Why Does My Python Recursive Function Sometimes Return None?

Why Python Recursive Function Sometimes Returns None

Unlike iterative functions, recursive functions rely on their recursive calls to return values. When the function is called multiple times, the return values need to be handled appropriately to obtain the desired output.

Consider the following Python code:

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

This function aims to find the greatest common divisor of two numbers using recursion. However, it may return None for certain inputs because the recursive call's return value is not handled correctly.

<code class="python">print(gcdIter(306, 136)) # Returns None
print(gcdIter(42, 84)) # Returns 42</code>

To resolve this, the code needs to pass the recursive call's return value back to the main function. By modifying the line gcdIter(a, b % a) to return gcdIter(a, b % a), the function will correctly return the greatest common divisor.

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

Alternatively, the function can be rewritten more compactly as:

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

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