Function doesn't return properly in recursive function?

Question

I was trying to write a recursive function that returns the collatz sequence of a number, but the code (shown below) doesn't work. How could this be circumvented and why does this happen?

If I write print(x) instead of return x, it can print fine and I can't figure out why it wouldn't be able to return x.

def collatz(x):
    if x[-1] == 1:
        return x
    elif x[-1] % 2 == 0:
        collatz(x + [x[-1]/2])
    elif x[-1] % 2 == 1:
        collatz(x + [3*x[-1] + 1])

You return x, but in recursive calls your should also return. Otherwise only innermost call returns x, but the outer ones don't return anything. Basically: collatz(x +... -> return collatz(x +... — h4z3
– h4z3, Commented Aug 21, 2019 at 8:56

Shahzad · Accepted Answer · 2019-08-21 08:58:16Z

2

You are missing the returns for the recursive calls. This is what you need:

def collatz(x):
    if x[-1] == 1:
        return x
    elif x[-1] % 2 == 0:
        return collatz(x + [x[-1]/2])
    elif x[-1] % 2 == 1:
        return collatz(x + [3*x[-1] + 1])

answered Aug 21, 2019 at 8:58

Shahzad

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Comments

zmbq · Accepted Answer · 2019-08-21 08:57:41Z

1

You forgot to return a value from the recursive calls. So each time you call recursively, you're actually returning None.

Add a return before each of the calls to collatz

answered Aug 21, 2019 at 8:57

zmbq

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