Day 31 : Python Program to Find Fibonacci Numbers 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)}.")

Code Explanation:

1. Factorial Function Definition:

def factorial(n):

    if n == 0 or n == 1:

        return 1

    else:

        return n * factorial(n - 1)

The function factorial(n) calculates the factorial of a number n recursively.

The base case for the recursion is when n is 0 or 1:

factorial(0) = 1

factorial(1) = 1

This is because the factorial of both 0 and 1 is defined as 1.

For any other value of n greater than 1, the function returns n * factorial(n - 1). This is the recursive call that keeps reducing n by 1 until it reaches 1.

For example:

factorial(5) will calculate 5 * factorial(4)

factorial(4) will calculate 4 * factorial(3)

factorial(3) will calculate 3 * factorial(2)

factorial(2) will calculate 2 * factorial(1)

Once factorial(1) is reached, it returns 1, and all the previous recursive calls return their results back, giving us the final answer.

2. Taking User Input:

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

This line takes an integer input from the user and stores it in the variable num.

input() reads input as a string, so int() is used to convert it into an integer.

3. Checking for Negative Input:

if num < 0:

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

else:

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

This block checks if the input number is negative.

If the number is negative, it prints: "Factorial is not defined for negative numbers." because the factorial is only defined for non-negative integers (0, 1, 2, 3, ...).

If the number is not negative, it calls the factorial() function and prints the result using print().

Example Execution:

Input:

Enter a number: 5

Step-by-Step Explanation:

The user inputs 5 which is stored in num.

The if condition checks if the number is negative (num < 0). Since 5 is not negative, it proceeds to the else block.

The factorial(5) function is called:

factorial(5) → 5 * factorial(4)

factorial(4) → 4 * factorial(3)

factorial(3) → 3 * factorial(2)

factorial(2) → 2 * factorial(1)

factorial(1) returns 1

Now, all the recursive calls return their results:

factorial(2) = 2 * 1 = 2

factorial(3) = 3 * 2 = 6

factorial(4) = 4 * 6 = 24

factorial(5) = 5 * 24 = 120

The final result 120 is printed: "The factorial of 5 is 120."

#source code --> clcoding.com