C语言 用户输入运算数和四则运算符，输出计算结果

用C语言中简单的语句可以实现四则运算：

1、if语句

#include<stdio.h>
int main()
{
    double num_left,num_right;
    char operate;
    printf("请输入四则运算:\n");
    scanf("%lf%c%lf",num_left,operate,num_right);

    if(operate=="+")
    {
        printf("结果是%f\n",num_left+num_right);
    }
    else if(operate=="-")
    {
        printf("结果是%f\n",num_left-num_right);
    }
    else if(operate=="*")
    {
        printf("结果是%f\n",num_left*num_right);
    }
    else if(operate=="/")
    {
        printf("结果是%f\n",num_left/num_right);
    }

    return 0;
}

2、switch...case...

#include<stdio.h>
int main()
{
    double num_left,num_right;
    char operate;
    printf("请输入四则运算:\n");
    scanf("%lf%c%lf",num_left,operate,num_right);

    switch(operate)
    case '+':
        printf("结果是%f\n",num_left+num_right);
        break;
    case '-':
        printf("结果是%f\n",num_left-num_right);
        break;
    case '*':
        printf("结果是%f\n",num_left*num_right);
        break;
    case '/':
        printf("结果是%f\n",num_left/num_right);
        break;
    return 0;
}