Factorial Calculation Program in C++

This program calculates the factorial of a given number using C++. The factorial of a non-negative integer n is the product of all positive integers less than or equal to n. It is denoted by n! and is defined as:

        n! = n * (n - 1) * (n - 2) * ... * 1

For example:

• 5! = 5 * 4 * 3 * 2 * 1 = 120
• 4! = 4 * 3 * 2 * 1 = 24
• 3! = 3 * 2 * 1 = 6

Program Code

#include <iostream>

// Function to calculate factorial
unsigned long long factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

int main() {
    int number;
    std::cout << "Enter a number to calculate its factorial: ";
    std::cin >> number;

    if (number < 0) {
        std::cout << "Factorial is not defined for negative numbers." << std::endl;
    } else {
        unsigned long long result = factorial(number);
        std::cout << "The factorial of " << number << " is " << result << "." << std::endl;
    }

    return 0;
}
    

Explanation

Here is a detailed explanation of the program:

1. Include the iostream Library

#include <iostream>
    

This line includes the iostream library, which is used for input and output operations.

2. Factorial Function

unsigned long long factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}
    

This function calculates the factorial of a given number recursively. If the number is 0 or 1, it returns 1 (base case). Otherwise, it calls itself with n-1 and multiplies the result with n (recursive case).

3. Main Function

int main() {
    int number;
    std::cout << "Enter a number to calculate its factorial: ";
    std::cin >> number;

    if (number < 0) {
        std::cout << "Factorial is not defined for negative numbers." << std::endl;
    } else {
        unsigned long long result = factorial(number);
        std::cout << "The factorial of " << number << " is " << result << "." << std::endl;
    }

    return 0;
}
    

The main function does the following:

1. Prompts the user to enter a number.
2. Reads the input number.
3. Checks if the number is negative. If so, it prints a message stating that factorial is not defined for negative numbers.
4. If the number is non-negative, it calls the factorial function and stores the result.
5. Prints the factorial of the entered number.