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Newsgroups: comp.lang.lisp
Subject: Re: How to reverse a list...
References: <5b829932.0112030950.3f96120b@posting.google.com> <3c0bf442$0$200$626a54ce@news.free.fr>
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From: Erik Naggum <erik@naggum.net>
Message-ID: <3216414699382977@naggum.net>
Organization: Naggum Software, Oslo, Norway
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Date: Tue, 04 Dec 2001 00:31:41 GMT
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* "Frederic Brunel" <brunel@mail.dotcom.fr>
| In my opinion, an elegant and typical functional way to do this is to use
| an auxiliary function taking a second argument to accumulate the result:
| 
| (defun reverse-aux (l acc)
|   (if (nilp l)
|       acc
|       (reverse-aux (cdr l)
|                    (append (list (car l)) acc))))
| 
| (defun my-reverse (l)
|   (reverse-aux l '()))
| 
| You could use the `label' special-form to declare the `reverse-aux'
| function as a local function within the `my-reverse' function or set the
| `acc' argument as an optional one!

  That would probably be elegant only in Scheme.

  Please note that (append (list elt) list) is functionally identical to
  (cons elt list), except that it copies and wastes the first argument.

  This is another way to do it:

(defun reverse-list (list)
  (do ((list list (cdr list))
       (rev () (cons (car list) rev)))
      ((endp list) rev)))

///
-- 
  The past is not more important than the future, despite what your culture
  has taught you.  Your future observations, conclusions, and beliefs are
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