From: Rick DeNatale <rick.denatale@...>
Date: 2007-04-30T11:37:12+09:00
Subject: Re: sorting by rand?

On 4/29/07, Peter Seebach <seebs@seebs.net> wrote:
> Browsing something at a bookstore recently, I saw an example of
> an array shuffle allegedly implemented with
>         x.sort { rand }
> or something to that effect.

> This seems sort of unlikely, to me, to be a correct shuffling
> algorithm.  I'm not even sure it's safe at all.  I know that
> doing the same thing to C qsort implementations occasionally
> causes one to blow up spectacularly, because its assumption
> that the comparison between any two items is constant gets
> broken.
>
> So, two vaguely related questions:
> 1.  Is there a reasonably sane way to do this, should I ever find
> myself wanting one?
> 2.  Am I right in guessing that handing garbage to Array#sort
> is probably crazy?

I believe that the "something to that effect was probably:

     x.sort_by {rand}

sort_by takes a block which normally takes one argument.  sort_by
calls this block once for each element of the enumeration and
associates that value with the element, it then sorts those values and
returns an array of the asssociated elements in the order of the
values. So for example:

(1..50).sort_by{|e| -e}
    ==>[50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37, 36,
35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19,
18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

The block without the argument just assignes a random value to each element.

Perfectly safe as far as I can see.

x.sort {rand}

doesn't really work, whether or not it's safe.  Rand without an
argument returns a float >= 0.0 and < 1.0, the block for sort is
expected to return -1, 0, or 1. Depending on the implementaiton of the
particular sort method, you might get different results.  For arrays,
it looks like you'll get the same results with repeated calls:

irb(main):020:0> (1..10).to_a.sort {rand}
=> [10, 6, 1, 7, 3, 8, 5, 9, 4, 2]
irb(main):021:0> (1..10).to_a.sort {rand}
=> [10, 6, 1, 7, 3, 8, 5, 9, 4, 2]

Whereas sort_by seems to do what's expected.

irb(main):022:0> (1..10).to_a.sort_by {rand}
=> [5, 9, 2, 1, 7, 6, 4, 10, 8, 3]
irb(main):023:0> (1..10).to_a.sort_by {rand}
=> [4, 8, 5, 6, 2, 3, 10, 7, 1, 9]

-- 
Rick DeNatale

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