書き初め@2016

multiple_of_9.ex

defmodule Kakizome do
  def multiple_of_nine?(x) when not is_integer(x), do: false
  # negative integer
  def multiple_of_nine?(n) when n < 0, do: multiple_of_nine?(-n)
  # 0, 1, 2, 3, 4, 5, 6, 7, 8 -> false
  def multiple_of_nine?(n) when n <= 8 , do: false
  def multiple_of_nine?(9), do: true
  def multiple_of_nine?(n) do
    # Is sum-of-digits-in-decimal multiple of 9 ?
    n |> Integer.to_string |> String.codepoints |> \
      Enum.map(fn(x) -> String.to_integer(x) end) |> \
      Enum.sum |> multiple_of_nine?
  end
end

2016は9の倍数なので