国外C语言程序设计英文lecture专家讲座

ComputerprogrammingLecture3Lecture3:OutlineProgramLooping[Kochan–chap.5]TheforStatementRelationalOperatorsNestedforLoopsIncrementOperatorProgramInputforLoopVariantsThewhileStatementThedoStatementThebreakStatementThecontinueStatementExecutingaprogramProgram=listofstatementsEntrypoint:thepointwheretheexecutionstartsControlflow:theorderinwhichtheindividualstatementsareexecutedStatement1Statement2Statement3Statement4Statement5Statement6Statement7Statement8StructureofaCprogram#include<stdio.h>intmain(void){intvalue1,value2,sum;value1=50;value2=25;sum=value1+value2;printf("Thesumof%iand%iis%i\n",value1,value2,sum);return0;}EntrypointofaCprogramSequentialflowofcontrolControllingtheprogramflowFormsofcontrollingtheprogramflow:ExecutingasequenceofstatementsRepeatingasequenceofstatements(untilsomeconditionismet)(looping)Usingatesttodecidebetweenalternativesequences(branching)Statement1Statement2Statement3Statement4Statement5Statement6Statement7Statement8ProgramLoopingLooping:doingonethingoverandoverProgramloop:asetofstatementsthatisexecutedrepetitivelyforanumberoftimesSimpleexample:displayingamessage100times:printf(“hello!\n”);printf(“hello!\n”);printf(“hello!\n”);…printf(“hello!\n”);printf(“hello!\n”);Repeat100times printf(“hello!\n”);Programlooping:enablesyoutodevelopconciseprogramscontainingrepetitiveprocessesthatcouldotherwiserequiremanylinesofcode!Theneedforprogramlooping#include<stdio.h>intmain(void){inttriangularNumber;triangularNumber=1+2+3+4+5+6+7+8;printf("Theeighthtriangularnumberis%i\n", triangularNumber);return0;}Whatifwehavetocomputethe200-th(1000-th,etc)triangularnumber?Exampleproblem:computingtriangularnumbers.(Then-thtriangularnumberisthesumoftheintegersfrom1throughn)InC:3differentstatementsforlooping:for,while,doExample–200thtriangularnumber

n=1triangularNumber=triangularNumber+nn<=200n=n+1yesnotriangularNumber=0PrinttriangularNumberStatementbeforeloopinit_expressionloop_conditionstatementloop_expressionStatementafterloopExample-for

/*Programtocalculatethe200thtriangularnumberIntroductionoftheforstatement*/#include<stdio.h>intmain(void){intn,triangularNumber;triangularNumber=0;for(n=1;n<=200;n=n+1) triangularNumber=triangularNumber+n;printf("The200thtriangularnumberis%i\n", triangularNumber);return0;}Theforstatement

for(init_expression;loop_condition;loop_expression) programstatement init_expressionProgramstatementloop_conditionLoopexpressionyesno12345Theforstatementfor(n=1;n<=200;n=n+1)

triangularNumber=triangularNumber+n;12345yesnoHowforworksTheexecutionofaforstatementproceedsasfollows:1.Theinitialexpressionisevaluatedfirst.Thisexpressionusuallysetsavariablethatwillbeusedinsidetheloop,generallyreferredtoasanindexvariable,tosomeinitialvalue.2.Theloopingconditionisevaluated.Iftheconditionisnotsatisfied(theexpressionisfalse–hasvalue0),theloopisimmediatelyterminated.Executioncontinueswiththeprogramstatementthatimmediatelyfollowstheloop.3.Theprogramstatementthatconstitutesthebodyoftheloopisexecuted.4.Theloopingexpressionisevaluated.Thisexpressionisgenerallyusedtochangethevalueoftheindexvariable5.Returntostep2.InfiniteloopsIt’sthetaskoftheprogrammertodesigncorrectlythealgorithmssothatloopsendatsomemoment!//Programtocount1+2+3+4+5#include<stdio.h>intmain(void){inti,n=5,sum=0;for(i=1;i<=n;n=n+1){ sum=sum+i;printf(“%i%i%i\n",i,sum,n);} return0;}Whatiswronghere?Doestheloopend?RelationaloperatorsOperatorMeaning==Isequalto!=Isnotequalto<Islessthan<=Islessorequal>Isgreaterthan>=IsgreaterorequalTherelationaloperatorshavelowerprecedencethanallarithmeticoperators:a<b+cisevaluatedasa<(b+c)ATTENTION!Donotconfuse:the“isequalto”operator==andthe“assignment”operator=ATTENTIONwhencomparingfloating-pointvalues!Only<and>comparisonsmakesense!Example–forwithabodyof2//Programtogenerateatableoftriangularnumbers#include<stdio.h>intmain(void){intn,triangularNumber;printf("TABLEOFTRIANGULARNUMBERS\n\n");printf("nSumfrom1ton\n");printf("------------------\n");triangularNumber=0;for(n=1;n<=10;++n){triangularNumber+=n;printf("%i%i\n",n,triangularNumber);}return0;}Thebodyoftheloopconsistsinablockof2statementsIncrementoperatorBecauseadditionby1isaverycommonoperationinprograms,aspecialoperatorwascreatedinCforthis.Incrementoperator:theexpression++nisequivalenttotheexpressionn=n+1.Decrementoperator:theexpression--nisequivalenttotheexpressionn=n–1Incrementanddecrementoperatorscanbeplacedinfront(prefix)orafter(postfix)theiroperand.Thedifferencebetweenprefixandpostfix:Example:ifn=4:a=n++leadstoa=4,n=5a=++nleadstoa=5,n=5Programinput

#include<stdio.h>intmain(void){intn,number,triangularNumber;printf("Whattriangularnumberdoyouwant?");scanf("%i",&number);triangularNumber=0;for(n=1;n<=number;++n) triangularNumber+=n;printf("Triangularnumber%iis%i\n",number, triangularNumber);return0;}Scanf:similartoprintf:firstargumentcontainsformatcharacters,nextargumentstellwheretostorethevaluesenteredatthekeyboardMoredetails->inalaterchapter!ReadsintegerfromkeyboardIt’spolitetodisplayamessagebeforeNestedloops#include<stdio.h>intmain(void){intn,number,triangularNumber,counter;for(counter=1;counter<=5;++counter){printf("Whattriangularnumberdoyouwant?");scanf("%i",&number);triangularNumber=0;for(n=1;n<=number;++n) triangularNumber+=n;printf("Triangularnumber%iis%i\n\n",number, triangularNumber);}return0;}Rememberindentations!forloopvariantsMultipleexpressions(commabetween…)

for(i=0,j=10;i<j;i++,j--)Omittingfields(semicolonhavetobestill…)

i=0;

for(;i<10;i++)Declaringvariables

for(inti=0;i=10;i++)Thewhilestatementwhile(expression) programstatementwhile(number<=0){printf(“Thenumbermustbe>0“);printf(“Giveanewnumber:“);scanf(“%i“,&number);}Thewhilestatementwhile(expression) programstatementstatementLoop_expressionyesnoLoopwiththetestinthebeginning!Bodymightneverbeexecuted!Example:

Aprogramtofindthegreatestcommondivisoroftwononnegativeintegervalues…Example-while

/*Programtofindthegreatestcommondivisoroftwononnegativeintegervalues*/#include<stdio.h>intmain(void){intu,v,temp;printf("Pleasetypeintwononnegativeintegers.\n");scanf("%i%i",&u,&v);while(v!=0){temp=u%v;u=v;v=temp;}printf("Theirgreatestcommondivisoris%i\n",u);return0;}Example:

Aprogramtoprintoutthedigitsofanumberinreverseorder…Example-while

//Programtoreversethedigitsofanumber#include<stdio.h>intmain(void){intnumber,right_digit;printf("Enteryournumber.\n");scanf("%i",&number);while(number!=0){right_digit=number%10;printf("%i",right_digit);number=number/10;}printf("\n");return0;}Example–whilenotquiteOK!

//Programtoreversethedigitsofanumber#include<stdio.h>intmain(void){intnumber,right_digit;printf("Enteryournumber.\n");scanf("%i",&number);while(number!=0){right_digit=number%10;printf("%i",right_digit);number=number/10;}printf("\n");return0;}Whathappensifyouenternumber=0?Thedostatementdo programstatementwhile(loop_expression);statementloop_expressionyesnoLoopwiththetestattheend!Bodyisexecutedatleastonce!Example–dowhile

//Programtoreversethedigitsofanumber#include<stdio.h>intmain(){intnumber,right_digit;printf("Enteryournumber.\n");scanf("%i",&number);do{right_digit=number%10;printf("%i",right_digit);number=number/10;}while(number!=0);printf("\n");return0;}Whichlooptochoose?Criteria:WhodeterminesloopingEntry-conditionloop->for,whileExit-conditionloop->doCriteria:Numberofrepetitions:Indefiniteloops->whileCountingloops->forInC,youcanactuallyrewriteanywhileasaforandviceversa!Example:whilevsfor

#include<stdio.h>intmain(void){intcount=1;while(count<=5){ printf("%i\n",count); ++count;}return0;}#include<stdio.h>intmain(void){intcount;for(count=1;count<=5; count++){ printf("%i\n",count);}return0;}ThebreakStatementCanb