选择结构

一.if-else选择结构

1

if (grade>=90)
printf("A");
else if (grade>=80)
printf("B");
else if (grade>=70)
printf("C");
else if (grade>=60)
printf("D");
else
printf("E");

2.条件运算符：? :

• 当 a=3, b=2 时，a>b ? a+b : a-b 的值为 a+b，即 5；
• 当 a=2, b=3 时，表达式的值为 a-b，即 -1

 

例：闰年的判断

void main()
{
int year, bleap = 0;
printf("please input the year: ");
scanf("%d", &year);
if(year % 100 != 0)
{
if(year % 4 == 0)
bleap = 1;
}
else if(year % 400 == 0)
bleap = 1;
bleap ?
printf("%d is a leap year!", year) :
printf("%d is not a leap year!", year);
}

or

void main()
{
int year;
printf("please input the year: ");
scanf("%d", &year);
if((year % 100 != 0 && year % 4 == 0) || year % 400 == 0)
printf("%d is a leap year!", year);
else
printf ("%d is not a leap year!", year);
}

二.switch-case选择结构

grade=getchar();
switch(grade)
{
case 'A': printf("85~100\n");
case 'B': printf("70~84\n");
case 'C': printf("60~69\n");
case 'D': printf("<60\n");
default: printf("error\n");
}

例：任意输入两个数，和一个运算符，输出运算结果

void main()
{
int op1, op2, oprtr;
scanf("%d%d", &op1, &op2); 
oprtr=getchar();
switch(oprtr)
{
case '+': printf("=%d", op1+op2); break;
case '-': printf("=%d", op1-op2); break;
case '*': printf("=%d", op1*op2); break;
case '/': printf("=%d", op1/op2); break;
default: printf("operator is invalid!");
}
}

void main()
{
int op1, op2, oprtr, result, done=1;
scanf("%d%d", &op1, &op2); 
operator=getchar();
switch(oprtr)
{
case '+': result=op1+op2; break;
case '-': result=op1-op2; break;
case '*': result=op1*op2; break;
case '/': result=op1/op2; break;
default: done=0;
}
if (done)
printf("%d%c%d=%d", op1, oprtr, op2, result);
else 
printf("operator is invalid!");
}

例：根据输入的月份，给出当月的天数

void main()
{
int m, d;
scanf("%d", &m);
switch (m) 
{
case 1: case 3: case 5: case 7: case 8: case 10: case 12: d=31; break;
case 4: case 6: case 9: case 11: d=30; break;
case 2: d=28; break;
default: d=0;
}
d > 0 ? printf("%d", d) : printf("invalid input!");
}