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to Raindrops in the Crystal Track

Published at Jul 13 2018 · 0 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".


Follow the setup instructions for Crystal here:


More help installing can be found here:


Making the Test Suit Pass

Execute the tests with:

$ crystal spec

In each test suite all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by changing pending to it.


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.


require "spec"
require "../src/*"

describe "Raindrops" do
  describe "#drops" do
    it "converts 1 to 1" do
      Raindrops.drops(1).should eq "1"

    pending "converts 3 to Pling" do
      Raindrops.drops(3).should eq "Pling"

    pending "converts 5 to Plang" do
      Raindrops.drops(5).should eq "Plang"

    pending "converts 7 to Plong" do
      Raindrops.drops(7).should eq "Plong"

    pending "converts 35 to PlangPlong" do
      Raindrops.drops(35).should eq "PlangPlong"

    pending "converts 52 to 52" do
      Raindrops.drops(52).should eq "52"

    pending "converts 105 to PlingPlangPlong" do
      Raindrops.drops(105).should eq "PlingPlangPlong"
class Raindrops
  @@factors = {
    3 => "Pling",
    5 => "Plang",
    7 => "Plong",

  def self.drops(n : Int) : String
    result = @@factors.select { |k| n.divisible_by?(k) }
    result.empty? ? n.to_s : result.values.join

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