From: Brian Candler <B.Candler@...>
Date: 2007-05-10T00:03:17+09:00
Subject: Re: Implementation of the object.sort method.

On Wed, May 09, 2007 at 11:01:58PM +0900, Jorge Domenico Bucaran Romano wrote:
> I want a demonstrative implementation of the sort method to see how the 
> callback (passed block) is handled, specifically the comparison result 
> <=>.
> 
> For example, in the following code:
> 
> puts [4,5,3,2,1].sort do |a,b|
>   -1
> end
> 
> puts [4,5,3,2,1].sort do |a,b|
>   1
> end
> 
> Both print the array sorted down up. I don't understand how this is 
> possible

It's an artefact of the quicksort algorithm. If you lie to it about how the
elements compare, as you are doing above, then you'll get strange results.

> so I wanted to see a demonstrative (but factual) implementation 
> of the sort method to see how this parameters are handled.

# Noddy sort
def mysort(arr, &blk)
  blk ||= proc { |a,b| a <=> b }  # default if no block passed

  (0...arr.size-1).each do |i|
    (i+1...arr.size).each do |j|
      arr[i],arr[j] = arr[j],arr[i] if blk.call(arr[i],arr[j]) > 0
    end
  end
  arr
end

p mysort([4,5,3,2,1]) { |a,b| puts "Comparing #{a} and #{b}"; a <=> b }

Please don't use this as an example of a good sort algorithm! But it shows
how to do callbacks.

B.