Help- Simple recursive function to build a list

[actuary77]

I am trying to write simple recursive function to build a list:


def rec(n,alist=[]):
_nl=alist[:]
print n,_nl
if n == 0:
print n,_nl
return _nl
else:
_nl=_nl+[n]
rec(n-1,_nl)

_nl=[]
_nl=rec(4)
print _nl

### shouldn't this work?

_nl=rec(4)




The output is:
4 []
3 [4]
2 [4, 3]
1 [4, 3, 2]
0 [4, 3, 2, 1]
0 [4, 3, 2, 1]
None
None


Question:
============
Why isn't the function returning a list?
Why is function returning None?

Any guidance from one of you pro's would be greatly appreciated.

Thanks,
DKoch
actuary77@eleme ntal-systems.com

[Roy Smith]

Re: Help- Simple recursive function to build a list

In article <mailman.3222.1 109731598.22381 .python-list@python.org >,
actuary77 <actuary77@elem ental-systems.com> wrote:
[color=blue]
> I am trying to write simple recursive function to build a list:
>
>
> def rec(n,alist=[]):
> _nl=alist[:]
> print n,_nl
> if n == 0:
> print n,_nl
> return _nl
> else:
> _nl=_nl+[n]
> rec(n-1,_nl)
>
> _nl=[]
> _nl=rec(4)
> print _nl
>
> ### shouldn't this work?
>
> _nl=rec(4)
>
>
>
>
> The output is:
> 4 []
> 3 [4]
> 2 [4, 3]
> 1 [4, 3, 2]
> 0 [4, 3, 2, 1]
> 0 [4, 3, 2, 1]
> None
> None
>
>
> Question:
> ============
> Why isn't the function returning a list?
> Why is function returning None?[/color]

I'm not 100% sure what you're trying to do here, but I suspect you left a
"return" out of the else block, i.e. "return rec(n-1, _nl)". That would
certainly explain why it's returning None.

What problem are you trying to solve, or are you just experimenting with
recursion?

[Kent Johnson]

Re: Help- Simple recursive function to build a list

actuary77 wrote:[color=blue]
> I am trying to write simple recursive function to build a list:
>
>
> def rec(n,alist=[]):
> _nl=alist[:]
> print n,_nl
> if n == 0:
> print n,_nl
> return _nl
> else:
> _nl=_nl+[n]
> rec(n-1,_nl)[/color]

should be
return rec(n-1,_nl)
[color=blue][color=green][color=darkred]
>>> def rec(n,alist=[]):[/color][/color][/color]
... _nl=alist[:]
... print n,_nl
... if n == 0:
... print n,_nl
... return _nl
... else:
... _nl=_nl+[n]
... return rec(n-1,_nl)
...[color=blue][color=green][color=darkred]
>>> _nl = rec(4)[/color][/color][/color]
4 []
3 [4]
2 [4, 3]
1 [4, 3, 2]
0 [4, 3, 2, 1]
0 [4, 3, 2, 1][color=blue][color=green][color=darkred]
>>> print _nl[/color][/color][/color]
[4, 3, 2, 1]

Kent