🚀 Day 93/150 – Recursive Factorial in Python

Recursion is a programming technique where a function calls itself to solve a smaller version of the same problem. One of the most common examples of recursion is calculating the factorial of a number.

The factorial of a positive integer n is the product of all positive integers from 1 to n.

Formula:

5! = 5 × 4 × 3 × 2 × 1 = 120

In this post, we'll explore four different ways to calculate the factorial of a number using recursion and related approaches.

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Method 1 – Basic Recursive Function

A recursive function calls itself until it reaches the base case.

def factorial(n):
    if n == 0 or n == 1:
        return 1

    return n * factorial(n - 1)

print(factorial(5))

Output

120
 
Explanation

If n is 0 or 1, the function returns 1.
• Otherwise, it returns n × factorial(n - 1).
  The function keeps calling itself until the base case is reached.

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Method 2 – Taking User Input

Calculate the factorial recursively using user input.

def factorial(n):
    if n == 0 or n == 1:
        return 1

    return n * factorial(n - 1)

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

print("Factorial:", factorial(num))








Sample Input

6

Output

Factorial: 720
 
Explanation

• The user enters a number.
• The recursive function calculates its factorial.
• The result is displayed.

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Method 3 – Recursive Function with Error Handling

Prevent negative numbers from being processed.

def factorial(n):
    if n < 0:
        return "Factorial is not defined for negative numbers."

    if n == 0 or n == 1:
        return 1

    return n * factorial(n - 1)

print(factorial(-3))









Output

Factorial is not defined for negative numbers.

Explanation

• The function first checks for negative numbers.
• If the input is negative, it returns an error message.
• Otherwise, recursion continues normally.

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Method 4 – Recursive Function with Default Argument

Use a default argument to calculate the factorial of 5 if no value is provided.

def factorial(n=5):
    if n == 0 or n == 1:
        return 1

    return n * factorial(n - 1)

print(factorial())
print(factorial(4))

Explanation

• The function first checks whether the position is negative.
• If it is, an error message is returned.
• Otherwise, recursion proceeds normally. 

Output

120
24
Explanation

Comparison of Methods

Method	Best For
Basic Recursion	Learning recursion
User Input	Interactive programs
Error Handling	Validating input
Default Argument	Optional function parameters

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🔥 Key Takeaways

• Recursion is a technique where a function calls itself.
• Every recursive function must have a base case to stop the recursion.
• Factorial is one of the best examples for understanding recursion.
• Always validate input before performing recursive calculations.
• While recursion is elegant, iterative solutions are often more memory-efficient for very large inputs.