From: GOTO Kentaro <gotoken@...>
Date: 2002-09-13T09:56:25+09:00
Subject: Re: Multi-level sort idiom?

At Fri, 13 Sep 2002 09:21:37 +0900,
Mike Stok wrote:
> The result of nonzero? does get returned, effectively nonzero?
> transforms any number except 0 to itself, and 0 is transformed to a
> false value, nil.

Historically, nonzero? was introduced for very this kind of sort (what
is the correct name?).  But Array#<=> was introduced in no time after
nonzero? was.  Tadayoshi Funaba, who is the inventor of nonzero? and
Array#<=>, said that that Array#<=> might obsolets nonzero?.  He
thought that a user however could choose favorite one [ruby-list:8546].
I prefer Array#<=> way. 

In 1.7.x, we can do
  x.sort_by{|a| [a[1],a[2],a[0]]}

instead of
  x.sort{|a,b| [a[1],a[2],a[0]] <=> [b[1],b[2],b[0]]}

and the former is faster than the latter because sort_by implements
Schwartzian transform(*1) in C. 

sort_by in Ruby is defined in shim(*2) for 1.6.x as follows:
  module Enumerable
    def sort_by
      ary = map { |i| [yield(i), i] }
      ary.sort! { |a, b| a[0] <=> b[0] }
      ary.map! { |i| i[1] }
    end
  end

(*1) http://www.5sigma.com/perl/schwtr.html
(*2) http://www.ruby-lang.org/~knu/cgi-bin/cvsweb.cgi/shim/ruby16/lib/features/ruby18/enumerable.rb

-- Gotoken