Day 33 : Python Program to Find the Factorial of a Number using Recursion

 

def factorial(n):

    if n == 0 or n == 1:

        return 1

    else:

    return n * factorial(n - 1)

num = int(input("Enter a number: "))

if num < 0:

    print("Factorial is not defined for negative numbers.")

else:

    print(f"The factorial of {num} is {factorial(num)}.")

#source code --> clcoding.com 

Code Explanation:

1. Function Definition: factorial(n)

This function computes the factorial of a given number 

𝑛

n using recursion.

def factorial(n):

    if n == 0 or n == 1:

        return 1

    else:

        return n * factorial(n - 1)

Base Case:

if n == 0 or n == 1:

    return 1

If 

𝑛

n is 0 or 1, the factorial is defined as 1. This serves as the stopping condition for the recursion.

Recursive Case:

return n * factorial(n - 1)

n by the result. This continues until the base case is reached.

How Recursion Works: 

factorial(5) = 5 * factorial(4)

factorial(4) = 4 * factorial(3)

factorial(3) = 3 * factorial(2)

factorial(2) = 2 * factorial(1)

factorial(1) = 1  (Base Case)

Substituting back:

factorial(5) = 5 * 4 * 3 * 2 * 1 = 120

Taking Input from the User

num = int(input("Enter a number: "))

Prompts the user to enter a number.

Converts the user input (a string) into an integer using int().

Handling Negative Input

if num < 0:

    print("Factorial is not defined for negative numbers.")

Factorial is defined only for non-negative integers.

If the input number is negative, the program prints an error message and does not proceed further.

Computing and Printing the Factorial

else:

    print(f"The factorial of {num} is {factorial(num)}.")

If the input is valid (a non-negative number):

The program calls the factorial function with the input num.

The result is printed in a formatted string: f"The factorial of {num} is {factorial(num)}.".