C语言程序基本的计算器功能

```c
#include <stdio.h>
// 函数声明
double add(double a, double b);
double subtract(double a, double b);
double multiply(double a, double b);
double divide(double a, double b);
int main() {
    double num1, num2, result;
    char operator;
    printf("请输入一个运算符 (+, -, *, /): ");
    scanf("%c", &operator);
    printf("请输入两个数字： ");
    scanf("%lf %lf", &num1, &num2);
    switch(operator) {
        case '+':
            result = add(num1, num2);
            break;
        case '-':
            result = subtract(num1, num2);
            break;
        case '*':
            result = multiply(num1, num2);
            break;
        case '/':
            if(num2 != 0.0) {
                result = divide(num1, num2);
            } else {
                printf("除数不能为0\n");
                return 1;
            }
            break;
        default:
            printf("错误的运算符\n");
            return 1;
    }
    printf("结果是： %.2lf\n", result);
    return 0;
}
// 函数定义
double add(double a, double b) {
    return a + b;
}
double subtract(double a, double b) {
    return a - b;
}
double multiply(double a, double b) {
    return a * b;
}
double divide(double a, double b) {
    return a / b;
}
```