From: Ari Brown <ari@...>
Date: 2007-04-24T11:24:57+09:00
Subject: Re: Shuffle Method

I'm not gonna lie. You're a genius to me. Not only did you spot my  
problem and help me fix it, but you provided not 1, not 3, not 4, but  
2 solutions to help me. You rock.

Meh! Too many commands I haven't learned yet. But thank you SOOOOOO  
MUCHHHH

On Apr 23, 2007, at 1:17 AM, Daniel Martin wrote:
>  def recurssive_shuffle some_array
>    n = some_array.length
>    new_array = []
>    while n != 0
>      r = rand(n)
>    helper_array = []
>      while some_array.length != (r + 1)
>        helper_array.push some_array.pop
>      end
>      new_array.push some_array.pop
>      n = (n - 1)
>      helper_array.each do |item|
>        some_array.push item
>      end
>    end
>    puts new_array
>  end
>
> However, it occurs to me that this algorithm is easier to express as:
>
>  # "recursive" only has one "s"
>  def recursive_shuffle some_array
>    new_array = []
>    while ! some_array.empty?
>      r = rand(some_array.length)
>      new_array.push some_array.delete_at(r)
>      helper_array = some_array.slice!(r..-1)
>      helper_array.reverse!
>      some_array.concat helper_array
>    end
>    puts new_array
>  end
>
> It took me a while to realize that you were reversing the bits you'd
> pulled off.
>
> I'll also note that the reversing doesn't affect the randomness of
> your shuffle at all.  The last three lines of that while loop
> (everything dealing with helper_array) could be dropped without
> affecting anything.  The sort_by { rand } method is almost certainly
> faster, or you could go and implement a standard Fisher-Yates in-place
> shuffle:
>
> # shuffles some_array in-place
> def shuffle some_array
>   2.upto(some_array.length) do |n|
>     r = rand(n)
>     some_array[n-1],some_array[r] = some_array[r],some_array[n-1]
>   end
>   some_array
> end
>
> (or open up Array, and add this method as shuffle!)

-------------------------------------------------------|
~ Ari
crap my sig won't fit