challenges.exs

challenges.exs

defmodule Challenge do
  # Problem 1
  def sum(list) do
    _sum(list, 0)
  end
  defp _sum([], total), do: total
  defp _sum([head|tail], total), do: _sum(tail, total+head)

  # Problem 2
  def alt(list1, list2) do
    _alt(list1, list2, [])
  end
  defp _alt([], [], combined), do: Enum.reverse combined
  defp _alt([head1|tail1], [head2|tail2], combined), do: _alt(tail1, tail2, [head2|[head1|combined]])

  # Problem 3
  def fib(count) do
    _fib([0, 1], count - 2)
  end
  defp _fib(list, count) when count > 0 do
    left = Enum.at(list, -2)
    right = Enum.at(list, -1)
    _fib(List.insert_at(list, -1, left + right), count - 1)
  end
  defp _fib(list, _), do: list

  # Problem 4
  def arrange(list) do
    Enum.sort(list, fn(x, y) ->
      first_digit = fn(n) ->
        n
        |> Integer.to_string
        |> String.at(0)
        |> String.to_integer
      end

      first_digit.(x) > first_digit.(y)
    end)
    |> Enum.join
    |> String.to_integer
  end
end