default values to hashes (from ruby tapas)

gistfile1.rb

# a default value of array allows us to implicitly initialize accessed fields
h = Hash.new{ |h ,k| h[k] = []}
h['a'] << "c"
h['a'] << "d"

>> h
{
  "a" => [
    [0] "c",
    [1] "d"
  ]
}