Lecture content - IDATDDA69/lectures/2016/01_introduction.pdf · Closure in Python Python has limited support for lamdas: single statement: numbers.map(lambda v: v * 2) But support

TDDA69DataandProgramStructureIntroductionCyrilleBerger

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LecturecontentCourseIntroductionIntroductiontothedifferentProgrammingParadigmThedifferentprogrammingparadigmsWhydifferentparadigms?IntroductiontoFunctionalProgrammingExpressionsFunctionsControlRecursionAdvantagesandinconvenientsoffunctionalprogramming

CourseIntroduction

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CoursegoalsDescribeaspectsofevaluationandexecutionindifferentlanguagemodelsExplainanddemonstratehowdesignchoicesaffecttheexpressivenessandefficacyofaprogramminglanguageAnalyzeandvalueprogramminglanguagesbasedontheirevaluationandcompilationstrategiesImplementprogramminglanguagesintheformofaninterpreterandacompiler

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Whydoyouneedtoknowhowprogramareinterpreted?

ItwillhelpyouunderstandwhyprogramminglanguageworksacertainwayandwhatarethelimitsNewprogramminglanguagesandinterpretersareconstantlybeingdevelopedExistinginterpretersareconstantlybeingdevelopedtoimproveperformance,security,addnewfeatures...

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ProgramminglanguagesGeneralpurposes:C,C++,Java,Python...Specialpurposes:Prolog,Matlab,R,Agent0...Scripting:JavaScript,VBA...Historical:Fortran,Lisp...

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Evolutionofprogramminglanguages

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Howisaprograminterpreted?Sourcecode Parser

Parser

AbstractSyntaxTree Treevisitor

Generator Sourcecode ...

Bytecode VirtualMachine

Assembler Assembly OperatingSystem CPU

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Facebook'sworkonthePHPintepreter

FacebookstartedwithPHPin2004Backatthetime,PHPwasthegoldstandardforwebsiteprogrammingandprototypingButthisiscausingproblemsandforpracticalreasonstheycannotchangeprogramminglanguage

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WhydidFacebookneedtodeveloptheirownPHPinterpreter?

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WhatdidFacebookdo?ThestandardPHPinterpreterisusingavirtualmachine(Zend)TheydevelopedatooltoconvertPHPtoC++ThentheydevelopedanewinterpreterthatdoJust-In-Time(JIT)compilation,calledHHVMTheyintroducedHack,avariantofPHPwithatypingsystem

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Andotherexamples...GooglewithJava,Dalvik,PythonwithCPythonvsQt'sJavaScript,switchingfromASTInterpretationtoJITandtoamixofJITandASTInterpretation...

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Listoflectures1IntroductionandFunctionalProgramming2ImperativeProgrammingandDataStructures3Parsing4Evaluation5ObjectOrientedProgrammingandtypessystem6Macrosand7VirtualMachinesandBytecode8GarbageCollectionandNativeCode9ConcurrentComputing10DeclarativeProgramming11Logic12Summary

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Book(s)StructureandInterpretationofComputerProgramsinPythonbyHalAbelson,JerrySussman,JulieSussmanandJohnDeneroStructureandInterpretationofComputerProgramsbyHalAbelson,JerrySussmanandJulieSussman

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Listoflabs1FunctionalProgramming2Supportingclassforaninterpreter3ECMAScriptInterpreter4Macros5Bytecode6Garbagecollector7SQpy

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Divisionoftime24hlectures(in12sessions)40hlabs(in20sessions)8htutorials(in488hhomework

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Lastyearevaluationandimprovments

Toomuchboilerplatting,LabsandlecturesarenotwellconnectedTwonewlabsQuizzesarenicebutteacherwaitfortoolongIntroduceatimeoutTeacherisshy,slidesaremostlyusedbytheteachertoknowwhattosayIntroducemoreinterractivityTheexamisworthmorethantwocredits

IntroductiontothedifferentProgrammingParadigm

Thedifferentprogrammingparadigms

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ProgrammingParadigm(2/2)declarative

functional

Imperative

Object-Oriented

LogicSymbolic

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DeclarativeExpresseslogicofcomputationwithoutcontrolflow:Whatshouldbecomputedandnothowitshouldbecomputed.Examples:XML/HTML,antlr4/yacc/regularexpressions,make/ants,SQL,...

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Declarative-Examples<b>Helloworld!</b>SELECTnameFROMstudentWHEREcourseeq'TDDA69'grammarHello;r:'hello'ID;ID:[a-z]+;WS:[''\t\r\n]+->skip;

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FunctionalComputationaretreatedasmathematicalfunctionwithoutchanginganyinternalstateExamples:Lisp,Scheme,

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Functional-Examples(print"HelloWorld")(take25(squares-of->(1491625364964...576625)

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ImperativeExpresshowcomputationareexecutedDescribescomputationintermofstatementsthatchangetheinternalstateExamples:C/C++,Pascal,Java,Python,JavaScript...

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Imperative-Examplesfor(vari=1;i<26;++i){varsq=i*i;console.log(sq)}#include<stdio.h>intmain(){charch;printf("Enteracharacter\n");scanf("%c",&ch);if(ch=='a'||ch=='A'||ch=='e'||ch=='E'||ch=='i'||ch=='I'||ch=='o'||ch=='O'||ch=='u'||ch=='U')printf("%cisavowel.\n",ch);elseprintf("%cisnotavowel.\n",ch);return0;}

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Object-OrientedBasedontheconceptofobjects,whicharedatastructurescontainingfieldsandmethodsProgramsaredesignedbymakingobjectsinteractwitheachothersExamples:C++,Java,C#,Python,Ruby,JavaScript...

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Object-Oriented-Programming#include<iostream>classCharacter:publicSymbol{public:Character(char_c):m_c(_c){}boolisVowel()const{returnch=='a'||ch=='A'||ch=='e'||ch=='E'||ch=='i'||ch=='I'||ch=='o'||ch=='O'||ch=='u'||ch=='U';}private:charm_c;};intmain(){charc;std::cout<<"Enteracharacter:\n";std::cin>>c;Characterch(c);if(ch.isVowel()){std::cout<<c<<"isavowel.\n";}else{std::cout<<c<<"isnotavowel.\n";}}

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OthersparadigmlanguagesLogicBasedonFormallogic:expressingfactsandrulesSymbolicAprogramcanmanipulateitsownformulasandcomponentsasiftheyaredataExample:prolog

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Logicprogramminglikes(mary,food).likes(mary,wine).likes(john,wine).likes(john,mary).|?-likes(mary,food).yes.|?-likes(john,wine).yes.|?-likes(john,food).no.

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Symbolicprogrammingd(X,X,1):-!./*d(X)w.r.t.Xis1*/d(C,X,0):-atomic(C)./*IfCisaconstant*//*thend(C)/dXis0*/d(U+V,X,R):-/*d(U+V)/dX=A+Bwhere*/d(U,X,A),/*A=d(U)/dXand*/d(V,X,B),R=A+B....d(sin(W),X,Z*cos(W)):-/*d(sin(W))/dX=Z*cos(W)*/d(W,X,Z)./*whereZ=d(W)/dX*/d(exp(W),X,Z*exp(W)):-/*d(exp(W))/dX=Z*exp(W)*/d(W,X,Z)./*whereZ=d(W)/dX*/...?-d(cos(2*X+1),X,what=2*sin(2*X

Whydifferentparadigms?

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Canyoudoeverythinginimperativeprogramming?

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Isthereaparadigmtorulethemall?

IntheoryyoucanprogrameverythinginC/C++andimperativeprogramming,orfunctionalprogramming...ButisthatAndisthat

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FunctionalvsImperativeDoubleallthenumbersinanarrayvarnumbers=Imperative:vardoubled=[]for(vari=0;i<numbers.length;i++){varnewNumber=numbers[i]*2doubled.push(newNumber)}Functional:vardoubled=numbers.map(function(n){returnn*2})

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DeclarativevsImperativeSelectallthedogsthatbelongstoaspecificDeclarative:SELECT*fromdogsINNERJOINownersWHEREdogs.owner_id=owners.idImperative:vardogsWithOwners=[]vardog,ownerfor(vardogindogs){for(varownerinowners){if(owner&&dog.owner_id==owner.id){dogsWithOwners.push({dog:dog,owner:owner})}}}

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FunctionalvsImperativeImperativelanguage(C/C++,Java...)BasicconstructsareimperativestatementsChangeexistingvalues,statesx=x+y=while(x>0)...FunctionalBasicconstructsaredeclarativeDeclarenewvaluesfunctionf(x){returnx+1;ComputationsareprimarilydonebyevaluatingexpressionsPureifallconstructsaredeclarative

IntroductiontoFunctionalProgramming Expressions

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Expressions(1/2)Primitiveexpressionstocapturethesimplestelementswewanttodescribenumbers,arithmeticexpressions...2 plus 'hello'

number operator stringCallexpressions:max ( 2 , 3 )

operator operand operand42

Expressions(2/2)Meansofcombiningsimpleelementsintocoumpoundonesmax(min(pow(3,5),-4),min(1,-2)*2)+6

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NamesMeansofabstractingelementsbynamingandmanipulatingthem2 plus 'hello'

namea plus b

name name namemax ( 2 , 3 )name

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AssignmentBindsnamestoa := 2

Nowahasthevalue2a plus 2

evaluatesto4

Functions

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Whatisafunction?Assignmentisasimplemeansofabstraction:bindsnamestovaluesFunctiondefinitionisamorepowerfulmeansofabstraction:bindsnamestoexpressions

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FunctiondefinitionAfunctiondefinitioncontains:AsignaturewhichdefineshowmanyargumentsafunctiontakesAbodywhichdefinesthecomputationperformedwhenthefunctioniscalledfunction <name> ( parameterslist )

return <returnexpression> ;

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Pureandnon-purefunctionPurefunctions:justreturnvalues:Math.abs(-2)->Math.pow(2,100)->1267650600228229401496703205376Non-purefunctions:havesideeffects:print(-2)->Butprint'-2'intheAsideeffectisnotavalue,itisanythingthathappensasaconsequenceofcallinga

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Whatisalambda?www.govote.at956729start

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ClosureandlambdaAlambdaisafunctionwithnonamevardoubled=numbers.map(function(n){returnn*2})AclosureisafunctionwithcapturedvariablesThisseemsimple,butthisisactuallyratherpowerful!functioncreate_function_multiplication(number){returnfunction(x){returnx*number;}}vardoubled=numbers.map(create_function_multiplication(2))

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ClosureinPythonPythonhaslimitedsupportforlamdas:singlestatement:numbers.map(lambdav:v*2)Butsupportnestedfunctions(andclosure):defcreate_function_multiplication(number):deffunction_multiplication(x):returnx*numberreturnfunction_multiplicationdoubled=numbers.map(create_function_multiplication(2))

Control

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Controlflow(1/2)Inimperativeprogramming,acontrolflowstatementexecutionresultinachoicebetweentwopathsExemple:if,while...Infunctionalprogramming,itisdoneusingaspecialfunction.Forinstance,inLisp:(COND(condition1result1)(condition2result2)...(TresultN))

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Controlflow(2/2)Thefunctionalwayinimperativelanguages:InJavaScript,C++,Java,Ruby...:condition?result1:result2;InPython:conditionifresult1elseresult2

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Whataboutloops?LoopconstructsisimperativeHowwouldyouimplementtheequivalentofaloopinfunctional?functionfactorial(n){varr=1;for(vari=2;i<=n;++i){r*=i;}returnr;}

Recursion

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Whatisrecursion?Afunctioniscalledrecursiveifthebodyofthatfunctioncallsitself,eitherdirectlyorindirectly.

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Factorial:theclassicalexample(1/2)

Factorialinfactorial::Integral->Integralfactorial0=1factorialn=n*factorial(n-1)FactorialinCommonLISP:(define(factorialn)(cond((=n0)1)(t(*n(factorial(-n1))))))

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Factorial:theclassicalexample(1/2)

Withaloop:functionfactorial(n){varr=1;for(vari=2;i<=n;++i){r*=i;}returnr;}Witharecursivefunctionfactorial(n){return(n===0)?1:n*factorial(n-1)}

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Recursionvsloopswhile(expression){do_something();}functionloop_something(args...){if(expression)return;else{do_something();loop_something(args...);}}(define(loop_somethingargs...)(cond(expression)value)(t(do_something)(loop_somethingargs...))

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Whentouserecursionratherthaniteration?(1/4)

deffactorial(n):if(n==0):return1else:returnn*factorial(n-1)Letstry:factorial(10)factorial(1000)

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Whentouserecursionratherthaniteration?(2/4)

Inmostprogramminglanguage,thenumberoffunctioncallislimitedbythesizeofthestacksys.getrecursionlimit()sys.setrecursionlimit(1003)factorial(1000)Tail-calloptimisation

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Whentouserecursionratherthaniteration?(3/4)

Callingafunctionisusuallymoreexpensivethanaloopdeffactorial2(n):r=1foriinrange(1,n+1):r=r*ireturnrtimeit.timeit("factorial(30)","from__main__importfactorial")timeit.timeit("factorial2(30)","from__main__importfactorial2")

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Whentouserecursionratherthaniteration?(4/4)

ItisamatterofRecursionisabitmoregeneralthanloopsWhenwalkingthroughatree

Advantagesandinconvenientsoffunctionalprogramming

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Noside-effectspurefunctionalInpurefunctional,callingafunctiononlyreturnavalueTheimplicationisthatcallingafunctionwiththesameargumentswillalwaysreturnthesamevalueIsthewithdrawfunctionpure(definebalance100)(define(withdrawamount)(if(>=balanceamount)(begin(set!balance(-balanceamount))balance)"Insufficientfunds"))www.govote.atentercode5445start

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VerificationandprovingToproveaprogramcorrect,wemustconsidereverythingaprogramdependsonInpurefunctionalprograms,dependenceonanydatastructureisexplicit

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Provingpropertiesinfunctionalprogramming

(define(powerbn)(cond((=n0)1)(t(*(powerb(-n1))))))Claim:foranyintegern≥0andanynumberb,(powerbn)=bⁿProof:1)Verifythebasecase:(powerb2)Assumethat(powerb(-n1))iscorrect3)Verifythat(powerbn)iscorrectassumingthat(powerb(-n1))iscorrect

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Provingpropertiesinimperativeprogramming

functionpower(b,n){intresult=1;for(inti=0;i<n;++i){result*=b;}returnresult;}Devisealoopinvariant:(n≥i)⋀(result=bⁱ)ProvethatitistrueforthefirstloopiterationProvethateachloopiterationpreservesitAssumethat(n≥i)⋀(result=bⁱ)Provethat(n≥j)⋀(result=bʲ)withj=i+

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ConcurencyConcurencyisoneofthecurrenthottopicinprogrammingThemainchallengeisdata-raceImperativeprogramsareverysensibletodata-racebecauseofstatesThereisnodata-raceinpurefunctionallanguagesalldataisimmutableallfunctionsarepure,withoutside-effects

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Summaryontheupsideoffunctionalprogramming

Themainadvantageisnoside-effectsVerificationandprovingConcurencyProductivity?Ericssonclaimsanincreaseinproductivitybetween9and25timeswhenusingtheirhome-grownversionofErlang

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Thedownsideoffunctionalprogramming

Inpractice,thereisverylimitedneedforprovingaprogramMostlyincriticalapplications:rocketcontrol,hospital...Andhowdoyouprovehardware?Performanceissues(rememberfunctioncallareexpensive)VerylimitedsupportMostprogrammingtasksrequirestates

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MyKeymessageaboutprogrammingparadigms

Bepragmatic,thereisnooneanswer!