C语言编程实现一个简单的计算器。此计算器应具备基本计算功能：加法、减法、乘法、除法、阶乘等运算，请利用模块化程序设计的思想设计实现。提示：可利用有参有返回值函数实现计算器的各项功能。

下面是一个简单的 C 语言计算器的实现，包括加法、减法、乘法、除法和阶乘等运算：

#include <stdio.h>

// 定义加法函数
double add(double a, double b) {
    return a + b;
}

// 定义减法函数
double subtract(double a, double b) {
    return a - b;
}

// 定义乘法函数
double multiply(double a, double b) {
    return a * b;
}

// 定义除法函数
double divide(double a, double b) {
    if (b != 0) {
        return a / b;
    } else {
        printf("Error: Division by zero!\n");
        return 0;
    }
}

// 定义阶乘函数
double factorial(double n) {
    double result = 1;
    if (n < 0) {
        printf("Error: Cannot compute factorial of a negative number!\n");
        return 0;
    } else {
        for (int i = 1; i <= n; i++) {
            result *= i;
        }
        return result;
    }
}

int main() {
    int choice;
    double num1, num2, result;
    printf("Welcome to the calculator program!\n");
    printf("Please select an operation:\n");
    printf("1. Addition\n");
    printf("2. Subtraction\n");
    printf("3. Multiplication\n");
    printf("4. Division\n");
    printf("5. Factorial\n");
    scanf("%d", &choice);
    switch (choice) {
        case 1:
            printf("Enter two numbers to add:\n");
            scanf("%lf %lf", &num1, &num2);
            result = add(num1, num2);
            printf("Result: %lf\n", result);
            break;
        case 2:
            printf("Enter two numbers to subtract:\n");
            scanf("%lf %lf", &num1, &num2);
            result = subtract(num1, num2);
            printf("Result: %lf\n", result);
            break;
        case 3:
            printf("Enter two numbers to multiply:\n");
            scanf("%lf %lf", &num1, &num2);
            result = multiply(num1, num2);
            printf("Result: %lf\n", result);
            break;
        case 4:
            printf("Enter two numbers to divide:\n");
            scanf("%lf %lf", &num1, &num2);
            result = divide(num1, num2);
            printf("Result: %lf\n", result);
            break;
        case 5:
            printf("Enter a number to compute its factorial:\n");
            scanf("%lf", &num1);
            result = factorial(num1);
            printf("Result: %lf\n", result);
            break;
        default:
            printf("Error: Invalid choice!\n");
            break;
    }
    return 0;
}

在程序运行时，用户可以选择要进行的操作，然后输入相应的数值，计算器将输出计算结果。例如，如果用户选择加法操作并输入数值 2 和 3，则程序将输出 5。