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Re: Python and Lisp : car and cdr

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On 6/19/2011 9:24 AM, Steven D'Aprano wrote:

> No. Each cell in a Lisp-style linked list has exactly two elements, and
> in Python are usually implemented as nested tuples:
>
> (head, tail)  # Annoyingly, this is also known as (car, cdr).
>
> where head is the data value and tail is either another Lisp-style list
> or a marker for empty (such as the empty tuple () or None).
>
> So a one-element linked list might be given as:
>
> (42, None)
>
> A two element list:  (42, (43, None))
> Three element list:  (42, (43, (44, None)))
>
> and so forth. So while you could harmlessly use a slice L[1:], there is
> no point, since L[1:] will have at most a single element.

It should be noted that the head element of any 'list' can also be a 
'list' (as with Python lists),

t = { { (1,None), (2,(3,None)) ), ( (4,(5,None)), (6,None) ) )

so that the structure is actually a tree, which is a much more general 
data structure than a true sequence of atoms. But TREP (for 
tree-processing) is not as catchy as LISP (for list processing).

-- 
Terry Jan Reedy

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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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