From: Rick DeNatale <rick.denatale@...>
Date: 2007-04-30T11:45:36+09:00
Subject: Re: sorting by rand?

On 4/29/07, Peter Seebach <seebs@seebs.net> wrote:
> In message <40D2708E-0D0C-42E4-82B3-E31D93481880@mac.com>, Gary Wright writes:
> >
> >On Apr 29, 2007, at 10:01 PM, Peter Seebach wrote:
> >> So, two vaguely related questions:
> >> 1.  Is there a reasonably sane way to do this, should I ever find
> >> myself wanting one?
> >
> >Short answer:   array.sort_by { rand }
> >
> >Longer answer: http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-
> >talk/168963
>
> Well, okay.  It works in practice.  Is it guaranteed to work, and is
> it actually reasonable to expect the output to be randomly shuffled?

See my longer explanation of sort_by which crossed in the e-mail.

> Just for comparison, I tried:
>
> l = a.length
> a.each_index { |x| a[x] = a[rand(l)] }
>
> I'm pretty sure I got the generic shuffle algorithm right (Knuth FTW),
> and if so, it produces quality random results in about 1/4 the time.

Looks suspicious to me, you are replacing each element of x in turn
with a randomly selected element, which means that you are likely to
end up with a different set of elements in the end.  One of the
requirements of shuffling an array would seem to be that

   shuffle(a).sort == a.sort

Something like:

   a.each_index {|x| y=rand; a[x], a[y] = a[y], a[x]}

would seem to be a better approach, but a.sort_by {rand} does the same
thing more concisely.


-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/