**π‘ Problem Formulation:** Finding the greatest common divisor (GCD) of two numbers is a common algorithmic problem. The GCD is the largest positive integer that divides both numbers without leaving a remainder. For instance, the GCD of 48 and 18 is 6. This article demonstrates how to compute the GCD of two numbers using various recursive methods in Python.

## Method 1: Euclidean Algorithm Using Subtraction

The Euclidean algorithm for GCD via subtraction is one of the oldest known algorithms. It involves recursively subtracting the smaller number from the larger one until both numbers are equal, which is the GCD. Functionally, this method takes two integers as input and returns their GCD.

Here’s an example:

def gcd_subtraction(a, b): if a == b: return a if a > b: return gcd_subtraction(a - b, b) return gcd_subtraction(a, b - a) print(gcd_subtraction(48, 18))

Output: 6

This code defines a recursive function named `gcd_subtraction()`

that computes the GCD of two numbers using the subtraction-based Euclidean algorithm. It recurses by reducing the larger number by the smaller one until they become equal, at which point the equal number is the GCD.

## Method 2: Euclidean Algorithm Using Modulus Operator

The Euclidean algorithm can also be implemented using the modulus operator, which is more efficient than the subtraction method. It recursively takes the modulus of the two numbers until the second number becomes zero. The first number at that point is the GCD. This method is quicker and handles larger inputs more effectively.

Here’s an example:

def gcd_modulus(a, b): if b == 0: return a return gcd_modulus(b, a % b) print(gcd_modulus(48, 18))

Output: 6

The function `gcd_modulus()`

computes the GCD by calling itself with the second number and the remainder of the first number divided by the second number. It stops when the second number is zero, at which time the first number is the GCD.

## Method 3: Optimized Euclidean Algorithm with Tail Recursion

This method optimizes the recursive Euclidean algorithm by using tail recursion. Tail recursion can help in improving performance and memory usage by allowing the Python interpreter to reuse stack frames. The logic is the same as the modulus method; however, the implementation may vary depending on the Python interpreter’s ability to optimize tail recursion.

Here’s an example:

def gcd_tail_recursive(a, b): return a if b == 0 else gcd_tail_recursive(b, a % b) print(gcd_tail_recursive(48, 18))

Output: 6

The function `gcd_tail_recursive()`

is a compact version of the modulus method, utilizing a conditional expression for the tail call. This implementation is more concise and can be optimized by the interpreter into an iterative loop to save stack memory.

## Method 4: Euclidean Algorithm Using Iteration as an Alternative

Even though the focus is on recursion, it might be useful to see its counterpart iterative Euclidean algorithm for comparison. It is not a recursive method but can inform understanding of the recursive approach. It tends to run extremely fast and with constant memory usage on all Python interpreters because there is no call stack overhead.

Here’s an example:

def gcd_iterative(a, b): while b: a, b = b, a % b return a print(gcd_iterative(48, 18))

Output: 6

The function `gcd_iterative()`

demonstrates the iterative approach. It uses a while-loop to perform the same logic as the recursive modulus method. This version is straightforward and efficient since it doesn’t involve any recursive calls, thus avoiding potential stack overflows for large inputs.

## Bonus One-Liner Method 5: Using Python’s Standard Library

Python’s standard library provides a built-in function `math.gcd()`

to compute the GCD of two numbers. It’s a simple one-liner that delegates the task to an efficient, well-tested library function.

Here’s an example:

import math print(math.gcd(48, 18))

Output: 6

The one-liner uses Python’s `math`

module, which includes a gcd function that is concise and reliable. This approach is ideal for those who seek simplicity and do not wish to implement GCD logic themselves.

## Summary/Discussion

**Method 1:**Subtraction-Based Euclidean Algorithm. Strengths: Intuitive understanding of the algorithm. Weaknesses: Less efficient with large numbers.**Method 2:**Modulus-Based Euclidean Algorithm. Strengths: More efficient than the subtraction method. Weaknesses: Still subject to the Python recursion depth limit for very large inputs.**Method 3:**Tail Recursive Method. Strengths: Python interpreters may optimize it. Weaknesses: Python does not officially support tail call optimization, so your mileage may vary.**Method 4:**Iterative Euclidean Algorithm. Strengths: Most efficient and reliable. Weaknesses: Not recursive (included for comparison purposes).**Method 5:**Python’s Standard Library. Strengths: Most straightforward and tested approach. Weaknesses: Not a custom solution, which might be required for educational or constrained environments.