From: Jim Weirich <jim@...>
Date: 2006-03-17T22:01:40+09:00
Subject: Re: Card tricks with Ruby

francisrammeloo@hotmail.com wrote:
>>> cards = cards.sort_by { rand }
> 
> How does this work??
> I thought the code block should yield either -1, 0 or +1, but rand
> yields a number between 0 and 1. Or am I mistaken?
> 

You are thing of the normal sort method which looks like this:

   cards.sort { |a, b|
     (a.suit <=> b.suit).nonzero? || (a.value <=> b.value)
   }

With sort_by, you need to return a key value that will be used as the 
comparison key for that particular value.

   cards.sort_by { |card|  [card.suit, card.value] }

Sort_by actually does a "Schwartzian Transform"[1] on the array to 
perform the sorting.

--
-- Jim Weirich

[1] A trick developed by Randall Schwartz to speed sorting in Perl.  See 
http://en.wikipedia.org/wiki/Schwartzian_Transform

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