Why Does My Recursive GCD Function Return None?

Understanding the Return Value in Python Recursive Functions

In recursive functions, it's crucial to handle the return value of the recursive call correctly to obtain the desired result. Let's examine the provided code to understand why it returns None in certain cases.

The gcdIter function aims to find the greatest common divisor (GCD) of two numbers using an iterative approach. However, the issue lies in the recursive call:

<code class="python">gcdIter(a, b%a) </code>

The function fails to assign the return value of the recursive call. By default, Python returns None when a function doesn't explicitly return a value. Thus, when the function is called recursively, the none value overwrites the correct GCD value.

To address this, we need to pass on the return value of the recursive call with the return keyword:

<code class="python">return gcdIter(a, b % a)</code>

Now, the function assigns the GCD value obtained from the recursive call, ensuring that the correct result is returned.

Additionally, the code can be further simplified by using multiple target assignment in the function definition:

<code class="python">def gcdIter(a, b):
    a, b = min(a, b), max(a, b)
    if b % a == 0:
        return a
    return gcdIter(a, b % a)</code>

Alternatively, a more compact version of the GCD iterative function can be expressed as follows:

<code class="python">def gcd_iter(a, b):
    return gcd_iter(b, a % b) if b else abs(a)</code>

By correcting the recursive call and using appropriate return values, the gcdIter function can effectively find the GCD of two numbers without returning None in any case.

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