文库发布：c语言06教程

Agenda:function(函数)VoidFunctionandvalue-returningfunctionUsingFunctionArgumentsandParametersUsingLocalVariablesinaFunctionFunctionPreconditions(前置条件)andPostconditions(后置条件)FunctionsEveryC++programmusthaveafunctioncalledmain.

Programexecutionalwaysbeginswithfunctionmain.

Anyotherfunctionsaresubprogramsthatmustbeexplicitlycalled.

WelcomePrintingAfriendofyoursisreturningfromalongtrip,andyouwanttowriteaprogramthatprintsthefollowingmessage:

**********************************************WelcomeHome!********************************************************************************************ModulesMainLevel0Print2LinesLevel1Print3LinesLevel1PrinttwolinesofasterisksPrint“WelcomeHome!”PrintfourPrint“***********************”Print“***********************”Print“***********************”Print“***********************”Print“***********************”Print“***********************”#include<stdio.h>

//程序WelcomevoidPrint2Lines();//Functionprototypes(函数原型)voidPrint4Lines();intmain(){Print2Lines();//Functioncall（函数调用）printf("WelcomeHome!\n");Print4Lines();//Functioncallreturn0;}CcodesvoidPrint2Lines()//Functionheading{printf("***************\n");printf("***************\n");}voidPrint4Lines()//Functionheading{printf("***************\n");printf("***************\n");printf("***************\n");printf("***************\n");}CcodesRevisedWelcomePrintingRevisethefunctionsothatthecallercanindicatethenumberoflinesandthenumberofasterisksperlineforprinting.Like:PrintLines(3,10);

//functioncallwillprintmessageasfollows:******************************#include<stdio.h>//程序NewWelcomevoidPrintLines(intnumLines,intnumPerLine);

//Functionprototypeintmain(){PrintLines(2,10);printf("WelcomeHome!\n");PrintLines(4,6);return0;}CcodesvoidPrintLines(intnumLines,intnumPerLine){

intcount1=1;//Loopcontrolvariableintcount2=1;

while(count1<=numLines){count2=1;

while(count2<=numPerLine)

{printf("*");count2++;

}printf(“\n”);count1++;}}CcodesRevisedWelcomePrinting(2)Revisethefunctionsothatthecallercanindicatethenumberoflinesandthenumberofasterisksperlineandthecharacterthatconsistsoftheline,forprinting.Like:PrintLines(3,10,‘#’);

//functioncallwillprintmessageasfollows:###############################include<stdio.h>//程序NewWelcome1voidPrintLines(intnumLines,intnumPerLine,charwhichChar);

intmain(){

PrintLines(2,10,'+');printf("WelcomeHome!\n");

PrintLines(4,6,'#');

return0;}CcodesvoidPrintLines(intnumLines,intnumPerLine,charwhichChar){

intcount1=1;//Loopcontrolvariableintcount2=1;

while(count1<=numLines){count2=1;

while(count2<=numPerLine)

{printf(“%c”,whichChar);count2++;

}printf(“\n”);count1++;}}CcodesFunctionCallsOnefunctioncallsanotherbyusingthenameofthecalledfunctionfollowedby()thatenclosesanargumentlist,whichmaybeempty.Afunctioncalltemporarilytransferscontrolfromthecallingfunctiontothecalledfunction.FunctionName

(ArgumentList)Theargumentlist(实参列表)isawayforfunctionstocommunicatewitheachotherbypassinginformation.Theargumentlistcancontain0,1,ormorearguments,separatedbycommas,dependingonthefunction.FunctionCallSyntaxTerminologyintmain(){

PrintLines(2,10,'+');:}voidPrintLines(…){:

printf(“%c”,whichChar);:}thecaller(callingblock)thecalledfunctionProblemWriteafunctiontocalculatethefollowing(xisaninteger):

#include<stdio.h>#include<math.h>double

f(intx);

//prototypevoidmain(){

int

x1=0;

int

x2;=9.1;

printf(“the1stresultof%dis:“,x1);

printf(“%f\n”,f(

x1

));

printf(“the2ndresultof%dis:“,x1);

printf(“%f\n”,f(

x2

));}argumentsCcodesdouble

f(

intx)

{

if(x==0)

return1.0;elsereturn1/pow(x,3)+1/pow(x,4);}TwoPartsofFunctionDefinitionbodyheadingTwoPartsofFunctionDefinitionbodyheadingintmain(){

PrintLines(2,10,'+');:}voidPrintLines(…){:

printf(“%c”,whichChar);:}bodyheadingdouble

f(

intx)

{

if(x==0)

return1.0;elsereturn1/pow(x,3)+1/pow(x,4);}Whatisinaheading?typeofvaluereturnednameoffunctionparameterlist（形参列表）voidPrintLines(intnumLines,intnumPerLine,charwhichChar

){:

cout<<whichChar;:}Whatisinaheading?typeofvaluereturnednameoffunctionparameterlistdouble

f(

intx);

voidPrintLines(intnumLines,intnumPerLine,charwhichChar);FunctionPrototype(函数原型)

Aprototypelookslikeaheadingbutmustendwithasemicolon,anditsparameterlistneedsonlytocontainthetypeofeachparameter.Thefollowing2typesareofthesameandareVALID.double

f(

int

);

voidPrintLines(

int,int,char

);SuccessfulFunctionCompilationBeforeafunctioniscalledinyourprogram,thecompilermusthavepreviouslyprocessedeitherthefunction’sprototypeorthefunction’sdefinition(headingandbody).

Why?

Forsuccessfulcompilation,thecompilerneedstheinformationcarriedbythefunctionprototypeorthefunctiondefinitionbeforethefunctioncall.Thisinformationincludes:----datatypeofthereturnedvalue----functionname----thenumberofparameters----datatypeofeachparameterFunctionCallsWhenafunctioniscalled,temporarymemoryisallocatedforitsvalueparameters,anylocalvariables,andforthefunction’snameifthereturntypeisnotvoid.

Flowofcontrolthenpassestothefirststatementinthefunction’sbody.

Thecalledfunction’sstatementsareexecuteduntila

returnstatement

(withorwithoutareturnvalue)orthe

closingbraceofthefunctionbodyisencountered.Thencontrolgoesbacktowherethefunctionwascalled.FunctionCalls：调用F函数：调用F函数：

被调用函数F某函数SummaryWhyweneedfunctions?Whatisafunctiondefinition?Whatpartsconsistofafunctiondefinition?Whatisafunctionprototype?Whyitisneededbythecompiler?Whatisafunctioncall?Whatwillhappenwhenafunctioncallismade?Asaprogrammer,youalwaysswitchbetweena“functioncreator”anda“functionuser”.Isthatright?Wherecomesthefunction?ReturnaValueorNOT?InC,avalue-returningfunctionreturnsonevalueofthetypespecifiedinitsheadingandprototype(calledthereturntype).Incontrast,avoid-functiondoesnotreturnanyvalue.ReturnaValueorNOT?void函数的调用是一条单独的语句，这是因为void函数并不会返回一个值，调用void函数是为了进行该函数的动作。返回值函数的调用一般使用在表达式中，这是因为需要使用该函数返回来的值。void函数的调用就像是一条命令；而返回值函数的调用一般是表达式的一部分。PrintLines(2,10,‘+’);

//void函数的调用是单独的语句printf(“%f\n”,f(x1));

//返回值函数的调用一般使用在表达式中ExampleWriteavoidfunctioncalledDisplayMessage(),whichyoucancallfrommain(),todescribethepollutionindexvalueitreceivesasaparameterYourcitydescribesapollutionindex lessthan35as“Pleasant”,

35through60as“Unpleasant”,

above60as“HealthHazard”voidDisplayMessage(intindex)

{

if(index<35)

printf(“Pleasant\n”);

elseif(index<=60)

printf(“Unpleasant\n”);

elseprintf(“HealthHazard\n”);}parameter

Ccodes#include<stdio.h>voidDisplayMessage(int);

//Prototypeintmain()

{

intpollutionIndex; printf(“Enterairpollutionindex:”);scanf(“%d”,&pollutionIndex);

DisplayMessage(pollutionIndex);

//Call

return0;} TheRestoftheProgram31argumentsReturnStatementreturn;

//NovaluetoreturnIsvalidonlyinthebodyblockofavoidfunction.Causescontroltoleavethefunctionandimmediatelyreturntothecallingblock,leavinganysubsequentstatementsinthefunctionbodyunexecuted.#include<stdio.h>voidDisplayMessage(int);

//Prototypeintmain() {

DisplayMessage(15);

//Functioncall

printf(“GoodBye\n“); return0;}voidDisplayMessage(intn) {printf(“Ihavelikedmathfor%dyears\n“,n);

return;

//explicitlyindicateswheretoreturn}VoidFunctionsStandAlone33intmain() {

charch;

scanf(“%c”,&ch);

DisplayMessage(ch);

//Functioncall

printf(“GoodBye\n“);

return0;}voidDisplayMessage(char

ch) {

if(ch==‘s’)

return;

//returnwhenneededelseprintf(“Thecharacteris:%c”,ch);}Sometimes…34DisplayMessagefunctionProgramwithSeveralFunctionsmainfunctionffunctionProgramwithSeveralFunctionsmainfunctionSquarefunctionCubefunctionProgramwithSeveralFunctionsmainfunction程序中函数之间的关系

函数定义即为函数。C++程序虽然由若干个函数组成，但各个函数是相互独立的，即各个函数定义是互相独立的（相互分离）。某函数的函数体中不能有另一个函数的定义。函数与函数之间的关系是：调用与被调用。除此以外，没有任何其它关系。程序中函数之间的关系intmain(){:intcube(intn){:}:} 错！程序中函数之间的关系intmain(){…cube(x

);:} intcube(intn){:}ArgumentListandParameterListAparameterlist

isthemeansusedforafunctiontoshareinformationprovidedbyargumentlistwiththeblockcontainingthecall.intmain(){

PrintLines(

2,

10

,'+‘

);}voidPrintLines(intnumLines,intnumPerLine,charwhichChar){

：}ClassifiedbyLocationArgumentsParametersAlwaysappearinafunctioncallwithinthecallingblock.Alwaysappearinthefunctionheading.SomeCbooksUsetheterm“actualparameters”forarguments.Thosebooksthenrefertoparametersas“formalparameters”43Example#include<math.h>#include<stdio.h>doublemulSqrt(int,int);intmain(){doublex;intx1=2,x2=3;

x=mulSqrt(x1,x2);

printf(“%f\n”,x);

return0;}doublemulSqrt(inta1,inta2){doublex;

x=a1*a2;

if(x<0)return-1.0;elsereturnsqrt(x);}44ArgumentListandParameterList一般情况下，实参的数量、位置与数据类型应该与形参一致。doublemulSqrt(inta1,inta2){……}mulSqrt(x1,x2)45每个实参的数据类型应该与相应位置的形参的数据类型一致，否则会引起自动类型转换。例如若mulSqrt的函数调用如下，其返回值是多少？

mulSqrt(2.1,3.4)？2.1和3.4自动转换成2和3并传递给mulSqrt函数里的形参a1和a2，从而令a1和a2的值为2和3。所以这个调用返回的值是sqrt(6)即2.44949。ArgumentListandParameterList46值形参相对应的实参可以是任何具有一个值的项目：常量、变量或表达式。例如

mulSqrt(3,4)//常量作实参

mulSqrt(x1,x2)//变量作实参

mulSqrt(x1,4)//常量、变量作实参

mulSqrt(2*3+1,x2)//表达式、变量作实参ArgumentListandParameterList47x=mulSqrt(x1,x2);实参与形参是不同的内存空间。调用时，实参值单向传递给形参变量。然后

，实参与形参就没有任何关系。

22main函数中的x1mulSqrt函数中的a1内存3main函数中的x2mulSqrt函数中的a2ArgumentListandParameterList348#include<stdio.h>voidvalPara(int);intmain(){intx=1;

valPara(x);

printf(“x=%d\n”,x);

return0;}voidvalPara(intx){x++;}运行程序屏幕将输出：

x=1ArgumentListandParameterList两个变量是否为同一变量，并不是看它们的变量名是否一样，而是看它们是否为同一个内存空间。QuestionsWhyisafunctionusedforatask? Tocutdownontheamountofdetailinyourprogram(encapsulation).Canonefunctioncallanotherfunction? Yes.Canafunctionevencallitself? Yes,thatiscalledrecursion(递归);itisveryusefulandrequiresspecialcareinwriting.AnAssertion(断言)Anassertionisatruth-valuedstatement--onethatiseithertrueorfalse(

notnecessarilyinC++code

)[即表示某种状况的语句]Examples

studentCount>0 sumisassigned&&count>0 response==‘y’or‘n’ 0.0<=deptSales<=25000.0 beta==beta@entry*2PreconditionsandPostconditionsAprecondition

isanassertiondescribingeverythingthatthefunctionrequirestobetrueatthemomentthefunctionisinvoked.

Apostcondition

describesthestateatthemomentthefunctionfinishesexecuting,providingthepreconditionistrue.Thecaller

isresponsibleforensuringtheprecondition,andthefunctioncodemustensurethepostcondition.

Forexample...Functioninterfaceandimplementation接口函数首部（声明）+前置条件：如何调用该函数后置条件+目的：该函数是用来干什么的设计函数设计接口：说明函数要实现的功能及如何调用该函数设计实现部分Functioninterfaceandimplementationvoidswap(/*inout*/int&firstInt,/*inout*/int&secondInt)//Precondition:firstIntandsecondIntareassigned//Postcondition:firstInt==secondInt@entry// &&secondInt==firstInt@entry{

：：}接口实现用户关注设计者关注函数的注释(comments)清晰规范的注释，这是良好而基本