hash_oddity.rb

# what is going on with Hash.new([])?

lines = [
  [1,2],
  [3,4],
  [5,6],
  [5,6.5],
  [7,8]
]

present = Hash.new([])
lines.map do |item|
  present[item.first] << item.last
  "some important value"
end

# present = {} ?!?!?!?!

present = {}
lines.map do |item|
  present[item.first] ||= []
  present[item.first] << item.last
  "some important value"
end
# present = {5=>[6, 6.5], 1=>[2], 7=>[8], 3=>[4]}