From: Michael Ulm <michael.ulm@...>
Date: 2005-12-06T22:24:01+09:00
Subject: Re: Shuffling an array, sort_by{rand}'s bias (was Re: need some Ruby magic)

Mauricio Fern�ndez wrote:

> On Tue, Dec 06, 2005 at 08:23:52AM +0900, ara.t.howard@noaa.gov wrote:
> 
>>the big difference is that a value will have rand called multiple times for
> 
> 
> That might be the very reason why it is ultimately much more biased than
> sort_by{ rand }... I'm keeping the latter.

Indeed,

ar = [0, 1, 2]; result = Hash.new(0)
100000.times {result[ar.sort {rand <=> rand}] += 1}
p result
# => {[2, 1, 0]=>25235,
       [2, 0, 1]=>12285,
       [1, 0, 2]=>12426,
       [0, 1, 2]=>25111,
       [1, 2, 0]=>12468,
       [0, 2, 1]=>12475}

Michael

-- 
Michael Ulm
R&D Team
ISIS Information Systems Austria
tel: +43 2236 27551-219, fax: +43 2236 21081
e-mail: michael.ulm@isis-papyrus.com
Visit our Website: www.isis-papyrus.com