[Tutor] Function returns 'None'

[Nick Raptis]

On 07/11/2010 04:59 PM, Dominik Danter wrote:
> Hello
>
> As en exercise I wrote the following function:
>
>
> def recursfac(x,carryover=1):
>     print 'x:',x,'carryover:', carryover
>     if x > 1:
>         carryover *= x
>         recursfac(x-1, carryover)
>     else:
>         return carryover
>
> print recursfac(3)
>
> Very much to my surprise I get the following output:
>
> x: 3 carryover: 1
> x: 2 carryover: 3
> x: 1 carryover: 6
> None
>
> Where did I go wrong?
>
Your problem is that you expect the "return" to exit the recursion 
altogether. Instead, carryover is passed to the previous level of the 
recursion, which has nothing more to execute and returns None.

So the first step to fix this would be to make sure that your function 
returns carryover no matter what:

def recursfac(x,carryover=1):
     print 'x:',x,'carryover:', carryover
     if x > 1:
         carryover *= x
         recursfac(x-1, carryover)
         return carryover
     else:
         return carryover

Or simply (to remove the code duplication):

def recursfac(x,carryover=1):
     print 'x:',x,'carryover:', carryover
     if x > 1:
         carryover *= x
         recursfac(x-1, carryover)
     return carryover

Now there's still one more problem. The output is this:

x: 3 carryover: 1
x: 2 carryover: 3
x: 1 carryover: 6
3

Why is it returning 3 istead of 6?

Well, the function didn't catch the returned carryover value on the way 
up, so it's just returns what it knows: the value of carryover that it 
self has computed.
Instead, you have to do this:

def recursfac(x,carryover=1):
     print 'x:',x,'carryover:', carryover
     if x > 1:
         carryover *= x
         carryover = recursfac(x-1, carryover)
     return carryover

And this returns
x: 3 carryover: 1
x: 2 carryover: 3
x: 1 carryover: 6
6

Done!

What you should learn from this is that, when doing recursion, figuring 
out what your function should do on the way up is as crucial as what you 
want it to do on the way down.

Nick