From: Francis Moreau <francis.moro@gmail.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Basic questions about elisp
Date: Thu, 5 Nov 2009 06:25:03 -0800 (PST) [thread overview]
Message-ID: <e99cc4e7-9e25-4368-a976-e4a8ed46f948@w19g2000yqk.googlegroups.com> (raw)
In-Reply-To: 87r5sdm8kn.fsf@lola.goethe.zz
On Nov 5, 1:59 pm, David Kastrup <d...@gnu.org> wrote:
> Francis Moreau <francis.m...@gmail.com> writes:
>
> > (defmacro x-nconc (l e)
> > `(if (null ,l) (setq ,l ,e) (nconc ,l ,e)))
>
> Actually, that's pretty stupid
thanks
> since it is exactly the same as
> (defmacro (l e) `(setq ,l (nconc ,l ,e)))
>
> It is less obscure to use nconc in the same manner as append, namely
> using the return value (and accepting the side-effect for efficiency's
> sake). And then you don't need your personal macro and get more
> readable code.
yes, I actually missed the use of the return value.
>
> The usual iteration would be something like
>
> (while lst
> <do something with (car lst)>
> <maybe append something to lst>
> (setq lst (cdr lst)))
>
> If you really want efficiency, you'll not append something to lst in
> order to avoid quadratic behavior. Then you'd rather do something like
>
> (let ((iter lst) app)
> (while
> (progn
> (while iter
> <do something with (car iter)>
> <maybe (push something app)>
> (setq lst (cdr lst)))
> app)
> (setq iter (nreverse app) app nil)))
>
hmm, it's probably my lack of elisp usage/knowledge, but it's terribly
hard to read for something really simple to achieve...
BTW, is '(setq lst (cdr lst)))' correct ?
> Note that this does not change the original list at all. If that's not
> what is desired, you can write the last setq as
>
> (setq iter (nreverse app) lst (nconc lst iter) app nil)
>
> This still is suboptimal since the nconc will repeatedly traverse the
> same elements.
Lists are really implemented as simply linked list ?
Thanks
next prev parent reply other threads:[~2009-11-05 14:25 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-11-05 11:13 Basic questions about elisp Francis Moreau
2009-11-05 11:50 ` Lennart Borgman
[not found] ` <mailman.10118.1257421858.2239.help-gnu-emacs@gnu.org>
2009-11-05 12:07 ` Francis Moreau
2009-11-05 12:44 ` Lennart Borgman
2009-11-05 12:59 ` David Kastrup
2009-11-05 14:25 ` Francis Moreau [this message]
2009-11-05 14:37 ` Pascal J. Bourguignon
2009-11-05 14:58 ` Pascal J. Bourguignon
2009-11-05 12:57 ` tomas
[not found] ` <mailman.10122.1257425638.2239.help-gnu-emacs@gnu.org>
2009-11-05 14:29 ` Francis Moreau
2009-11-05 14:41 ` Pascal J. Bourguignon
2009-11-05 15:06 ` David Kastrup
2009-11-06 16:03 ` Francis Moreau
2009-11-06 16:49 ` David Kastrup
2009-11-06 20:53 ` Francis Moreau
2009-11-06 21:18 ` Pascal J. Bourguignon
2009-11-07 14:49 ` Francis Moreau
2009-11-07 17:50 ` Pascal J. Bourguignon
2009-11-08 9:46 ` tomas
[not found] ` <mailman.10266.1257674088.2239.help-gnu-emacs@gnu.org>
2009-11-09 20:51 ` Francis Moreau
2009-11-08 15:18 ` Francis Moreau
2009-11-08 16:58 ` tomas
2009-11-08 17:12 ` Pascal J. Bourguignon
2009-11-09 21:04 ` Francis Moreau
2009-11-10 12:11 ` Joost Kremers
2009-11-10 14:16 ` Francis Moreau
2009-11-10 18:53 ` David Kastrup
2009-11-06 5:06 ` Barry Margolin
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