#include <stdio.h>

int main()
{
    float data1, data2;
    char op;

    while (3 == scanf("%f%c%f", &data1, &op, &data2) )
    {
        float result;

        printf("%.6lf%c%.6lf=", data1, op, data2);

        switch (op)
        {
        case '+':
            result = data1 + data2;
            break;
        case '-':
            result = data1 - data2;
            break;
        case '*':
            result = data1 * data2;
            break;
        case '/':
            result = data1 / data2;
            break;
        default:;
        }

        printf("%.6lf\n", result);
    }
    return 0;
}


2.操作符增加和”%”，操作数增加非负浮点数（如2.0，816.73等）

取余是对整数而言的，浮点数哪来的取余呢？
   程序也有错误吧：while (3 == scanf("%f%c%f", &data1, &op, &data2) )  输入字符应该在第一个输入，不然你输浮点数后的空格就会赋给op了

case '%':
            result = (int)data1 % (int)data2;
            break;
这样可以吗？？

while (3 == scanf("%f%c%f", &data1, &op, &data2) )
什么意思这句，从来没看见这么用过？？