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Re: [PATCH] use tail pointer for LOOP (Was: Re: O(N^2) behavior in LOOP)
From: |
tomas |
Subject: |
Re: [PATCH] use tail pointer for LOOP (Was: Re: O(N^2) behavior in LOOP) |
Date: |
Wed, 16 Jun 2010 19:44:20 +0200 |
User-agent: |
Mutt/1.5.15+20070412 (2007-04-11) |
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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.
For the very curious (and to add some scientific varnish to this ;-),
here's my Emacs:
GNU Emacs 23.1.91.1 (i486-pc-linux-gnu, GTK+ Version 2.12.12)
of 2010-01-11 on elegiac, modified by Debian
Enjoy -- and may this keep you too from doing more important things ;-)
Regards
- -- tomás
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- Re: [PATCH] use tail pointer for LOOP (Was: Re: O(N^2) behavior in LOOP),
tomas <=
- Re: [PATCH] use tail pointer for LOOP, David Kastrup, 2010/06/16
- Re: [PATCH] use tail pointer for LOOP, tomas, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, Thien-Thi Nguyen, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, tomas, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, Thien-Thi Nguyen, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, tomas, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, Thien-Thi Nguyen, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, Thien-Thi Nguyen, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, Stefan Monnier, 2010/06/17
- Re: [PATCH] use tail pointer for LOOP, David Kastrup, 2010/06/18