From: Gareth Adams <gareth.adams@...>
Date: 2006-11-29T22:55:37+09:00
Subject: Array#group_by

I posted this on the Rails list, but then I realised it would make more sense
over here:

-------

Just thought I'd share a function which has been really useful to me, for 2 
reasons

1) It could easily help someone else
2) People might be able to make it work better or point out where it's not 
optimal

class Array
  # Groups elements of an array based on a user-defined condition
  # 
  # Each element from this array is passed to the block, and the returned value
  # determines the key under which it will be stored in the output hash
  #
  # If no block is given then the array indices will be used as the hash keys
  def group_by
    hsh = {}
    self.dup.each_with_index do |element, i|
      if block_given?
        key = yield element
      else
        key = i
      end
      hsh[key] ||= []
      hsh[key] << element
    end
    hsh
  end
end