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Ch5SelectionStatementsProgrammingConstruct1）Wedividethebasicprogrammingconstructintothreetypes:SequentialStructure、SelectionStructureandLoopStructure.2）FlowChart(流程图)：isagraphicalrepresentationinwhichspecificsymbolsandnecessarylettersareusedtodescribethesteps.3）Theflowchartaboutthethreebasicprogrammingconstructisasfollows：ABconditionABconditionASequentialstructureSelectionStructureSelectionStructure1、relationoperatorandrelationexpression

2、logicaloperatorsandlogicalexpression3、ifstatements4、switchstatements5、examplesChapterOutlineThreeforms：

（1）if（expression）statements

（2）if（expression）statements1

elsestatements2

（3）if（expression1）statements1elseif(expression2)

statements2elseif(expression3)

statements3

elseif（expressionm）statementsmelsestatementsnexexex第五章选择结构If语句的形式BriefSummary

Forme1：if（expression）statementsexpressionstatementsF(0)T(not0)ex：

if(x>y)printf(“%d”,x);Firstly,solvetheexpression.Ifthevalueoftheexpressionisnotzero，thenthecorrespondingststementsequenceisexcuted.Ifnoneoftheexpressionisfalse,thenexcutethestatementsafterifderectly.第五章选择结构If语句的形式exampleexpressionstatements1statements2TFex：if(x>y)printf(“%d”,x);elseprintf(“%d”,y);Form2：if（expression）statements1

elsestatements2第五章选择结构If语句的形式exampleexpression1expression2expression3expression4statements1statements2statements3statements4statements5FFFFTTTTex：if(number>500)cost=0.15;elseif(number>300)cost=0.10;elseif(number>100)cost=0.075;elseif(number>50)cost=0.05;elsecost=0;Form3：nested-ifstatements第五章选择结构If语句的形式example（1）Theexpressionofthreeformsbehindif-statementsalwaysislogicalorrelationexpression.DCLofthreeif-statementsforms：（2）Inform2andforme3，thereisasemicoloninfrontofelse,alsointheendofallstatements.（3）Behindiforelse,itmayincludeonlyoneoperatestatements,alsoincludemanystatements,butshouldbedrawedtogetherwithbrace.({})第五章选择结构If语句的形式Ex:#include<stdio.h>main(){intx,y;scanf(“%d,%d”,&x,&y);if(x>y)x=y;y=x;elsex++;y++;printf(“%d,%d\n”,x,y);}CompileError!ex： if(a+b<c&&b+c>a&&c+a>b) { s=0.5*(a+b+c); area=sqrt(s*(s-a)*(s-b)*(s-c)); printf(“area=%6.sf”,area); } elseprintf(“itisnotatrilateral”);

Notice： Thebrace(“}”)’souterface

neednotsemicolon。Becauseinthebrace({})thereisafullcompoundstatement.第五章选择结构If语句的形式

originalprogram:main(){floata,b,t;scanf(“%f,%f”,&a,&b);if(a>b){t=a;a=b;b=t;}printf(“%5.2f,%5.2f”,a,b);}ex1：Inputtwerealnumbers，pleaseoutputtherenumbersaccordingtotheorderoflittletobig.Needthreefloatvariables:

a,b,t第五章选择结构If语句举例ex2：Input

three

real

numbers，please

output

there

numbers

according

to

the

order

of

little

to

big.originalprogram:main() {floata,b,c,t; scanf(“%f,%f,%f”,&a,&b,&c); if(a>b){t=a;a=b;b=t;} if(a>c){t=a;a=c;c=t;} if(b>c){t=b;b=c;c=t;} printf(“%5.2f,%5.2f,%5.2f”,a,b,c); }第五章选择结构If语句举例Return#include<stdio.h>main(){inta,b;printf("Enterintegera:");scanf("%d",&a);printf("Enterintegerb:");scanf("%d",&b);

if(a==b)printf("a==b\n");elseprintf("a!=b\n");}ex3：Iputtwonumbers,decidethemtobeequallyornot.Result：Enterintegera:12

Enterintegerb:12a==bResult：Enterintegera:12

Enterintegerb:9a!=b第五章选择结构If语句嵌套举例ReturnGeneralform： expression1？expression2：expression3Conditionoperatorcommandsthreeoperateobjects,itisaoperatoralonelyinClanguage.ex：max=(a>b)?a:b;Equalto：

if(a>b)max=a;elsemax=b;第五章选择结构条件运算符Conditionoperator

DCL：（1）Executionorder：Firstly,solvetheexpression1，

Ifthevalueisture,thensolvetheexpression2,thismomenthereisthevalueofwholeexpressions.Ifthevalueisfalse,thensolvetheexpression3,thismomenthereisthevalueofwholeexpressions.ex：max=(a>b)?a:b;Ifa=3,b=4;max=4；Ifa=6，b=4;max=6；第五章选择结构条件运算符（2）Conditionoperatorovermatchassignedoperator,butunderrelationandarithmeticoperator.（3）Conditionoperator’scombineddirecionisfromrighttoleft.ex1：max=(a>b)?a:b;max=a>b?a:b;ex2：max=a>b?a:b+1;max=a>b?a:(b+1);ex3：max=a>b?a:c>d?c:d；max=a>b?a:（c>d?c:d）;Supposea=1,b=2,c=3,d=4第五章选择结构条件运算符（4）Conditionoperatorcann’treplceordinaryif-statements,onlywhentheembedstatementsinitisassignedstatements(andtwobranchesassigntothesamevariable).ex：

if(a>b)printf(“%d”,a);elseprintf(“%d”,b);printf(“%d”,a>b?a:b);（5）Thetypeofexpression1andexpression2canbesame,alsobedifferent.ex：

intx;x?‘a’:‘b’;ex：x>y?1:1.5;第五章选择结构条件运算符Ex5:Inputaletter，iudgeitisacapitalletterornot，ifis，thenchangeittolower-caseletter；ifnot，thennothingchanged。Finally,outputthisletter。main(){charch;scanf(“%c“,&ch);ch=(ch>='A‘&&ch<='Z‘)?(ch+32):ch;printf(“%c“,ch);}第五章选择结构条件运算符Returnex6：Function-1（x<0)0(x=0)1(x>0)y=Writeaprogramme，inputthevalueofx.Thenouputy.main(){floatx;inty;scanf(“%f”,&x);if(x<0)y=-1;if(x==0)y=0;if(x>0)y=1;printf(“\n%f,%d”,x,y);}第五章选择结构If语句嵌套举例ReturnAlgorithm2：inputxifx<0y=-1else：

ifx=0y=0ifx>0y=1outputyAlgorithm3：inputxifx<0y=-1else：

ifx=0y=0else：y=1ouputy第五章选择结构If语句嵌套举例p1：main(){floatx;inty;scanf(“%f”,&x);

if(x<0)y=-1;elseif(x==0)y=0;elsey=1;printf(“x=%f,y=%d\n”,x,y);}p2：modifyto：if(x>=0)if(x>0)y=1;elsey=0;elsey=-1;p3：modifyto：y=-1;if(x!=0)if(x>0)y=1;elsey