Avatar of davearonson

davearonson's solution

to Raindrops in the Elixir Track

Published at Jul 13 2018 · 5 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Convert a number to a string, the contents of which depend on the number's factors.

  • If the number has 3 as a factor, output 'Pling'.
  • If the number has 5 as a factor, output 'Plang'.
  • If the number has 7 as a factor, output 'Plong'.
  • If the number does not have 3, 5, or 7 as a factor, just pass the number's digits straight through.


  • 28's factors are 1, 2, 4, 7, 14, 28.
    • In raindrop-speak, this would be a simple "Plong".
  • 30's factors are 1, 2, 3, 5, 6, 10, 15, 30.
    • In raindrop-speak, this would be a "PlingPlang".
  • 34 has four factors: 1, 2, 17, and 34.
    • In raindrop-speak, this would be "34".

Running tests

Execute the tests with:

$ elixir raindrops_test.exs

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

For more detailed information about the Elixir track, please see the help page.


A variation on a famous interview question intended to weed out potential candidates. http://jumpstartlab.com

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("raindrops.exs", __DIR__)

ExUnit.configure(exclude: :pending, trace: true)

defmodule RaindropsTest do
  use ExUnit.Case

  # @tag :pending
  test "1" do
    assert Raindrops.convert(1) == "1"

  @tag :pending
  test "3" do
    assert Raindrops.convert(3) == "Pling"

  @tag :pending
  test "5" do
    assert Raindrops.convert(5) == "Plang"

  @tag :pending
  test "7" do
    assert Raindrops.convert(7) == "Plong"

  @tag :pending
  test "6" do
    assert Raindrops.convert(6) == "Pling"

  @tag :pending
  test "9" do
    assert Raindrops.convert(9) == "Pling"

  @tag :pending
  test "10" do
    assert Raindrops.convert(10) == "Plang"

  @tag :pending
  test "14" do
    assert Raindrops.convert(14) == "Plong"

  @tag :pending
  test "15" do
    assert Raindrops.convert(15) == "PlingPlang"

  @tag :pending
  test "21" do
    assert Raindrops.convert(21) == "PlingPlong"

  @tag :pending
  test "25" do
    assert Raindrops.convert(25) == "Plang"

  @tag :pending
  test "35" do
    assert Raindrops.convert(35) == "PlangPlong"

  @tag :pending
  test "49" do
    assert Raindrops.convert(49) == "Plong"

  @tag :pending
  test "52" do
    assert Raindrops.convert(52) == "52"

  @tag :pending
  test "105" do
    assert Raindrops.convert(105) == "PlingPlangPlong"

  @tag :pending
  test "12121" do
    assert Raindrops.convert(12121) == "12121"
defmodule Raindrops do

  @noises %{ 3 => "Pling", 5 => "Plang", 7 => "Plong" }

  @doc """
  Returns a string based on raindrop factors.

  - If the number contains 3 as a prime factor, output 'Pling'.
  - If the number contains 5 as a prime factor, output 'Plang'.
  - If the number contains 7 as a prime factor, output 'Plong'.
  - If the number does not contain 3, 5, or 7 as a prime factor,
    just pass the number's digits straight through.
  @spec convert(pos_integer) :: String.t
  def convert(number) do
    make_noises(number) || Integer.to_string(number)

  defp make_noises(number) do
    @noises |> Map.keys
            |> Enum.filter(&(rem(number, &1) == 0))
            |> Enum.map_join(&(@noises[&1]))
            |> presence

  defp presence("" ), do: nil
  defp presence(str), do: str


Community comments

Find this solution interesting? Ask the author a question to learn more.
Avatar of davearonson

And now even more elixirish, transforming Map.keys(@noises) to @noises |> Map.keys....

Avatar of frbaroni

Woow, this is amazing!

Avatar of ana-balica

This is brilliant. Thanks for providing this solution!

Avatar of davearonson

Thanks! I didn't really consider it all that brilliant -- the main thing that lets it work so simply is the presence function, which I simply swiped from Ruby on Rails. (I first looked to see if Elixir or Erlang had such a thing, but apparently not.) Great illustration of the importance of exposure to lots of different languages (which is one reason I'm doing this track in the first place), frameworks, etc.

Avatar of rchavarria

I think that make_noises(number) || Integer.to_string(number)

looks very JavaScript-ish.

Just joking, very nice solution. I didn't know I could use and or operator (||) like that. And I love that @noises name.

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