Returning Functions or Values: recursive python functions

Question

Would anyone be willing to explain why the first does not work, but the second does?

In the first, the function calculates the final, adjusted value...

# returns None
def _pareRotation(degs):
    if degs > 360:          
        _pareRotation(degs - 360)
    else:
        print "returning %s" % degs
        return degs

...but returns None:

print _pareRotation(540)
>> returning 180
>> None

However, if we flip things a bit and return the function...

# returns expected results
def _pareRotation(degs):
    if degs < 360:          
        print "returning %s" % degs     
        return degs
    else:
        return _pareRotation(degs - 360)

...it works as expected:

print _pareRotation(540)
>> returning 180
>> 180

Mostly, wondering what causes that None to get ejected from the recursive loop?

Moses Koledoye · Accepted Answer · 2016-07-12 19:08:30Z

4

You're not returning in the first case:

def _pareRotation(degs):
    if degs > 360:          
        _pareRotation(degs - 360)
#      ^

answered Jul 12, 2016 at 19:08

Moses Koledoye

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2 Comments

Two-Bit Alchemist Over a year ago

There should be a canonical question for this if there isn't already. This kind of recursion failure comes up 5 times a day on this tag at least.

ghukill Over a year ago

Good grief, and there it is. Thank you. I agree @Two-BitAlchemist, but there might be some subtle variance in the questions. I had trouble grokking the idea that the target variable -- let's say final_degs -- would in a sense "become" the recursive function. When I run final_degs = _pareRotation(540), and I print or use final_degs, I'm actually using a variable that has gone through a few transformations (function to int). Is that an accurate way of thinking about it?

Hallowizer · Accepted Answer · 2016-07-12 19:10:35Z

0

Yeah, you aren't returning in the first case, and also I think it should be %d, for int.

answered Jul 12, 2016 at 19:10

Hallowizer

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