Introduction

The factorial of a number is the product of all positive integers less than or equal to that number. It is commonly denoted by n!. Factorials are widely used in combinatorics, algebra, calculus, and many other fields of mathematics.

The factorial of a number n is defined as:

• n! = n × (n-1) × (n-2) × ... × 1
• For example, 5! = 5 × 4 × 3 × 2 × 1 = 120
• Special case: 0! = 1 (by definition).

In this program, we will calculate the factorial of a given number using Python programming language.

Objective

The objective of this program is to compute the factorial of a user-supplied number. This will demonstrate basic programming concepts such as loops, functions, and input/output in Python.

Python Code

# Python program to calculate the factorial of a number

# Function to calculate factorial
def factorial(n):
    if n == 0 or n == 1:
        return 1
    else:
        result = 1
        for i in range(2, n + 1):
            result *= i
        return result

# Input from the user
number = int(input("Enter a number to find its factorial: "))

# Checking for negative numbers
if number < 0:
    print("Factorial is not defined for negative numbers.")
else:
    # Calculating factorial and displaying the result
    print(f"The factorial of {number} is {factorial(number)}")

Explanation of the Program

Let’s break down how this program works:

• Function Definition: The function factorial(n) calculates the factorial of a number. If n is 0 or 1, it returns 1 (since 0! = 1 and 1! = 1). Otherwise, it uses a for loop to multiply the integers from 2 to n together.
• User Input: The program prompts the user to input a number using the input() function. This number is then converted to an integer using int().
• Negative Numbers: The program checks if the input number is negative. Since factorials are only defined for non-negative integers, it will display an error message if the user enters a negative number.
• Displaying the Result: If the input is valid, the program calls the factorial() function and prints the result in a user-friendly format.

How to Run the Program

1. Install Python: Make sure Python is installed on your machine. You can download it from the official website: python.org.
2. Create a Python File: Create a new file with a .py extension, such as factorial_program.py.
3. Write the Code: Copy and paste the Python code provided above into your Python file.
4. Run the Program: Open a terminal or command prompt, navigate to the directory where your Python file is located, and run the program using the command: python factorial_program.py.
5. Enter a Number: When prompted, enter a number and the program will display its factorial.