From: Niklas Frykholm <niklas@...>
Date: 2001-10-09T06:17:38+09:00
Subject: [ruby-talk:22262] Re: [Newbie] Switching from Perl : suffling

On Tue, Oct 09, 2001 at 04:58:24AM +0900, Dave Thomas wrote:
> How about something like
> 
>      class Array
>        def shuffle
>          each_index {|i| o = rand(size); self[i], self[o] = self[o], self[i] }
>        end
>      end

That code in fact results in a slightly biased shuffle. The easiest
way to realize that is probably this:

Suppose that we have an array with 3 items, then there are 3! = 6
different shuffles. Your code can take one of 3*3*3 = 27 different
paths. Each path ends in a particular shuffle. But since 6 does not
divide 27, the shuffles cannot be equally likely.

Or try

    count = Hash.new(0)
    100000.times {a = [1,2,3]; a.shuffle; count[a]+=1}
    p count

Here is a fixed shuffle

    class Array
        def shuffle!
            each_index {|i| o=rand(size-i); self[i], self[i+o] = \
                self[i+o], self[i]}
        end
    end

// Niklas