Things I tend to forget in ruby.

group_arr.rb

#!/usr/bin/env ruby

arr = [ 
  [:a, [1,2,2,2,2,1]], 
  [:b, [3,3,3,3,3,4,4,4]],
  [:c, [8,8,8,8,8,8,8]]]

headers = arr.collect{|el| el[0]}

headers.each do |h|
  puts h
  p arr.group_by{|x| x[0] == h}[true][0]
end

arr = (1..100).to_a

arr.group_by{|i| i%3} 

# Above
# => {1=>[1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 
#         43, 46, 49, 52, 55, 58, 61, 64, 67, 70, 73, 76, 79, 
#         82, 85, 88, 91, 94, 97, 100], 
#     2=>[2, 5, 8, 11, 14, 17, 20, 23, 26, 29, 32, 35, 38, 41, 
#         44, 47, 50, 53, 56, 59, 62, 65, 68, 71, 74, 77, 80, 
#         83, 86, 89, 92, 95, 98], 
#     0=>[3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 
#         45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 
#         84, 87, 90, 93, 96, 99]}
#