Elixir 練習問題 ListsAndRecursion-7

以前に作った span 関数と内包表記を使って 2 から n までの素数。

defmodule MyList do
  def span(from, to) when from == to, do: [to]
  def span(from, to),                 do: [from | span(from + 1, to)]

  def primes(n) do
    for x <- span(2, n), is_prime(x), do: x
  end

  defp is_prime(2), do: true
  defp is_prime(3), do: true
  defp is_prime(n) do
    m = div(n, 2)
    Enum.all?(Enum.map(span(2, m), fn x -> rem(n, x) != 0 end))
  end
end

^o^ > iex practice_10_3.exs
Eshell V8.0  (abort with ^G)
Interactive Elixir (1.3.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> MyList.primes(50)
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]