第四章 選擇結(jié)構(gòu)程序設(shè)計(jì)

第四章選擇結(jié)構(gòu)程序設(shè)計(jì)學(xué)習(xí)目的：掌握關(guān)系運(yùn)算符和邏輯運(yùn)算符的運(yùn)算規(guī)則及優(yōu)先級(jí)關(guān)系

掌握條件語句的語句格式和執(zhí)行順序?qū)W會(huì)進(jìn)行選擇結(jié)構(gòu)程序設(shè)計(jì)重點(diǎn)難點(diǎn)：條件語句的嵌套選擇結(jié)構(gòu)程序的分析方法和設(shè)計(jì)方法

4.1關(guān)系運(yùn)算符和關(guān)系表達(dá)式關(guān)系運(yùn)算符

<<=>>=(優(yōu)先級(jí)相同）高

==!=(優(yōu)先級(jí)相同）低

算術(shù)運(yùn)算符（高）關(guān)系運(yùn)算符賦值運(yùn)算符（低）例：

c>a+bc>b==aa==b<ca=b>c關(guān)系表達(dá)式概念：由關(guān)系運(yùn)算符將兩個(gè)表達(dá)式連接起來的式子如：d+a>b+c(a>b)>(c>d)

關(guān)系表達(dá)式的值真以1代表

假以0代表例：當(dāng)a=3,b=2,c=1求f=a>b>c的值當(dāng)a=5,b=4,c=2求a>b!=c的值4.2邏輯運(yùn)算符和邏輯表達(dá)式邏輯運(yùn)算符

&&(邏輯與）||(邏輯或）!(邏輯非）邏輯運(yùn)算符的運(yùn)算規(guī)則優(yōu)先級(jí)：！&&||(高）

>（低）

！（高）算術(shù)運(yùn)算符關(guān)系運(yùn)算符

&&和||賦值運(yùn)算符（低）例(a>b)&&(x>y)(a==b)&&(x<y)!a||(a>b)邏輯表達(dá)式邏輯運(yùn)算結(jié)果邏輯值：真（1）假（0）運(yùn)算量的判斷：假（0）真（非0）例：a=6,b=4,c=2,

求表達(dá)式！(a-b)+c-1&&b+c/2的值

4.3if語句if語句的三種形式形式一格式：

if(expression)statement

執(zhí)行過程：exprstatement非0=0例：if(x>y)

printf(“%d”,x);形式二

格式：if(expression)statement1elsestatement2執(zhí)行過程：：exprstatement1statement2非0=0例：if(x>y)max=x;elsemax=y;形式三：格式：執(zhí)行過程：if(expr1)statement1elseif(expr2)statement2elseif(expr3)statement3…...elseif(exprm)statementm

elsestatementnexpr1statemnt1非0=0expr2expr3statemntnstatemnt3statemnt2非0非0=0=0例：if(salary>1000)index=0.4;elseif(salary>800)index=0.3;elseif(salary>600)index=0.2;elseif(salary>400)index=0.1;elseindex=0;說明：if后面的表達(dá)式類型任意語句可以是復(fù)合語句

if(x)if(x!=0)if(!x)if(x==0)如：if(a==b&&x==y)printf(“a=b,x=y”);if(3)printf(“OK”);if(‘a(chǎn)’)printf(“%d”,’a’);例考慮下面程序的輸出結(jié)果:#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!例：求一個(gè)數(shù)的絕對值#include<stdio.h>main(){intx,y;

printf("Enteraninteger:");

scanf("%d",&x);y=x;

if(y<0)y=-y;

printf("\ninteger:%d--->absolutevalue:%d\n",x,y);}運(yùn)行：Enteraninteger:-12integer:-12--->absolutevalue:12

比較兩個(gè)數(shù)，求其中的大者

main(){inta,b;

printf("Enterintegera:");

scanf("%d",&a);

printf("Enterintegerb:");

scanf("%d",&b);

if(a>b)

printf(“themaxnumberis:%d”,a);else

printf(“themaxnumberis:%d”,b);}求y=x(x+2)2<x<=102x-1<x<=2x-1x<=-1main(){intx,y;scanf(“%d”,&x);If(x>2&&x<=10)y=x*(x+2);elseif(x>-1&&x<=2)y=2*x;elseif(x<=-1)y=x-1;elsey=-1;If(y!=-1)printf(“%d”,y);elseprintf(“error”);}4.4If語句的嵌套if(expr1)if(expr2)statement1elsestatement2內(nèi)嵌ifif(expr1)if(expr2)statement1elsestatement3

內(nèi)嵌ifif(expr1)statement1elseif(expr3)statement3elsestatement4內(nèi)嵌ifif(expr1)if(expr2)statement1elsestatement2elseif(expr3)statement3elsestatement4內(nèi)嵌if內(nèi)嵌if注：if~else配對原則：缺省{}時(shí)，else總是和它上面離它最近的未配對的if配對if(……)if(……)if(……)else…...else…...else…...結(jié)果：-1main(){inta=2,b=3,c;c=a;if(a>b)c=1;elseif(a==b)c=0;elsec=-1;

printf(“%d”,c);}對函數(shù)y=-1(x<0)0(x=0)1(x>0)編程序，輸入x的值，輸出y的值程序1：程序2：main(){intx,y;

scanf(“%d”,&x);if(x<0)y=-1;elseif(x==0)y=0;elsey=1;printf(“x=%d,y=%d\n”,x,y);}main(){intx,y;

scanf(“%d”,&x);if(x>=0)if(x>0)y=1;elsey=0;elsey=-1;printf(“x=%d,y=%d\n”,x,y);}程序3：程序4：main(){intx,y;

scanf(“%d”,&x);y=-1;if(x!=0)if(x>0)y=1;elsey=0;

printf(“x=%d,y=%d\n”,x,y);}main(){intx,y;

scanf(“%d”,&x);y=0;if(x>=0)if(x>0)y=1;elsey=-1;

printf(“x=%d,y=%d\n”,x,y);}讀程序?qū)懡Y(jié)果main(){inta=1,b=3,c=5,d=4;if(a<b)if(c<d)x=1;elseif(a<c)if(b<d)x=2;elsex=3;elsex=6;elsex=7;printf(“x=%d”,x);}結(jié)果：x=24.5switch語句（開關(guān)分支語句）一般形式：

switch(表達(dá)式){caseE1:語句組1;break;caseE2:語句組2;break;…….caseEn:語句組n;break;[default:語句組n+1;]}switch表達(dá)式語句組1語句組2語句組n語句組…...E1E2Endefaultcase說明：E1,E2,…En是常量表達(dá)式,且值必須互不相同語句標(biāo)號(hào)作用，必須用break跳出case后可包含多個(gè)可執(zhí)行語句，且不必加{}switch可嵌套多個(gè)case可共用一組執(zhí)行語句如：……case‘A’:case‘B’:case‘C’:

printf(“score>60\n”);break;……..例switch(score){case5:printf(“Verygood!”);case4:printf(“Good!”);case3:printf(“Pass!”);case2:printf(“Fail!”);default:printf(“dataerror!”);}運(yùn)行結(jié)果：score為5時(shí)，輸出：

Verygood!Good!Pass!Fail!dataerror!例

main(){intx=1,y=0,a=0,b=0;switch(x){case1:switch(y){case0:a++;break;case1:b++;break;}case2:a++;b++;break;case3:a++;b++;}

printf(“\na=%d,b=%d”,a,b);}運(yùn)行結(jié)果：a=2,b=1例根據(jù)輸入字母輸出字符串#include<stdio.h>main(){intc;

printf("Entermornorhorother:");c=getchar();switch(c){case'm':printf("\nGoodmorning!\n");break;case'n':printf("\nGoodnight!\n");break;case'h':printf("\nHello!\n");break;default:printf("\n????????\n");break;}}4.6條件運(yùn)算符一般格式表達(dá)式1？表達(dá)式2:表達(dá)式3

說明：執(zhí)行順序優(yōu)先級(jí)結(jié)合方向表達(dá)式的類型例：設(shè)intx=1,y=0;x-->(y+x)?10:12.5>y++?‘A’:‘Z’例：intx=0,y=1;x++*x--!=‘y’?8-4:y

4.7程序舉例：例1寫一個(gè)程序，判斷某一年是否閏年Year被4整除Year被100整除Year被400整除YLeap=1NLeap=0YNLeap=1YNleap=0YN輸出閏年輸出非閏年leapmain(){intyear,leap;

scanf(“%d”,&year);if(year%4==0){if(year%100==0){if(year%400==0)leap=1;elseleap=0;}elseleap=1;}elseleap=0;if(leap)

printf(“%dis”,year);else

printf(“%disnot“,year);printf(“aleapyear.\n”);}程序：例2：求ax2+bx+c=0方程式的根main(){floata,b,c,disc,x1,x2,real,imag;else

scanf(“%f,%f,%f”,&a,&b,&c);{real=-b/(2*a);

printf(“theequation”);imag=sqrt(-disc)/(2*a);

if(fabs(a)<=1e-6)printf(“hascomplexroot:\n);

printf(“isnotaquadratic\n”);printf(%8.4f+%8.4fi\n,real,imag);

elseprintf(%8.4f-%8.4fi\n”,real,imag);disc=b*b-4*a*c;}}

if(fabs(disc)<=1e-6)

printf(“hastwoequalroots:%8.4f\n”,-b/(2*a));elseif(disc>1e-6){x1=(-b+sqrt(disc))/(2*a);x2=(-b-sqrt(disc))/(2*a);

pr