Chapter7Functions

7.1IntroductionAfunctionisablockofstatementscalledbynametocarryoutaspecifictaskInordertoreducethecomplexityofprograms,theyhavetobebrokenintosmaller,lesscomplexparts.FunctionsandclassesarethebuildingblocksofaC++program.functionsinthestandardlibrary:built-in,pre-writteninC++Line11callsthefunctionsqrt()tocalculatethesquarerootofthevalueinthevariablen7.1IntroductionC++allowsaprogrammertowritefunctionstoaddtothosealreadyathandinthestandardlibrary.Likevariables,functionsmustbedeclaredbeforetheyareused.Line7declaresstarstobeafunctionThefirstvoidonline7declaresthetypeofthefunctionstars()Thesecondvoidinformsthecompilerthat‘stars’willnotreceiveanydatafromthecallingprogram.Thesecondvoidisoptional.-----prototypeofthefunctionstars()----7.1IntroductionProgramExampleP7BLines22to26definethefunctionLine22:functionheaderLine24-25:functionbody7.2Functionarguments（函數實參)Functionstar()candisplay11asterisks,thenhowtodisplayavariablenumberofasterisks?Newfunctionstars():takeavaluepassedtoitanddisplaythenumberofasterisksspecifiedinthatvalue.7.2FunctionargumentsProgramExampleP7CRemarkThisnumberiscalledanargumentReceivedbytheparameternumdeclaredasanintegerinline23RemarkTheparametersofafunctionareknownonlywithinthefunction.Line25ofthefunctionstars()nowusesthevariablenumtodecidehowmanytimes*isdisplayed.7.2FunctionargumentsAnewprogramspecifies:thenumbercharactertodisplaydisp_chars()usestwoparameters:num(thenumberoftimestodisplayacharacter)ch(thecharactertodisplay)7.2FunctionargumentsThevariablenamesusedintheprototypeareoftenthesameasthoseusedfortheparametersinthefunctionheader.Itisgoodpracticetoplacecommentsafterthefunctionprototypetodescribethefunctionanditsparameters.Theprototypeandtheaccompanyingcommentsareknownasthefunctioninterface.7.3Defaultparametervalues（默認的形參值）Afunctionparameterthatisnotpassedavaluecanbeassignedadefaultvalue.Thesecondargumentisomitted.Thesetwoareequivalent.Ifaparameterisprovidedwithadefaultvalue,alltheparameterstoitsrightmustalsohaveadefaultvalue.√×7.4ReturningavaluefromafunctionProgramExampleP7ETodefineandcallafunctionwithreturnvalue,youshouldnotice:functionprototypefunctionheader7.4ReturningavaluefromafunctionThegeneralformatofthereturnstatement:Examples:Thesetwoblocksareequivalent:7.4ReturningavaluefromafunctionAfunctioncallcanbeusedanywhereinaprogramwhereavariablecanbeused7.5Inlinefunctions（內聯函數）Inlinefunctionscanreducethefunctionprocessingoverheads.Tomakeafunctioninline,precedethefunctionprototypewiththekeywordinline.Whenaninlinefunctionisusedinaprogram,thecompilerreplaceseverycalltothefunctionwiththeprogramstatementsinthefunctionIngeneral,considerinliningafunctionwhenthefunctioncontains1~3linesofcode.7.6PassingargumentsbyvaluePassingbyvalueAcopyoftheargumentvaluesispassedtothefunctionparametersThevalueoftheargumentcannotbechangedwithinthefunctionAcopyofthevalueofaispassedtopChangingthevalueofphasnoeffectona7.6PassingargumentsbyvalueThevalueoftheparametercanbepreventedfromchangewithinafunctionbymakingitaconstant.Todothis,placethekeywordconstbeforetheparameterinthefunctionprototypeandfunctionheader.7.7Passingargumentsbyreference(引用)Areferenceisasynonymoranaliasforanexistingvariable.Areferencetoavariableisdefinedbyadding&afterthevariable’sdatatype.Areferencemustalwaysbeinitializedwhenitisdefined

risareferenceton

nandrbothrefertothesamevalue

risnotacopyofn,butismerelyanothernamefornAchangetonwillalsoresultinachangetor7.7PassingargumentsbyreferenceProgramExampleP7GaandprefertothesamestoragelocationChangingthevalueofpalsochangesthevalueofa7.7PassingargumentsbyreferenceProgramExampleP7HArgumentspassedbyreferencelocalvariableThevariabletempisalocalvariabletothefunctionswap_vals().Localvariablesareknownonlywithinthefunctionwheretheyaredefined.7.8Passingaone-dimensionalarraytoafunctionArrayscanonlybepassedbyreferencetoafunction.Toavoidtheoverheadofcopyingalltheelementsofanarray,thatpassingbyvaluewouldentailProgramExampleP7I:containsafunctionsum_array()thatsumstheelementsofanintegerarraypassedtoitfrommain().7.8Passingaone-dimensionalarraytoafunctionLine16callssum_array()tocalculatethesumofthevaluesinthearrayvalues.Theargumentsarethenameandthenumberofelementsinthearray.[and]arenecessarytoindicatethattheparameterisareferencetoanarray.Thenumberofelementsisnotrequiredinthebracketsforaone-dimensionalarraytohandledifferentsizearray.Constinformsthecompilerthatwithinthefunctionsum_array(),arrayisread-onlyandcannotbemodified.Inline20arrayisareferencetovalues.Becausearrayscanonlybepassedbyreference,&isnotrequired.ProgramExampleP7J7.8Passingaone-dimensionalarraytoafunctionInsteadofcommentslistingtheparametersandreturnvalue,preandpostconditioncommentsareused.Inthepreconditioncomment,theconditionsthatmustexistbeforeafunctionmaybesuccessfullycalledarelisted.Inthepostconditioncomment,theconditionsthatwillexistafterthefunctioniscalledarelisted.ProgramExampleP7J…continued7.8Passingaone-dimensionalarraytoafunction7.8Passingaone-dimensionalarraytoafunction7.8Passingaone-dimensionalarraytoafunction7.9Passingamulti-dimensionalarraytoafunctionWhenamulti-dimensionalarrayispassedtoafunction,thefunctionparameterlistmustcontainthesizeofeachdimensionofthearray,exceptthefirst.7.9Passingamulti-dimensionalarraytoafunction7.9Passingamulti-dimensionalarraytoafunctionTheelementsofatwo-dimensionalarrayarestoredrowbyrowincontiguousmemorylocations.Anyelementarray[i][j]inprogramP7Khasanoffseti*2+jfromthestartingmemorylocationofarray[0][0].Inordertocalculatetheoffset,thenumberofcolumns(=2)isrequired;hencetheneedforthecompilertoknowthevalueoftheseconddimension7.10PassingastructurevariabletoafunctionPassingastructurevariabletoafunctionpassacopyofthemembervaluestothatfunction,thismeansthatthevaluesinthestructurevariablecannotbechangedwithinthefunction.Thevaluesinastructurevariablecanonlybechangedfromwithinafunctionifthevariableispassedbyreferencetothefunction.ProgramExampleP7L:demonstratespassingastructurevariablebyvalueandpassingastructurevariablebyreferencetotwodifferentfunctions.7.10PassingastructurevariabletoafunctionProgramExampleP7LWhenastructuretemplateisdefinedoutsidemain(),itmakesthestructuretemplateglobal.Thismeansthatthestructuretemplateisknowninmain()andindisplay_student_data()andget_student_data().7.10PassingastructurevariabletoafunctionProgramExampleP7L…continuedWhenastructurevariableispassedbyvaluetoafunction,theentirestructuredatamustbecopiedtothefunctionparameter.Foralargestructure,thisisasignificantoverheadanditispreferabletouseareference.Iftheargumentisn‘tchangedbythefunction,theconstkeywordshouldbeused.7.10PassingastructurevariabletoafunctionProgramExampleP7L…continued7.11PassingastringtofunctionThesameconsiderationsshouldbekeptinmindwhenusingstringsasargumentsaswhenusingstructurevariablesasarguments,i.e.passingastringbyvaluemeanscopyingallthecharactersofthestringtoafunctionparameter.Toavoidthisoverheaditispreferabletopassstringsbyreference.7.11.1PassingaC++stringtoafunctionProgramExampleP7MdemonstratespassingaC++stringbyconstreferencetoafunctionthatcountsthenumberofvowelsinthestring.7.11Passingastringtofunction7.11Passingastringtofunction7.11.2PassingaC-stringtoafunctionTouseC-stringsinsteadofC++stringsinprogramP7M,thefollowingmodificationsmustbemadetotheprogram:7.12Recursion（遞歸）Recursionisaprogrammingtechniqueinwhichaproblemcanbedefinedintermsofitself.Thetechniqueinvolvessolvingaproblembyreducingtheproblemtosmallerversionsofitself.Thefactorialofapositiveintegeristheproductoftheintegersfrom1throughtothatnumber.(a)0!=1.Thisiscalledthebasecase.(b)Forapositiveintegern,factorialnisntimesthefactorialofn-1.Thisiscalledthegeneralcaseclearlyindicatesthatfactorialisdefinedintermsofitself.7.12RecursionUsingthedefinition,factorial3iscalculatedasfollows:Thevalueofnis3so,using(b)above,3!=3*2!Nextfind2!Heren=2so,using(b)again,2!=2*1!Nextfind1!Heren=1so,using(b)again,1!=1*0!Nextfind0!Inthiscaseusing(a),0!isdefinedas1.Substitutingfor0!gives1!=1*1=1.Substitutingfor1!gives2!=2*1!=2*1=2.Finally,substitutingfor2!gives3!=3*2!=3*2=6.7.12RecursionProgramExampleP7N7.12RecursionProgramExampleP7NNotethat●Everyrecursivefunctionmusthaveatleastonebasecasewhichstopstherecursion●Thegeneralcaseeventuallyreducestoabasecase.7.12RecursionThefactorialfunctioncouldbewrittenusingiterationTherecursiveversionwillexecutemoreslowlythantheiterativeequivalentbecauseoftheaddedoverheadofthefunctioncalls.Theadvantageoftherecursiveversionisthatitisclearerbecauseitfollowstheactualmathematicaldefinitionoffactorial.7.13Functionoverloading（函數重載）Functionoverloadingisusedwhenthereisaneedfortwoormorefunctionstoperformsimilartasks,butwhereeachfunctionrequiresadifferentnumberofargumentsand/ordifferentargumentdatatypes.Usingdifferentfunctionswiththesamenameinaprogramiscalledfunctionoverloadingandthefunctionsarecalled

overloadedfunctions.Functionoverloadingrequiresthateachoverloadedfunctionhaveadifferentparameterlist,i.e.adifferentnumberofparametersoratleastoneparameterwithadifferentdatatype.7.13FunctionoverloadingProgramExampleP7OThecompilerdecideswhichofthetwosum_array()functionstocallbasedonmatchingargumentswithparameters.7.13FunctionoverloadingProgramExampleP7O…continued7.14Storageclassesautoandstatic

（auto和static存儲類型）7.14.1autoThevariablesdefinedinsideafunctionareauto(automatic)bydefault.Everytimeafunction(includingmain())isentered,storageforeachautovariableisallocated.Whenthefunctioniscompleted,theallocatedstorageisfreed,andanyvaluesintheautovariablesarelost.Suchvariablesareknownaslocalvariablesandareknownonly

withinthefunctioninwhichtheyaredefined.Thevariablesvar1andvar2havebeendefinedwiththestorageclassauto.Thekeywordautomaybeomitted.7.14Storageclassesautoandstatic7.14.2staticstaticvariables,likeautovariables,arelocaltothefunctioninwhichtheyaredefined.staticvariablesareallocatedstorageonlyonceandsoretaintheirvaluesevenafterthefunctionterminates.ProgramExampleP7P7.14StorageclassesautoandstaticProgramExampleP7P…continuedVariablestatic_varwasinitializedonlyonceandretaineditsvaluebetweeneachfunctioncall,i.e.thevalueofstatic_varwasincrementedeverytimeany_func()wascalled.Theautovariableauto_varwascreatedandinitialisedto0everytimethefunctionwasenteredandthenincrementedto1.7.15Thescopeofavariable（變量的作用域）Thescopeofavariablereferstothepartoftheprograminwhichavariablecanbeaccessed.blockscopeglobalscope7.15.1BlockscopeAblockisoneormorestatementsenclosedinbraces{and}thatalsoincludesvariabledeclarations.Avariabledeclaredinablockisaccessibleonlywithinthatblock.7.15ThescopeofavariableBlockscope7.15Thescopeofavariable…continued7.15ThescopeofavariableVariablesdeclaredinsidetheparenthesesofaforareaccessiblewithintheparentheses,aswellasinthestatement(s)containedintheforloop.7.15Thescopeofavariable7.15.2GlobalscopeAvariabledeclaredoutsidemain()isaccessiblefromanywherewithintheprogramandisknownasaglobalvariable.Becauseglobalvariablesareknown,andthereforecanbemodified,withineveryfunction,theycanmakeaprogramdifficulttodebugandmaintain.Globalvariablesarenotasubstituteforfunctionarguments.Apartfromitsownlocalvariables,afunctionshouldhaveaccessonlytothedataspecifiedinthefunctionparameterlist.7.15Thescopeofavariable7.15.3ReusingavariablenameItispermissibletogiveavariablethesamenameasanothervariableinanotherblock.Thisisknownasnamereuse(名重用).ProgramP7QIfavariableisdeclaredinaninnerblockandifavariablewiththesamenameisdeclaredinasurroundingblock,thevariableintheinnerblockhidesthevariableofthesurroundingblock.Ifaglobalvariableishiddenbyalocalvariable,theglobalvariablecanstillbeaccessedusingtheunaryscoperesolutionoperator::7.15ThescopeofavariableProgramP7Q…continued7.15ThescopeofavariableProgramP7Q…continued7.16Mathematicalfunctions（數學函數）Touseanyofthemathematicalfunctionsplacethestatement#include<cmath>atthestartoftheprogram.7.16.1SometrigonometricfunctionsProgramExampleP7R：demonstratessin(),cos()andtan()functions.7.16MathematicalfunctionsProgramExampleP7R7.16Mathematicalfunctions7.16.2Pseudo-randomnumberfunctions（偽隨機數函數）

Tousethepseudo-randomgeneratingfu