From: David Kastrup <dak@gnu.org>
To: emacs-devel@gnu.org
Subject: Re: [PATCH] use tail pointer for LOOP
Date: Wed, 16 Jun 2010 20:10:48 +0200 [thread overview]
Message-ID: <87fx0msv9z.fsf@lola.goethe.zz> (raw)
In-Reply-To: 20100616174420.GA2847@tomas
tomas@tuxteam.de writes:
> On Sat, May 29, 2010 at 08:49:13PM -0400, Daniel Colascione wrote:
>> On 5/29/10 8:45 PM, Ken Raeburn wrote:
>
> [...]
>
>> > Is it any faster to build the list in order? (Simply avoiding nreverse
>> > obviously makes things a little faster, but are you doing more work each
>> > time around the loop to maintain and use the tail pointer?)
>>
>> It's only a little bit more work to use the tail pointer [...]
>
> This has intrigued me for quite a while.
>
> Since I really should be doing tax declarations, I couldn't hold back for
> longer -- here is my (very unscientific) approach, to help you all
> procrastinate a bit too:
>
> | (defun copy1 (lst)
> | "Build up a copy of lst by consing up in reverse order, then
> | reversing"
> | (let ((res))
> | (while lst
> | (setq res (cons (car lst) res)
> | lst (cdr lst)))
> | (nreverse res)))
> |
> | (defun copy2 (lst)
> | "Build up a copy of lst by consing up in order, keeping a tail
> | pointer"
> | (when lst
> | (let ((res) (tail))
> | (setq res (cons (car lst) nil)
> | tail res
> | lst (cdr lst))
> | (while lst
> | (setcdr tail (cons (car lst) nil))
> | (setq tail (cdr tail)
> | lst (cdr lst)))
> | res)))
> |
> | (defun runtwo (n)
> | (let ((lst))
> | (while (> n 0)
> | (setq lst (cons n lst)
> | n (1- n)))
> | (garbage-collect)
> | (cons (benchmark-run (copy1 lst))
> | (benchmark-run (copy2 lst)))))
> |
> | (runtwo 1000000)
>
> (Maybe the tail pointer version could be done more elegantly: I'd be
> delighted to be taught more :)
>
> Turns out that the nreverse version is a tad faster (on my hardware, one
> of those Atom based netbooks, in case it matters) -- about 2.1 versus
> 2.7 seconds for a list of size 10^6. Garbage collections are comparable.
Did you byte-compile?
--
David Kastrup
next prev parent reply other threads:[~2010-06-16 18:10 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-29 21:56 O(N^2) behavior in LOOP Daniel Colascione
2010-05-29 22:06 ` Daniel Colascione
2010-05-29 22:14 ` Lennart Borgman
2010-05-29 22:35 ` Geoff Gole
2010-05-29 23:58 ` [PATCH] use tail pointer for LOOP (Was: Re: O(N^2) behavior in LOOP) Daniel Colascione
2010-05-30 0:45 ` Ken Raeburn
2010-05-30 0:49 ` Daniel Colascione
2010-06-16 17:44 ` tomas
2010-06-16 18:10 ` David Kastrup [this message]
2010-06-17 5:10 ` [PATCH] use tail pointer for LOOP tomas
2010-06-17 7:18 ` Thien-Thi Nguyen
2010-06-17 9:22 ` tomas
2010-06-17 10:03 ` Thien-Thi Nguyen
2010-06-17 14:05 ` tomas
2010-06-17 15:16 ` Thien-Thi Nguyen
2010-06-17 10:12 ` Thien-Thi Nguyen
2010-06-17 20:48 ` Stefan Monnier
2010-06-18 7:07 ` David Kastrup
2010-06-18 13:40 ` Stefan Monnier
2010-05-30 17:05 ` Štěpán Němec
2010-05-30 17:09 ` Daniel Colascione
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/emacs/
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87fx0msv9z.fsf@lola.goethe.zz \
--to=dak@gnu.org \
--cc=emacs-devel@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.savannah.gnu.org/cgit/emacs.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).