From: "Pascal J. Bourguignon" <pjb@informatimago.com>
To: help-gnu-emacs@gnu.org
Subject: Re: random predicate function
Date: Mon, 13 Dec 2010 16:26:03 +0100 [thread overview]
Message-ID: <87mxo9r8xg.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: mailman.0.1292251427.11097.help-gnu-emacs@gnu.org
Tyler Smith <tyler.smith@eku.edu> writes:
> Hi,
>
> I'm trying to write a function that will randomly sort the paragraphs in
> a region. It seems to work ok, except that it doesn't seem very random.
> I think the function I'm using to randomly generate true and false
> values is sub-optimal. It often generates long strings of 't' or nil,
> such that either the paragraph order doesn't change at all for multiple calls
> to the function, or it simply reverses the order each time I call it.
>
> Any suggestions welcome! Thanks.
>
> Tyler
>
> Here is the function:
You shoud not use sort to randomize, because it's suboptimal
[ O(n*log(n)) at best instead of O(n) ].
And foremost, you should not use a predicate that is not a total order
because this usually gives invalid results.
Google for: shuffle algorithm.
You could instead put your paragraphs in a vector and use:
(defun shuffle (vector)
"Re-orders randomly the vector."
(loop
for i from (1- (length vector)) downto 1
do (rotatef (aref vector i) (aref vector (random i)))))
to shuffle them and then re-insert them.
--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.
next parent reply other threads:[~2010-12-13 15:26 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.0.1292251427.11097.help-gnu-emacs@gnu.org>
2010-12-13 15:26 ` Pascal J. Bourguignon [this message]
2010-12-13 17:16 ` random predicate function Ted Zlatanov
2010-12-13 17:48 ` Pascal J. Bourguignon
2010-12-15 14:51 ` Ted Zlatanov
2010-12-15 15:20 ` Pascal J. Bourguignon
2010-12-15 16:44 ` Ted Zlatanov
2010-12-15 17:28 ` Pascal J. Bourguignon
2010-12-15 18:39 ` Ted Zlatanov
2010-12-15 18:04 ` Drew Adams
2010-12-13 18:17 ` Tyler Smith
[not found] ` <mailman.2.1292264248.1009.help-gnu-emacs@gnu.org>
2010-12-13 18:38 ` Pascal J. Bourguignon
2010-12-13 19:05 ` Tyler Smith
2010-12-13 14:43 Tyler Smith
2010-12-13 15:37 ` Gary
2010-12-13 16:08 ` Tyler Smith
2010-12-13 16:16 ` Erik Iverson
2010-12-13 16:50 ` Tyler Smith
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