lecture 12: pointers to functions...

Hank Childs, University of OregonNovember 7, 2019

Lecture 12:Pointers to Functions

Complexity

__. __ .__ __. __ ___ ___ ______ ___ .__ __. _______ _______ ___ .___________. ___| | | | | \ | | | | \ \ / / / | / \ | \ | | | \ | \ / \ | | / \| | | | | \| | | | \ V / | ,----' / ^ \ | \| | | .--. | | .--. | / ^ \ `---| |----` / ^ \| | | | | . ` | | | > < | | / /_\ \ | . ` | | | | | | | | | / /_\ \ | | / /_\ \| `--' | | |\ | | | / . \ __ | `----.__ / _____ \ | |\ | | '--' | | '--' | / _____ \ | | / _____ \\______/ |__| \__| |__| /__/ \__\ (_ ) \______(_ ) /__/ \__\ |__| \__| |_______/ |_______/ /__/ \__\ |__| /__/ \__\

_______.___________..______ __ __ ______ .___________. __ __ .______ _______ _______. / | || _ \ | | | | / || || | | | | _ \ | ____| / | | (----`---| |----`| |_) | | | | | | ,----'`---| |----`| | | | | |_) | | |__ | (----` \ \ | | | / | | | | | | | | | | | | | / | __| \ \

.----) | | | | |\ \----.| `--' | | `----. | | | `--' | | |\ \----.| |____.----) ||_______/ |__| | _| `._____| \______/ \______| |__| \______/ | _| `._____||_______|_______/

TalkaboutQ1frommidterm

Project2G

QuizonTuesday,Nov12

• TherewillbeaquizTuesdayNov12.• Itwillbeworth3points.• ItwillbeonfileI/O.• Nonotes.• Expectingyoutoknowfopen/fread/fwrite/etc.

Reading:Chapters5and6

• Iwillbefollowingthetextbookmorecloselythenextfewweeks

Callbyvalue/callbyreference

• Referstohowparametersarepassedtoafunction.– Callbyvalue:sendthevalueofthevariableasafunctionparameter• Sideeffectsinthatfunctiondon’t affectthevariableinthecallingfunction

– Callbyreference:sendareference(pointer)asafunctionparameter• Sideeffectsinthatfunctionaffectthevariableinthecallingfunction

CallbyValue

Cdoesnotcarethatfoohasan

argumentvariablecalled“x”andmainhasan

automaticvariablecalled“x”.TheyhaveNOrelation.

Callbyreference

Preprocessor

• Preprocessor:– takesaninputprogram– producesanotherprogram(whichisthencompiled)

• Chasaseparatelanguageforpreprocessing– DifferentsyntaxthanC– Usesmacros(“#”)

macro(“macroinstruction”):ruleforreplacinginputcharacterswithoutputcharacters

#include

• compilercanonlycompileonefileatafile• takesanotherfileandincludesitinthecurrentfile

• thefileisa“header”file– itcontainsfunctionprototypes– afunctionprototypedeclaresafunctionexists,butnothowitisimplemented

Demonstrate#include

#include<stdio.h>int main(){printf("Helloworld\n");

}

%gcc –Eprintf.c

FunctionPrototype

PreprocessorPhases

• Resolve#includes• Conditionalcompilation(#ifdef)• Macroreplacement• Specialmacros

#definecompilation

Thisisanexampleofmacroreplacement.

#defineviagcc command-lineoption

Conflicting–Dand#define

Conditionalcompilation

Conditionalcompilationcontrolledviacompilerflags

FunctionPointers

• Idea:– Youhaveapointertoafunction– Thispointercanchangebasedoncircumstance–Whenyoucallthefunctionpointer,itislikecallingaknownfunction

FunctionPointerExample

FunctionPointerExample#2

Functionpointer Partoffunctionsignature

Don’tbescaredofextra‘*’s…theyjustcomeaboutbecauseofpointersintheargumentsorreturnvalues.

Simple-to-ExoticFunctionPointerDeclarations

void(*foo)(void);void(*foo)(int **,char***);char**(*foo)(int **,void(*)(int));

Thesesometimescomeuponinterviews.

FunctionPointersvs Conditionals

Whataretheprosandconsofeachapproach?

Callbacks

• Callbacks:functionthatiscalledwhenaconditionismet– Commonlyusedwheninterfacingbetweenmodulesthatweredevelopedseparately.

– …librariesusecallbacksanddeveloperswhousethelibraries“register”callbacks.

Callbackexample

Callbackexample

WhatisaDataStructure?

WhatisaDataStructure?

• Datastructuredefinitions– Textbook:“asystematicwaytoorganizedata”–Wikipedia:“dataorganization,managementandstorageformatthatenablesefficientaccessandmodification”

WeAreAlreadyFamiliarWithSomeDataStructures

• Arrays• FromPython– List– Tuple– Dictionary– Set

KeyConcept

• Itorganizesdata• Itenablesefficientaccess–Whatdoesefficientmean??

Example:DataStructure:ArraysOperation:Search

IsItEfficient?:TwoSub-questions

• 1)Howlongdoesthistaketorun?– (howdowemeasurethis?)

• 2)Couldwedoitwithlessoperations?

IsItEfficient?:TwoSub-questions

• 1)Howlongdoesthistaketorun?– (howdowemeasurethis?)

• 2)Couldwedoitwithlessoperations?

HowLongDoesThisTakeToRun?

• Oneanswer:timeit!

Unixcommand:time

Courtesy:Sventek textbook

Anotheroption:addtimingstoyourprogram!

• (note:nowtakingafewslideaside)

gettimeofday

(therearelotsofUnixsystemcalls,whichdolotsofdifferentthings)

gettimeofday example

gettimeofday example

gettimeofday example

gettimeofday example

HowLongDoesThisTakeToRun?

• Oneanswer:timeit!• Question:whatifIchangethedata?–Whatifwehave8000studentsinsteadof80students?

• Newidea:analyzetheprogram

AnalyzetheProgram

• Howmanyoperationsdoesthisperform?• Assumestrcmp takes50operations• AndnumStudents is80• Then~80*50=4000operation

BigONotation(1/3):ImportantComputerScienceConcept

• Ifinputdatasizeis“N”,thenyoucandescribehowmanyoperationsoccurusingN

• If“numStudents”isN,andstrcmp takes50operations,then50N

BigONotation(2/3):ImportantComputerScienceConcept

• Idea#1behindBigO:don’tworryaboutconstants• Idea#2behindBigO:justsayhowmanyoperationswithrespecttoN(numberofdataelements)

• Answer:O(N)– Thisissosimple,almostnotuseful… neednewexample!

BigONotation(3/3):ImportantComputerScienceConcept

• Thisiscalled“AsymptoticAnalysis”–Why?… whathappenswhenNgets“large”?

Morecomplexexample

• Howmanyoperationsnow?• numStudents*numStudents*50(about)• à O(n2)

IsThisBetter?

• Howmanyoperationsnow?• numStudents*numStudents*25(about)• STILLà O(n2)

ComparingOurTwoImplementations

• Answer:secondversionisabouttwiceasfast– ButbothO(n2),somaybenotahugedifference

Newdatastructure:sortedarray

• Likeanarray,buteverythingissorted• Unsortedarray– int X[6]={4,1,3,7,9,2};

• Sortedarray– int X[6]={1,2,3,4,7,9};

CanWeImproveOnThisOneWithAnUnsortedArray?

• Probablynot,but…

ButWeCanImproveWithaSortedArray…

Howmanyoperationsdidthistake?

• StartwithNelements• After1iteration,N/2elements• After2iterations,N/4elements• After3iterations,N/8elements• After4iterations,N/16elements• Etc…• Afterlog2Niterations,1element• à O(logn)

Buttherewasworktotakeanarrayandmakeasortedarray

• Howlongdoesittaketosort?• Sort:complexityvaries:O(nlogn)toO(n2)– Project2Bexample:O(n2)– Mostrealworld:O(nlogn)

• So:wecoulddoIsStudentInClass onunsortedarrayforO(n)

• Or:sortarray(costO(nlogn))anddoIsStudentInClass forO(logn)

• à weonlywanttodoasortifweplantocallIsStudentInClass abunchoftimes

CanwegetabetterBigOforthisone?

• ‘for’loopovernumStudents:O(n)• Combined:O(nlogn)+O(n)à O(nlogn)– (asymptoticanalysis)

Courtesy:Sventek textbook

IsItEfficient?:TwoSub-questions

• 1)Howlongdoesthistaketorun?– (howdowemeasurethis?)

• 2)Couldwedoitwithlessoperations?

• Unsortedarraysweremoreefficientforsearch,buttherewasworkincreatingthem– Differentdatastructuresareefficientfordifferentoperations

– Hence,wehavealotofthem