A snippet I used for explaining the map/reduce paradigm

map_reduce.rb

>> words = "foo bar baz qux foo bar lala" 
#=> "foo bar baz qux foo bar lala"
>> # map (user-supplied)
..   mapped = words.split(/ /).map { |w| [w,1] } 
#=> [["foo", 1], ["bar", 1], ["baz", 1], ["qux", 1], ["foo", 1], ["bar", 1], ["lala", 1]]
>> # aggregate (built-in)
..   aggregated = mapped.group_by { |w, c| w } 
#=> {"foo"=>[["foo", 1], ["foo", 1]], "bar"=>[["bar", 1], ["bar", 1]], "baz"=>[["baz", 1]], "qux"=>[["qux", 1]], "lala"=>[["lala", 1]]}
>> # reduce (user-supplied)
>> aggregated.each_with_object({}) { |(k, v), h| h[k] = v.map(&:last).inject(:+) } 
#=> {"foo"=>2, "bar"=>2, "baz"=>1, "qux"=>1, "lala"=>1}