From: "trans.  (T. Onoma)" <transami@...>
Date: 2004-12-16T13:49:53+09:00
Subject: Re: [ANN] rand.rb 0.9: Random access methods for Enumerables

On Wednesday 15 December 2004 05:12 pm, Maarten Boonen wrote:
| Neil Spring schreef:
| > On Dec 14, 2004, at 7:24 PM, Ilmari Heikkinen wrote:
| >> rand.rb is a library for picking random elements and shuffling.
| >> This work is licensed under the same terms as Ruby itself.
| >
| > umm,  could you maybe replace:
| >
| >   def shuffle!
| >     sort!{rand <=> 0.5}
| >   end
| >
| > with:
| >
| >   def shuffle!
| >     each_index {|j| i = rand(size-j); self[j], self[j+i] = self[j+i],
| > self[j]}
| >     self
| >   end
|
| I made this a function a few days ago and it seems to be a lot faster.
| Not sure if it's completely correct though.
|
| def shuffle!
|  a = 0
|  while(a<length)
|   b = (length*Kernel.rand).to_i
|   tmp = self[a]
|   self[a] = self[b]
|   self[b] = tmp
|   a+=1
|  end
|  self
| end

Well, lets see... a couple of equivalent transformations and:

  def shuffle!
    ln = length
    ln.times do |a|
      b = Kernel.rand(ln)
      self[a], self[b] = self[b], self[a]
    end
    self
  end

If you look closely, it is almost the very same thing. The real difference 
lies in the the subtraction on the index being fed into #rand. I think this 
is to improve the randomness a bit.

How much faster does it run?

T.