Returns a hash, which keys are evaluated result from the block, and values are arrays of elements in enum corresponding to the key.

(1..6).group_by {|i| i%3} #=> {0=>[3, 6], 1=>[1, 4], 2=>[2, 5]}

This is included in Ruby 1.9 www.ruby-doc.org/core-1.9/classes/Enumerable.html#...

Implementation from Ruby on Rails: trunk/activesupport/lib/active_support/core_ext/enumerable.rb [rev 5334]

Source Code

# File merb/core_ext/enumerable.rb, line 43
def group_by
  inject({}) do |groups, element|
    (groups[yield(element)] ||= []) << element
    groups
  end
end