c++做计算器，没有思路啊啊

问题：请开发一个小小的计算器，当用户输入两整数，计算器可以直接给出和差积商

供参考：

#include <stdio.h>
#include <string.h>
 
void plus(double a, double b)
{
    printf("%g + %g = %g\n", a, b, a + b);
}
void minus(double a, double b)
{
    printf("%g - %g = %g\n", a, b, a - b);
}
void multiply(double a, double b)
{
    printf("%g * %g = %g\n", a, b, a * b);
}
void divide(double a, double b)
{        
    printf("%g / %g = %.2f\n", a, b, a / b);
}
int main(void)
{
    int  ret;
    char in[64] = { 0 }, x[16], y[16], op[10];
    double a, b;//参加运算的两个数
    while (1)
    {
        gets(in);//输入算式
        if (strcmp(in, "exit") == 0) break; //输入“exit”时，退出计算器
        ret = sscanf(in, "%[0-9.] %[^0-9. ] %[0-9.]", x, op, y);
        if (ret == 3) {
            sscanf(x, "%lf", &a);
            sscanf(y, "%lf", &b);
            if (strcmp(op, "+") == 0)
                plus(a, b);
            else if (strcmp(op, "-") == 0)
                minus(a, b);
            else if (strcmp(op, "*") == 0)
                multiply(a, b);
            else if (strcmp(op, "/") == 0 && b != 0)
                divide(a, b);
            else
                printf("error!Please input again.\n");
        }
        else
            printf("error!Please input again.\n");
    }
    return 0;
}