Lecture14

PointersICProgrammingLanguageOutlineBasicsofPointersPointersandArraysPointersandStringsPointerArraysPointersandFunctionsWhatisaPointer?PointerisoneofdatatypesofCPointertypevariableholdaspecificaddressofthememoryIfapointervariableholdanaddressofanothervariable,wesayitpointstothatvariableVariableAddressWhenavariableisdefined,fixedunitsofthememorywillbeallocatedforitbasedonitstypeTheaddressofthefirstunitissetasthevariable’saddress,normallyshownasHexformatPleasenotethatthevariablevalueandthevariableaddressaredifferentconceptsVariableAddress0X100X110X120X130X140X150X160X170000000000000000000000000000001100000000000000000000000000000101allocatedforvariableaallocatedforvariablebinta=3,b=5;DefinitionSyntax:

Type*Name;orType*Name;int*p;int*q;Apointerholdsnotonlytheaddress,butthetypeofthedataDifferenttypepointerswillnotbeabletodotheassignmentoperationsInitializationSyntax

Type*Pointer_Name=&Variable_Name;&isanoperatorgivestheaddressofanobjectExample1Definitionwhat’sthedifferencesofthefollowingstatements?int*p,q;intp,*q;int*p,*q;InitializationUninitializedpointersmightcauseunsafeoperations.youcaninitializeapointerbyNULL

int*p=NULL;NULLisaconstantdefinedinstandardlibrary.Itisavaluethatguaranteednottobepointedtoanylocationinmemory.*Operator*ispointingoperator(dereferenceoperator)Ifppointstotheintegerx,*pwillequalsxExample2Pleasenote*operatorisdifferentfromthe*wheretodefineapointerPointerAssignmentThepointerswithsametypecanassignitsvaluetoeachother inta; int*p=&a; int*q=p; *q=2;PointerAssignmentYoumaydefineapointerwithtypevoid.Suchasvoid*p;Apointeroftypevoid*cancontainanaddressofanytype.Anytypesofpointerscanbepassedaroundasavalueoftypevoid*,andalsocanbecasttotheappropriatetypewhenneeded.PointersandArraysPointersandarrayshavecloserelations,whichmaketheoperationsofarrayscanbefulfilledbypointerstoimprovetheperformanceoftheprograms.Allelementsofanarrayareallocatedconsequentmemoryunitsonebyone.intscore[3];intallscore[10][3];PointersandArraysThenameofanarrayislikeapointer,itholdstheaddressofthefirstelementofthearrayExample3,4Pleasenotearraynameisnotapointervariable,youcannotassignavaluetoit PointersandArraysApointerisabletopointtoanarray,oranelementofanarray.Example5 AddressArithmeticApointervariablecandolimitedarithmeticoperations:addition&subtractionAddressarithmeticisnotlikeaintegerarithmetic,itusestepastheoperationunitStepisthenumberofallocatedmemoryforthevariable,suchas4forinteger,and1forcharAddressArithmeticIfpointerppointstothesecondelementofanarray,p+1willpointstothethirdelement,andp-1willpointstothefirstoneExample6,7PointersandArraysTherearetwowaystoreferaelementofanarray,indexandpointerIfppointstoanarraya,thefollowingfourvaluesareequivalenta[i] *(a+i) *(p+i) p[i] Example8PointersandArraysAlthougharraysandpointerscanbeusedinterchangeablysometimes,youshouldalwayskeepinmindthatarraysarenotpointers.Thereareessentialdifferencesbetweenthem.Youcanchangethevalueofapointer,butyoucan’tchangetheaddressreferencedbyanarrayname.Example9PointersandArraysIftwopointersarethesametype,theycanbecomparedusingrelationoperators<>==!=Considertwopointersp,qpointtothemembersofanarrayExample10PointersandStringsApointercanpointtoastring,itpointstothefirstcharacterofthestringactually.Pleasenoteifapointerpointstoaconstantstring,youCANNOTchangethevalueofthestring.Example11,12PointerArraysTheelementsofthearrayarepointertypesSyntax:Type*Array_Name[Capacity]；Eachelementofthearraycontainsanaddress,whichpointstoaobject.Example14,15PointerArraysYoucandefineapointertopointtoapointerarraybyusingadoublepointer,whichislike:int**p;Example16PointersPointtoPointersAPointercanpointtoanotherpointer.Syntax:

Type**Name；Example17PointerArgumentsYoucandefineafunction’sformalargumentsasthetypeofpointers.Thenyoushouldpassanaddressastheactualargumenttocallthisfunction.Example18PointerArgumentsArrayscanactasformalvariablesofafunction.Forexample,reverse(chars[]).Theabsenceofthearray’scapacityispermittedhere.Youcanuseapointertoreplacethearraytoactastheargumentherelikereverse(char*s).Example19,20,21,22mainFunctionwithArgumentsItispossibleformainfunctiontohavetwoarguments,oneisaninteger,andanotherisapointerarray.

main(intargc,char*argv[])Command-lineargumentsItcomesfromtheoldcommandlineoperatingsystems.Suchascopya.txtC:\codemainFunctionwithArgumentsargcholdthenumberofallcommand-lineargumentsincludecommanditself.argvholdthevalueofallcommand-linearguments,whicharealistofstrings.Example23,24PointerReturnTypePointertypecanbeusedasthereturntypeoffunctions.Syntax:

Type*Name(Arguments){…… returnPointerType；}Example25PointersPointtoFunctionsPointerscanalsopointtofunctionsbyholdtheaddressofthefunctions.Syntax:

Return_Type(*Pointer_Name)()；Ifyoudonotenclose*Pointer_Namebyu