#include "stdio.h"
#include "math.h"
void main()
{float data1,data2;
char a;
printf("input:");
scanf("%f,%f,%c",&data1,&data2,&a);
switch (a)
{case '+':printf("%f%c%f=%f\n",data1,a,data2,data1+data2);break;
case '-':printf("%f%c%f=%f\n",data1,a,data2,data1-data2);break;
case '*':printf("%f%c%f=%f\n",data1,a,data2,data1*data2);break;
case '/':
{
if (data2==0)
{
printf("不存在\n");
}
else
{
printf("%f%c%f=%f\n",data1,a,data2,data1/data2);
}
}break;
default:printf("无结果\n");
}
}

char类型也可以看作是整型，就是字符的ASCII码。
如getchar()的返回值是int型，getchar函数的返回值是输入字符的ASCII码。

看看标准是怎么说的？switch( 任何integer类型的表达式 )

integer类型又是什么呢，标准说其包含 char, signed integer types, unsigned integer types, enumerated types 四种。
signed integer types 包含 signed char, short, int, long, long long
unsigned integer types 包含 _Bool, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long