MapReduce.rb

Expecte.rb

{1=>[1,1],
 3=>[1,1,1,1],
 5=>[1,1,1]
 7=>[1,1],
 2=>[1,1,1]}

Groupby.rb

require 'pp'

nums = [1,3,5,7,2,3,1,3,2,5,2,3,5,7]
maps = nums.map { |e| {:key=>e, :value=>1} }

hashes = maps.group_by { |hash| hash[:key] }
pp hashes

Result.rb

{1=>[{:key=>1, :value=>1}, {:key=>1, :value=>1}],
 3=>
  [{:key=>3, :value=>1},
   {:key=>3, :value=>1},
   {:key=>3, :value=>1},
   {:key=>3, :value=>1}],
 5=>[{:key=>5, :value=>1}, {:key=>5, :value=>1}, {:key=>5, :value=>1}],
 7=>[{:key=>7, :value=>1}, {:key=>7, :value=>1}],
 2=>[{:key=>2, :value=>1}, {:key=>2, :value=>1}, {:key=>2, :value=>1}]}

ผมเล่นด้วย

nums = [1,3,5,7,2,3,1,3,2,5,2,3,5,7]
hash = nums.each_with_object(Hash.new {[]}) { |v, hash| hash[v] <<= 1 }