From: Martin DeMello <martindemello@...>
Date: 2003-07-31T02:52:21+09:00
Subject: collect_by

For all you FPers out there - is this a defined function in any
language's standard library? And if so, what is it called?

module Enumerable
  def collect_by

    h = {}
    each {|i|
      j = yield i
      (h[j] ||= []) << i
    }
    h

    # or, as a one-liner
    # inject({}) {|c, e| (c[yield e] ||= []) << e; c}
  end
end

a = [1,2,3,4,5,6,7,8,9,10]
p a.collect_by {|i| i % 2} #=> {0=>[2, 4, 6, 8, 10], 1=>[1, 3, 5, 7, 9]}

martin