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Groups > comp.lang.python > #7960

Re: Python and Lisp : car and cdr

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Ethan Furman <ethan@stoneleaf.us> writes:

>> def car(L):
>>     return L[0]
>> def cdr(L):
>>     return L[1]
>
> IANAL (I am not a Lisper), but shouldn't that be 'return L[1:]' ?

Not for the linked list implementation he presented.

>> def length(L):
>>     if not L: return 0
>>     return 1 + length(cdr(L))
>
> How is this different from regular ol' 'len' ?

len would just return 2 for every linked list, and would raise an
exception for empty list (represented by None in Lie's implementation).

A more Pythonic implementation would represent the linked list as a
first-class objects with car and cdr being attributes, allowing for
fairly natural expression of __len__, __iter__, etc.  For example:

class List(object):
    __slots__ = 'car', 'cdr'

    def __init__(self, it=()):
        it = iter(it)
        try:
            self.car = it.next()
        except StopIteration:
            pass
        else:
            self.cdr = List(it)

    def __len__(self):
        if not hasattr(self, 'cdr'):
            return 0
        return 1 + len(self.cdr)

    def __iter__(self):
        head = self
        while hasattr(head, 'cdr'):
            yield head.car
            head = head.cdr

    def __repr__(self):
        return "%s(%r)" % (type(self).__name__, list(self))

>>> l = List([1, 2, 3])
>>> l
List([1, 2, 3])
>>> l.car
1
>>> l.cdr
List([2, 3])
>>> l.cdr.cdr.car
3
>>> l.cdr.cdr.cdr
List([])
>>> tuple(l)
(1, 2, 3)

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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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