recursion

[John Machin]

On Sep 14, 10:04 pm, Marc 'BlackJack' Rintsch <bj_... at gmx.net> wrote:
> On Fri, 14 Sep 2007 13:40:17 +0200, Gigs_ wrote:
> > sorry i think that i express wrong. having problem with english
>
> > what i mean is how python knows to add all thing at the end of recursion
>
> Because you have written code that tells Python to do so.  ;-)
>
> >  >>> def f(l):
> >      if l == []:
> >          return []
> >      else:
> >          return f(l[1:]) + l[:1]
>
> > f([1,2,3])
>
> > recursion1   f([2,3]) + [1]
>
> > recursion2   f([3]) + [2]  or [2, 1]?
>
> > recursion3   f([]) + [3] or   [3, 2, 1]
>
> Both alternatives in recursion 2 and 3 are wrong.  You have to simply
> replace the function invocation by its result which gives:
>
>     f([1, 2, 3])
> r1  f([2, 3]) + [1]
> r2  f([3]) + [2] + [1]
> r3  f([]) + [3] + [2] + [1]
> r4  [] + [3] + [2] + [1]
>
> And now the calls return:
>
> r3  [3] + [2] + [1]
> r2  [3, 2] + [1]
> r1  [3, 2, 1]
>
> > i dont get all this
>
> >  >>> def f(l):
> >      if l == []:
> >    print l
> >          return []
> >      else:
> >          return f(l[1:]) + l[:1]
>
> >  >>> f([1,2,3])
> > []
> > [3, 2, 1]  # how this come here? how python save variables from each
> > recursion?
>
> There is not just one `l` but one distinct `l` in each call.
>

I reckon that what the OP wants is a simple explanation of how
function calls use a stack mechanism for arguments and local
variables, and how this allows recursive calls, unlike the good ol'
FORTRAN IV of blessed memory. Perhaps someone could oblige him?

I'd try but it's time for my beauty sleep :-)
<yawn>
Good night all
John