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Re: Python and Lisp : car and cdr

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On 06/18/11 00:45, Franck Ditter wrote:
> Hi, I'm just wondering about the complexity of some Python operations 
> to mimic Lisp car and cdr in Python...
> 
> def length(L) :
>   if not L : return 0
>   return 1 + length(L[1:])
> 
> Should I think of the slice L[1:] as (cdr L) ? I mean, is the slice
> a copy of a segment of L, or do I actually get a pointer to something
> inside L ? Is the above function length O(n) or probably O(n^2) ? 
> Where are such implementation things (well) said ?
> 
> Thanks,
> 
>      franck

Your function does not mimic Lisp's car/cdr. This one does:


def car(L):
    return L[0]
def cdr(L):
    return L[1]
def length(L):
    if not L: return 0
    return 1 + length(cdr(L))

L = (a, (b, (c, (d, None))))
length(L)

is O(n)

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Thread

Python and Lisp : car and cdr Franck Ditter <franck@ditter.org> - 2011-06-17 16:45 +0200
  Re: Python and Lisp : car and cdr Ian Kelly <ian.g.kelly@gmail.com> - 2011-06-17 09:29 -0600
  Re: Python and Lisp : car and cdr Nobody <nobody@nowhere.com> - 2011-06-18 15:34 +0100
  Re: Python and Lisp : car and cdr Lie Ryan <lie.1296@gmail.com> - 2011-06-19 16:00 +1000
    Re: Python and Lisp : car and cdr Ethan Furman <ethan@stoneleaf.us> - 2011-06-19 05:56 -0700
      Re: Python and Lisp : car and cdr Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-06-19 13:24 +0000
        Re: Python and Lisp : car and cdr Terry Reedy <tjreedy@udel.edu> - 2011-06-19 12:20 -0400
          Re: Python and Lisp : car and cdr Teemu Likonen <tlikonen@iki.fi> - 2011-06-19 19:38 +0300
      Re: Python and Lisp : car and cdr Hrvoje Niksic <hniksic@xemacs.org> - 2011-06-19 16:26 +0200
    Re: Python and Lisp : car and cdr Chris Angelico <rosuav@gmail.com> - 2011-06-19 23:23 +1000
    Re: Python and Lisp : car and cdr "Elias Fotinis" <efotinis@yahoo.com> - 2011-06-19 16:24 +0300
    Re: Python and Lisp : car and cdr Ethan Furman <ethan@stoneleaf.us> - 2011-06-19 08:07 -0700
    Re: Python and Lisp : car and cdr Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-06-19 11:36 -0700

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