms(e.g.C++,Java,C#,Python,etc.)



• ControlFlow

18

ProgramminginPython• Ourprogrammingenvironment– Pythonprogramminglanguage

– PyCharm,anintegrateddevelopmentenvironment(IDE)

– Terminal

19

ProgramminginPython• ToprograminPython– Composeaprogrambytypingitintoafilenamed,say,helloworld.py

– Run(orexecute)theprogrambytypingpython helloworld.py intheterminalwindow

20

Editor(PyCharm)

compiler/interpreter(python)

helloworld.py Hello, World

InputandOutput• Bird’s-eyeviewofaPythonprogram

– Inputtypes:command-linearguments,standardinput,fileinput

– Outputtypes:standardoutput,fileoutput,graphicaloutput,audiooutput

21

my_program.pyinput output

InputandOutput• Command-lineargumentsaretheinputswelistafteraprogram

namewhenweruntheprogram

$ python my_program.py arg_1 arg_2 ... arg_n

• Thecommand-lineargumentscanbeaccessedwithinaprogram,suchasmy_program.py above,viathearray(akalist)sys.argv1assys.argv[1], sys.argv[2], . . . , sys.argv[n]

• Thenameoftheprogram(my_program.py)isstoredinsys.argv[0]

22

1ThesysmoduleprovidesaccesstovariablesandfunctionsthatinteractwiththePythoninterpreter

InputandOutputimport sys

print('Hi, ', end='')print(sys.argv[1], end='')print(’. How are you?')

23

$ python useargument.py Alice Hi, Alice. How are you? $ python useargument.py Bob Hi, Bob. How are you? $ python useargument.py CarolHi, Carol. How are you?

useargument.py

1.Pythonislikeacalculator 2.Avariableisacontainer

4.Aprogramisarecipe3.Differenttypescannotbecompared

24

1.PythonisLikeaCalculator

25

YouTypeExpressions.PythonComputesTheirValues.• 5• 3+4• 44/2• 2**3• 3*4+5*6• (72– 32)/9*5

26

Pythonhasanaturalandwell-definedsetofprecedencerulesthatfullyspecifytheorderinwhichtheoperatorsareappliedinanexpression• Forarithmeticoperations,multiplication

anddivisionareperformedbeforeadditionandsubtraction

• Whenarithmeticoperationshavethesameprecedence,theyareleftassociative,withtheexceptionoftheexponentiationoperator**,whichisrightassociative

• Wecanuseparenthesestooverrideprecedencerules

AnExpressionisEvaluatedFromtheInsideOut

• HowmanyexpressionsareinthisPythoncode?

(72– 32)/9.0*5

anexpression values

(72 – 32)/9.0 *5(40)/9.0 *540 /9.0 *54.44 *522.2

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AnotherEvaluationExample

(72 – 32)/(9.0 *5)(40)/(9.0 *5)40 /(9.0 *5)40 /(45.0)40 /45.0.888

28

2.AVariableisaContainer

29

Avariableisanameassociatedwithadata-typevalue

VariablesHoldValues

• Recallvariablesfromalgebra:– Letx=2…– Lety=x…

• Toassignavariable,use“varname =expression”pi = 3.14pivar = 6*10**2322 = x #Error!

• Notallvariablenamesarepermitted!

Nooutputfromanassignmentstatement

30

• Variablenamesmustonlybeoneword(asinnospaces)

• Variablenamesmustbemadeupofonlyletters,numbers,andunderscore(_)

• Variablenamescannotbeginwithanumber

ChangingExistingVariables(“re-binding”or“re-assigning”)

x = 2xy =yx = 5xy

• “=”inanassignmentisnot apromiseofeternalequality– Thisisdifferent thanthemathematicalmeaningof“=”

• Evaluatinganexpressiongivesanew(copyofa)number,ratherthanchanginganexistingone

2x

31

HowanAssignmentisExecuted

1. Evaluatetheright-handsidetoavalue2. Storethatvalueinthevariable

x = 2print(x)y = xprint(y)z = x + 1print(z)x = 5print(x)print(y)print(z)

Stateofthecomputer: Printedoutput:

223523

x:2y:2z:3

x:5

Tovisualizeaprogram’sexecution:http://pythontutor.com

32

MoreExpressions:Conditionals(valueisTrue orFalse)22 > 4 #condition,orconditional22 < 4 #condition,orconditional22 == 4 …x = 100 #Assignment,not conditional!22 = 4 #Error!x >= 5x >= 100x >= 200not Truenot (x >= 200)3<4 and 5<64<3 or 5<6temp = 72water_is_liquid = (temp > 32 and temp < 212)

Numericoperators:+,*,**Booleanoperators:not,and,orMixedoperators:<,>=,==

33

MoreExpressions:strings• Astringrepresentstext

– 'Python'– myString = "BBM 101-Introduction to Programming"– ""

• Emptystringisnotthesameasanunboundvariable– "" and‘’arethesame

• Wecanspecifytab,newline,backslash,andsinglequotecharactersusingescapesequences’\t’,’\n’,’\\’,and’\’’,respectively

Operations:• Length:

– len(myString)

• Concatenation:– "Hacettepe" + " " + ' University'

• Containment/searching:– 'a' in myString– "a" in myString

34

Stringsruler1 = '1'ruler2 = ruler1 + ' 2 ' + ruler1 ruler3 = ruler2 + ' 3 ' + ruler2 ruler4 = ruler3 + ' 4 ' + ruler3 print(ruler1)print(ruler2)print(ruler3)print(ruler4)

35

11211213121121312141213121

3.DifferentTypescannotbeCompared

36

anInt = 2aString = "Hacettepe" anInt == aString # Error

TypesofValues

• Integers(int):-22,0,44– Arithmeticisexact– Somefunnyrepresentations:12345678901L

• Realnumbers(float,for“floatingpoint”):2.718,3.1415– Arithmeticisapproximate,e.g.,6.022*10**23

• Strings(str):"I love Python"," "

• Truthvalues(bool,for“Boolean”):True,False

GeorgeBoole37

OperationsBehavedifferentlyonDifferentTypes3.0 + 4.03 + 43 + 4.0"3" + "4" #Concatenation3 + "4" #Error3 + True #Error

Moral:Pythononlysometimes tellsyouwhenyoudosomethingthatdoesnotmakesense.

38

OperationsonDifferentTypes

15.0 / 4.0 3.75 3.7515 / 4 3.75 315.0 / 4 3.75 3.7515 / 4.0 3.75 3.75

15.0 // 4.0 3.015 // 4 315.0 // 4 3.015 // 4.0 3.0

39

BeforePythonversion3.5,operandusedtodeterminethetypeofdivision.

Python3.5 Python2.x

/:Division//:IntegerDivision

TypeConversion

float(15) 15.0int(15.0) 15int(15.5) 15int("15") 15str(15.5) 15.5float(15) / 4 3.75

40

AProgramisaRecipe

41

DesigntheAlgorithmBeforeCoding• Weshouldthink(designthealgorithm)beforecoding

• Algorithmicthinkingisthelogic.Also,calledproblemsolving

• Codingisthesyntax

• Makethisahabit

• Somestudentsdonotfollowthispracticeandtheygetchallengedinalltheircoursesandcareers!

42

WhatisaProgram?• Aprogramisasequenceofinstructions

• Thecomputerexecutesoneaftertheother,asiftheyhadbeentypedtotheinterpreter

• Savingyourworkasaprogramisbetterthanre-typingfromscratch

43

x = 1y = 2x + yprint(x + y)print("The sum of", x, "and", y, "is", x+y)

Theprint() Statement

• Theprint statementalwaysprintsoneline– Thenextprintstatementprintsbelowthatone

• Write0ormoreexpressionsafterprint,separatedbycommas– Intheoutput,thevaluesareseparatedbyspaces

• Examples:x = 1y = 2print(3.1415)print(2.718, 1.618)print()print(20 + 2, 7 * 3, 4 * 5)print("The sum of", x, end="")print(" and", y, "is", x+y)

44

3.14152.718 1.618

22 21 20The sum of 1 and 2 is 3

Toavoidnewline

Exercise:ConvertTemperatures• Makeatemperatureconversionchartasthefollowing

• FahrenheittoCentigrade,forFahrenheitvaluesof:-40,0,32,68,98.6,212

• C=(F- 32)× 5/9

• Output:Fahrenheit Centigrade-40 -40.00 -17.777832 0.068 20.098.6 37.0212 100.0

• YouhavecreatedaPythonprogram!

• (Itdoesn’thavetobethistedious,anditwon’tbe.)

45

Expressions,Statements,andPrograms• Anexpressionevaluatestoavalue

3 + 4pi * r**2

• Astatementcausesaneffectpi = 3.14159print(pi)

• Expressionsappearwithinotherexpressionsandwithinstatements(fahr – 32) * (5.0 / 9)print(pi * r**2)

• Astatementmaynot appearwithinanexpression3 + print(pi) #Error!

• Aprogramismadeupofstatements– Aprogramshoulddosomethingorcommunicateinformation

46

1.Pythonislikeacalculator 2.Avariableisacontainer

4.Aprogramisarecipe3.Differenttypescannotbecompared

47

ProgrammingLanguages• Aprogramminglanguageisa“language”towriteprogramsin,

suchasPython,C,C++,Java

• Theconceptofprogramminglanguagesarequitesimilar

• Python:

• Java:

• Pythonissimpler!That’swhywearelearningitfirstJ

48

print("Hello, World!")

public static void main(String[] args) {System.out.println("Hello, World!");

}

EvolutionofProgrammingLanguages

49

The2017TopProgramming Languages

• https://spectrum.ieee.org/computing/software/the-2017-top-programming-languages 50

51

http://carlche

o.com/startcoding



• ControlFlow

52

Repeatingyourself

Makingdecisions

53

TemperatureConversionChartRecalltheexercisefromthepreviouslecture

fahr = 30cent = (fahr -32)/9.0*5print(fahr, cent)fahr = 40cent = (fahr -32)/9.0*5print(fahr, cent)fahr = 50cent = (fahr -32)/9.0*5print(fahr, cent)fahr = 60cent = (fahr -32)/9.0*5print(fahr, cent)fahr = 70cent = (fahr -32)/9.0*5print(fahr, cent)Print("All done")

54

Output:30 -1.1140 4.4450 10.060 15.5570 21.11All done

TemperatureConversionChartAbetterwaytorepeatyourself:

55

for f in [30,40,50,60,70]:print(f, (f-32)/9.0*5)

print("All done")

Loopbodyisindented

Alist

Indentationissignificant

for loop

Executethebody5times:• oncewithf=30• oncewithf=40• oncewithf=50• oncewithf=60• oncewithf=70

loopvariableoriterationvariable

Output:30 -1.1140 4.4450 10.060 15.5570 21.11All done

Colonisrequired

i = 1print(i)i = 4 print(i)i = 9print(i)

HowaLoopisExecuted:TransformationApproach

for i in [1,4,9]:print(i)

Stateofthecomputer: Printedoutput:

149

i:1i:4i:9

Idea:convertafor loopintosomethingweknowhowtoexecute

1. Evaluatethesequenceexpression2. Writeanassignmenttotheloop

variable,foreachsequenceelement

3. Writeacopyoftheloopaftereachassignment

4. Executetheresultingstatements

56

for i in [1,4,9]:print(i)

HowaLoopisExecuted:DirectApproach

Printedoutput:

149

i:1i:4i:9

CurrentlocationinlistStateofthecomputer:

1. Evaluatethesequenceexpression2. Whiletherearesequence

elementsleft:a) Assigntheloopvariabletothenext

remainingsequenceelementb) Executetheloopbody

57

TheBodycanbeMultipleStatements

Executewholebody,thenexecutewholebodyagain,etc.

for i in [3,4,5]:print("Start body")print(i)print(i*i)

Convention:oftenusei orj asloopvariableifvaluesareintegersThisisanexceptiontotherulethatvariablenamesshouldbedescriptive

Output:Startbody39Startbody416Startbody525

NOT:StartbodyStartbodyStartbody34591625

loopbody:3statements

58

IndentationinLoopisSignificant

• Everystatementinthebodymusthaveexactlythesameindentation• That’showPythonknowswherethebodyendsfor i in [3,4,5]:

print("Start body")print(i)

print(i*i)

• Comparetheresultsoftheseloops:for f in [30,40,50,60,70]:

print(f, (f-32)/9.0*5)print("All done")

for f in [30,40,50,60,70]:print(f, (f-32)/9.0*5)print("All done")

Error!

59

TheBodycanbeMultipleStatements

Howmanystatementsdoesthisloopcontain?

for i in [0,1]:print("Outer", i)for j in [2,3]:print(" Inner", j)print(" Sum", i+j)

print("Outer", i)

Whatistheoutput?

Output:Outer0Inner2Sum2Inner3Sum3

Outer0Outer1Inner2Sum3Inner3Sum4

Outer1

loopbody:3statements“nested”

loopbody:2statements

60

Keyidea:1. Assigneachsequenceelementtotheloopvariable2. Duplicatethebody

UnderstandLoopsThroughtheTransformationApproach

for i in [0,1]:print("Outer", i)for j in [2,3]:print(" Inner", j)

i = 0print("Outer", i)for j in [2,3]:print(" Inner", j)

i = 1print("Outer", i)for j in [2,3]:print(" Inner", j)

i = 0print("Outer", i)j = 2print(" Inner", j)j = 3print(" Inner", j)i = 1print("Outer", i)for j in [2,3]:print(" Inner", j)

61

FixThisLoop

# Goal: print 1, 2, 3, …, 48, 49, 50for tens_digit in [0, 1, 2, 3, 4]:for ones_digit in [1, 2, 3, 4, 5, 6, 7, 8, 9]:print(tens_digit * 10 + ones_digit)

Whatdoesitactuallyprint?Howcanwechangeittocorrectitsoutput?

Moral:Watchoutforedgeconditions (beginningorendofloop)

62

SomeFixes# Goal: print 1, 2, 3, …, 48, 49, 50

for tens_digit in [0, 1, 2, 3, 4]:for ones_digit in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:print(tens_digit * 10 + ones_digit + 1)

for tens_digit in [0, 1, 2, 3, 4]:for ones_digit in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:print(tens_digit * 10 + ones_digit)

for tens_digit in [1, 2, 3, 4]:for ones_digit in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:print(tens_digit * 10 + ones_digit)

print 50

• Analyzeeachoftheabove63

Reusingloopvariable(don’tdothis!)

TestYourUnderstandingofLoopsPuzzle1:

for i in [0,1]:print(i)

print(i)

Puzzle2:i = 5for i in []:

print(i)

Puzzle3:for i in [0,1]:

print("Outer", i)for i in [2,3]:

print(" Inner", i)print("Outer", i)

innerloopbody

outerloopbody

Outer0Inner2Inner3Outer3Outer1Inner2Inner3Outer3

011

Output:

(nooutput)

64

TheRangeFunction

Asanimplicitlist:for i in range(5):

… body …

range(5) =[0,1,2,3,4]

range(1,5) =[1,2,3,4]

range(1,10,2) =[1,3,5,7,9]

Thelist[0,1,2,3,4]

Upperlimit(exclusive)

Lowerlimit(inclusive)

step(distancebetweenelements)

65

DecomposingaListComputation

• Tocomputeavalueforalist:– Computeapartialresultforallbutthelastelement– Combinethepartialresultwiththelastelement

Example:sumofalist:

[3,1,4,1,5,9,2,6,5]

Listb

Lista

sum(Lista)=sum(Listb)+5sum(Listb)=sum(Listc)+6…sum(Listy)=sum(Listz)+3sum(emptylist)=0

Listc

Listy

Listz

66

HowtoProcessaList:OneElementataTime• Acommonpatternwhenprocessingalist:

result = initial_valuefor element in list:result = updated result

use result

• initial_value isacorrectresultforanemptylist

• Aseachelementisprocessed,result isacorrectresultforaprefixofthelist

• Whenallelementshavebeenprocessed,result isacorrectresultforthewholelist

# Sum of a listresult = 0for element in mylist:result = result + element

print result

67

SomeLoops# Sum of a list of values, what values?result = 0for element in range(5): # [0,1,2,3,4]

result = result + elementprint("The sum is: " + str(result))

# Sum of a list of values, what values?result = 0for element in range(5,1,-1):

result = result + elementprint("The sum is:", result)

# Sum of a list of values, what values?result = 0for element in range(0,8,2):

result = result + elementprint("The sum is:", result)

# Sum of a list of values, what values?result = 0size = 5for element in range(size):

result = result + elementprint("When size = " + str(size) + ", the result is " + str(result))

68

Thesumis:10

5,4,3,2Thesumis:14

0,2,4,6Thesumis:12

0,1,2,3,4Whensize=5,theresultis10

divisorpattern.py: Acceptintegercommand-lineargumentn.Writetostandardoutputann-by-n tablewithanasteriskinrowi andcolumnjifeitheri dividesjorjdividesi.import sys

n = int(sys.argv[1])for i in range(1, n + 1):

for j in range(1, n + 1):if (i % j == 0) or (j % i == 0):

print(‘* ‘, end=‘‘) else:

print(‘ ‘, end=‘‘) print(i)

69

Nesting

divisorpattern.py: Accept integer command-line argument n. Write to standard output an n-by-n table with an asterisk in row iand column j if either i divides j or j divides i.

import stdioimport sys

n = int(sys.argv [1])for i in range(1, n + 1):

for j in range(1, n + 1):if (i % j == 0) or (j % i == 0):

stdio.write(’* ’)else:

stdio.write(’ ’)stdio.writeln(i)

$ python divisorpattern.py 3* * * 1* * 2* * 3

$ python divisorpattern.py 10* * * * * * * * * * 1* * * * * * 2* * * * 3* * * * 4* * * 5* * * * 6* * 7* * * * 8* * * 9* * * * 10

Variable trace (n = 3)

i j output----------------1 1 ’* ’1 2 ’* ’1 3 ’* 1\n’2 1 ’* ’2 2 ’* ’2 3 ’ 2\n’3 1 ’* ’3 2 ’ ’3 3 ’* 3\n’

12 / 18

ExamplesofListProcessing

• Productofalist:result = 1for element in mylist:result = result * element

• Maximumofalist:result = mylist[0]for element in mylist:result = max(result, element)

• Approximatethevalue3by1+2/3+4/9+8/27+16/81+…=(2/3)0 +(2/3)1 +(2/3)2 +(2/3)3 +…+(2/3)10result = 0for element in range(11):result = result + (2.0/3.0)**element

result = initial_valuefor element in list:result = updated result

Thefirstelementofthelist(countingfromzero)

70

ExercisewithLoops• Writeasimpleprogramtoaddvaluesbetweentwogiveninputsa,b

• e.g.,ifa=5,b=9,itreturnssumof(5+6+7+8+9)• Hint:wedidsome‘algorithmicthinking’and‘problemsolving’here!

71

a=5b=9total = 0for x in range(a, b+1):

total += xprint(total)

AnotherTypeofLoops• Thewhile loopisusedforrepeatedexecutionaslongasanexpressionistrue

72

n = 100s = 0counter = 1while counter <= n:

s = s + countercounter += 1

print("Sum of 1 until %d: %d" % (n,s))

Sum of 1 until 100: 5050

MakingDecisions• Howdowecomputeabsolutevalue?

abs(5) = 5

abs(0) = 0

abs(-22) = 22

73

AbsoluteValueSolutionIf thevalueisnegative,negateit.Otherwise,usetheoriginalvalue.

val = -10

# calculate absolute value of valif val < 0:

result = - valelse:

result = val

print(result)

val = -10

if val < 0:print(- val)

else:print(val)

74

Inthisexample,result willalwaysbeassignedavalue.

Anotherapproachthatdoesthesamethingwithoutusingresult:

AbsoluteValueSolutionAswithloops,asequenceofstatementscouldbeusedinplaceofasinglestatementinsideanifstatement:

val = -10


result = - valprint("val is negative!")print("I had to do extra work!")

else:result = valprint("val is positive")

print(result)75

AbsoluteValueSolutionWhathappenshere?

val = 5


result = - valprint("val is negative!")

else:for i in range(val):

print("val is positive!")result = val

print(result)

76

AnotherifItisnotrequiredthatanythinghappens…

val = -10

if val < 0:print("negative value!")

77

Whathappenswhenval =5?

Executiongetshereonlyif“height>100”isfalseAND“height>50”istrue

TheifBodycanbeAnyStatements

# height is in kmif height > 100:print("space")

else:if height > 50:print("mesosphere")

else:if height > 20:print("stratosphere")

else:print("troposphere")

Writtendifferently,butmoreefficient!# height is in kmif height > 100:print("space")

elif height > 50:print("mesosphere")

elif height > 20:print("stratosphere")


Writtendifferently!# height is in kmif height > 50:if height > 100:print("space")

else:print("mesosphere")



thenclause

elseclause

t

f t

f

0 10 20 30 40 50 60 70 80 90 100

troposphere stratosphere mesosphere spacekmaboveearth

Executiongetshereonlyif“height>100”isfalse

78

Executiongetshereonlyif“height<=100”istrueAND“height>50”istrue

Version1





thenclause

elseclause

t

e t

e

Executiongetshereonlyif“height<=100”istrue

0 10 20 30 40 50 60 70 80 90 100


Version1





0 10 20 30 40 50 60 70 80 90 100


Version2

if height > 50:if height > 100:print("space")

else:print("mesosphere")



0 10 20 30 40 50 60 70 80 90 100


Version3if height > 100:print("space")




ONEoftheprintstatementsisguaranteedtoexecute:whicheverconditionitencountersfirstthatistrue

0 10 20 30 40 50 60 70 80 90 100


OrderMatters# version 3if height > 100:print("space")




# broken version 3if height > 20:print("stratosphere")


elif height > 100:print("space")


Tryheight=72onbothversions,whathappens?

0 10 20 30 40 50 60 70 80 90 100


Version3# incomplete version 3if height > 100:print("space")



Inthiscaseitispossiblethatnothingisprintedatall,when?

0 10 20 30 40 50 60 70 80 90 100


WhatHappensHere?# height is in kmif height > 100:print("space")

if height > 50:print("mesosphere")

if height > 20:print("stratosphere")


Tryheight=72

0 10 20 30 40 50 60 70 80 90 100


ThethenClauseor theelseClauseisExecuted

speed = 65limit = 70if speed <= limit:

print("Good job, safe driver!")else:

print("You owe $", speed/fine)

86Whatifwechangespeedto50?

Thebreak Statement• Thebreak statementterminatesthecurrentloopandresumesexecutionatthenextstatement

87

for letter in 'hollywood':if letter == 'l':

breakprint ('Current Letter :', letter)

Current Letter : hCurrent Letter : o

Thecontinue Statement• Thecontinue statementinPythonreturnsthecontroltothebeginningofthewhileloop.

88

for letter in 'hollywood':if letter == 'l':

continueprint ('Current Letter :', letter)

Current Letter : hCurrent Letter : oCurrent Letter : yCurrent Letter : wCurrent Letter : oCurrent Letter : oCurrent Letter : d

Introduction to Programming I - Hacettepebbm101/lectures/lec3-python... · Lecture Overview •Programming languages (PLs) •Introduction to Python and Programming •Control Flow

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