From: Stefan Rusterholz <apeiros@...>
Date: 2008-08-24T19:17:11+09:00
Subject: Re: Array.shuffle/Array.shuffle!

Jeff Moore wrote:
> This works but I can't escape the nagging sense that there's a more
> concise approach (for one thing, it looks like there are way too
> transient objects to me).
> 
> Anyone have a better idea?
> 
> 
> class Array
> 
>   def shuffle
>     t_self = self.dup
>     t_size = self.size
>     result=[]
>     t_size.times { result << t_self.slice!(rand(t_self.size)) }
>     result
>   end
> 
>    def shuffle!
>     t_self = self.dup
>     t_size = self.size
>     self.clear
>     t_size.times { self << t_self.slice!(rand(t_self.size)) }
>     self
>   end
> 
> end
> 
> 
> Regards...

If you need a less predictable one than the sort_by { rand } then I 
suggest this:

class Array
  # Shuffle the array
  def shuffle!
    n = length
    for i in 0...n
      r = Kernel.rand(n-i)+i
      self[r], self[i] = self[i], self[r]
    end
    self
  end

  # Return a shuffled copy of the array
  def shuffle
    dup.shuffle!
  end
end

This shuffle is known as fisher-yates/knuth shuffle and is afaik by 
current means considered impossible to predict. Also its complexity is 
O(n) compared to the O(nlogn) of sort_by (which for many cases probably 
still is faster in ruby since it runs mainly in C, I didn't bench it, 
though).

As of 1.8.7 you have those methods already natively provided by ruby.

Regards
Stefan (apeiros)
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