Why Does `gcdIter` Return `None` When Calculating GCD?

Understanding the Puzzling Behavior of Python Recursive Functions: Why Does gcdIter Return None?

When dealing with recursive functions, it's crucial to grasp their return mechanism. Consider the following gcdIter function, intended to calculate the greatest common divisor (GCD) of two numbers. However, for certain input values (e.g., 306 and 136), it unexpectedly returns None.

The issue lies in the recursive part of the code:

gcdIter (a,b%a)

While the function makes a recursive call to itself with the relevant parameters, it fails to handle the return value of that call. To retain the GCD result, the return value must be passed back up the chain:

return gcdIter (a,b%a)

Additionally, it's prudent to use multi-target variable assignments to streamline the code:

def gcdIter(a, b):
    a, b = min(a, b), max(a, b)
    if b % a == 0:
        return a
    return gcdIter(a, b % a)

By addressing these issues, gcdIter will consistently return the GCD for any given input values, as expected.

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