An iterative algorithm for Fibonacci numbers

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The problem is that your return y is within the loop of your function. So after the first iteration, it will already stop and return the first value: 1. Except when n is 0, in which case the function is made to return 0 itself, and in case n is 1, when the for loop will not iterate even once, and no return is being execute (hence the None return value).

To fix this, just move the return y outside of the loop.

Alternative implementation

Following KebertX’s example, here is a solution I would personally make in Python. Of course, if you were to process many Fibonacci values, you might even want to combine those two solutions and create a cache for the numbers.

def f(n):
    a, b = 0, 1
    for i in range(0, n):
        a, b = b, a + b
    return a

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