From: Erik Veenstra <erikveen@...>
Date: 2008-09-09T17:43:10+09:00
Subject: Re: Is this a sensible implementation for Array#group_by ?

Indeed, Facets does have an Enumerable#group_by. And it has an
Enumerable#cluster_by as well. And the latter is the one you're
looking for, because you want an Array and not a Hash.

Group_by uses each, because it's faster than inject.

gegroet,
Erik V. - http://www.erikveen.dds.nl/

----------------------------------------------------------------

 module Enumerable
   def group_by
      res        = {}
      each{|e| (res[yield(e)] ||= []) << e}
      res
    end

   def cluster_by(&block)
     #group_by(&block).values        # In case of unsortable keys.
     group_by(&block).sort.transpose.pop || []
   end
 end

----------------------------------------------------------------

 a       = %w(apple banana pear plum nectarine orange melon)

 a.group_by{|e| e.length}        # ==> {5=>["apple", "melon"],
6=>["banana", "orange"], 9=>["nectarine"], 4=>["pear", "plum"]}
 a.cluster_by{|e| e.length}      # ==> [["pear", "plum"], ["apple",
"melon"], ["banana", "orange"], ["nectarine"]]

----------------------------------------------------------------