@TOC[ 第四章 选择结构程序设计]

##例 4.1

#include <stdio.h>
#include <math.h>
int main()
{
double a,b,c,disc,x1,x2,p,q;
scanf("%lf,%lf,%lf",&a,&b,&c);
disc=b*b-4*a*c;
if(disc<0)
{
printf("此方程无实根\n");
}
else
{
p=-b/(2.0*a);
q=sqrt(disc)/(2.0*a);
x1=p+q;
x2=p-q;
printf("此方程有实根:\nx1=%7.2f\nx2=%7.2f\n",x1,x2);
}
return 0;
}

运行结果如下：

##例4.2 输入两个实数，按由小到大的顺序输出。

#include<stdio.h>
int main()
{
float a,b,t;
printf("输入两个实数：");
scanf("%f,%f",&a,&b);
if(a>b)
{
t=a;
a=b;
b=t;
}
printf("%5.2f,%5.2f\n",a,b);
return 0;
}

##例4.3输入a,b,c三个数，按由小到大的顺序输出。

#include<stdio.h>
int main()
{
float a,b,c,t;
printf("请输入三个数:");
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\n",a,b,c);
return 0;
}

运行结果如下:

##例4.4

#include <stdio.h>
int main()
{
char ch;
scanf("%c",&ch);
ch=(ch>='A'&&ch<='Z')?(ch+32):ch;
printf("%c\n",ch);
return 0;
}

运行结果如下：

##例4.6

#include <stdio.h>
int main()
{
char grade;
scanf("%c",&grade);
printf("Your grade");
switch(grade)
{
case 'A':printf("85~100\n");break;
case 'B':printf("70~84\n");break;
case 'C':printf("60~69\n");break;
case 'D':printf("<60\n");break;
default:printf("enter data error!\n");
}
return 0;
}

运行结果如下：

##例4.7

#include <stdio.h>
int main()
{
void action1(int,int),action2(int,int);
char ch;
int a=15,b=23;
ch=getchar();
switch(ch)
{
case 'a':
case 'A':action1(a,b);break;
case 'b':
case 'B':action2(a,b);break;
default:putchar('\a');
}
return 0;
}
void action1(int x,int y)
{
printf("x+y=%d\n",x+y);
}
void action2(int x,int y)
{
printf("x*y=%d\n",x*y);
}

运行结果如下：

##例4.8 写一个程序，判断某一年是否为闰年。

#include <stdio.h>
int main()
{
int year,leap;
printf("enter year:");
scanf("%d",&year);
if(year%4==0)
{
if(year%100==0)
{
if(year%400==0)
leap=1;
else
leap=0;
}
else leap=1;
}
else
leap=0;
if(leap)
printf("%d is",year);
else
printf("%d is not",year);
printf("a leap year.\n");
return 0;
}

运行结果如下：

##例4.9

#include <stdio.h>
#include <math.h>
int main()
{
double a,b,c,disc,x1,x2,realpart,imagpart;
scanf("%lf,%lf,%lf",&a,&b,&c);
printf("The equation");
if(fabs(a)<=1e-6)
printf("is not a quadratic\n");
else
{
disc=b*b-4*a*c;
if(fabs(disc)<=1e-6)
printf("has tow equal roots:%8.4f\n",-b/(2*a));
else
if(disc>1e-6)
{
x1=(-b+sqrt(disc)/(2*a));
x2=(-b-sqrt(disc)/(2*a));
printf("has distinct real roots:%8.4f and%8.4f\n",x1,x2);
}
else
{
realpart=-b/(2*a);
imagpart=sqrt(-disc)/(2*a);
printf("has complex roots:\n");
printf("%8.4f+%8.4fi\n",realpart,imagpart);
printf("%8.4f-%8.4fi\n",realpart,imagpart);
}
}
return 0;
}

运行结果如下：



##例4.10

#iclude<stdio.h>
int main()
{
int c,s;
float p,w,d,f;
printf("please enter price,weight,discount:");
scanf("%f,%f,%d",&p,&w,&s);
if(s>=300) c=12;
else          c=s/250;
switch(c)
{
case 0: d=0;break;
case 1: d=2;break;
case 2:
case 3: d=5;break;
case 4:
case 5:
case 6:
case 7: d=8;break;
case 8:
case 9:
case 10:
case 11: d=10;break;
case 12: d=15;break;
}
f=p*w*s*(1-d/100);
printf("freight=%10.2f\n",f);
return 0;
}

运行结果如下：