!!

发个简单些的.
#include<stdio.h>
main()
{
    float x,y;
    char ch;
    printf("输入格式:2+5\n");
    scanf("%f%c%f",&x,&ch,&y);
    switch(ch)
    {
    case '+':printf("%f\n",x+y);
        break;
    case '-':printf("%f\n",x-y);
        break;
    case '*':printf("%f\n",x*y);
        break;
    case '/':if (y!=0)
                 printf("%f\n",x/y);
        else
            printf("除数不能为零\n");
        break;
    default:printf("输入有误\n");
    }
    return 0;
}