文档介绍：选择型程序设计
if语句(条件选择语句)
if语句的三种形式
形式一:
格式:if (expression)
statement
执行过程:
expr
statement
非0
=0
例:if (x>y)
printf(“%d”,x);
expr
statement1
statement2
非0
=0
形式二:
格式:if (expression)
statement1
else
statement2
执行过程:
例:if (x>y) max=x;
else max=y;
第五章 C的分枝程序设计
形式三:
格式:
if ( expr1 ) statement1
else if (expr2 ) statement2
else if (expr3 ) statement3
…...
[ else statementn ]
expr1
statemnt1
非0
=0
expr2
expr3
statemntn
statemnt3
statemnt2
非0
非0
=0
=0
执行过程:
例:if (salary>1000) index=;
else if (salary>800) index=;
else if (salary>600) index=;
else if (salary>400) index=;
else index=0;
如:if(a==b&&x==y) printf(“a=b,x=y”);
if(3) printf(“OK”);
if(‘a’) printf(“%d”,’a’);
说明:
if后面的表达式类型任意
语句可以是复合语句
if(x) if(x!=0)
if(!x) if(x==0)
例考虑下面程序的输出结果:
#include <>
main()
{ int x,y;
scanf(“%d,%d”,&x,&y);
if(x>y)
x=y; y=x;
else
x++; y++;
printf(“%d,%d\n”,x,y);
}
Compile Error!
/**/
#include <>
main()
{ int x,y;
printf("Enter an integer:");
scanf("%d",&x);
y=x;
if(y<0)
y= -y;
printf("\ninteger:%d--->absolute value:%d\n",x,y);
}
例求一个数的绝对值
运行:Enter an integer:-12
integer:-12--->absolute value :12
/**/
#include <>
main()
{ int a,b;
printf("Enter integer a:");
scanf("%d",&a);
printf("Enter integer b:");
scanf("%d",&b);
if(a==b)
printf("a==b\n");
else
printf("a!=b\n");
}
例输入两个数并判断两数相等否
运行:Enter integer a:12
Enter integer b:12
a==b
运行:Enter integer a:12
Enter integer b:9
a!=b
/**/
#include <>
main()
{ char c;
printf("Enter a character:");
c=getchar();
if(c<0x20) printf("The character is a control character\n");
else if(c>='0'&&c<='9') printf("The character is a digit\n");
else if(c>='A'&&c<='Z') printf("The character is a capital letter\n");
else if(c>='a'&&c<='z') printf("The character is a lower letter\n");
else printf("The character is other character\n");
}
例判断输入字符种类
运行:Enter a character:
The character is a control character
运行:En