Exercism

Instructions

Test suite

Solution

Given a number, find the sum of all the unique multiples of particular numbers up to but not including that number.

If we list all the natural numbers below 20 that are multiples of 3 or 5, we get 3, 5, 6, 9, 10, 12, 15, and 18.

The sum of these multiples is 78.

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby sum_of_multiples_test.rb

To include color from the command line:

ruby -r minitest/pride sum_of_multiples_test.rb

Source

A variation on Problem 1 at Project Euler http://projecteuler.net/problem=1

Submitting Incomplete Solutions

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

sum_of_multiples_test.rb

require 'minitest/autorun'
require_relative 'sum_of_multiples'

# Common test data version: 1.2.0 fb5b0a1
class SumOfMultiplesTest < Minitest::Test
  def test_multiples_of_3_or_5_up_to_1
    # skip
    sum_of_multiples = SumOfMultiples.new(3, 5)
    assert_equal 0, sum_of_multiples.to(1)
  end

  def test_multiples_of_3_or_5_up_to_4
    skip
    sum_of_multiples = SumOfMultiples.new(3, 5)
    assert_equal 3, sum_of_multiples.to(4)
  end

  def test_multiples_of_3_up_to_7
    skip
    sum_of_multiples = SumOfMultiples.new(3)
    assert_equal 9, sum_of_multiples.to(7)
  end

  def test_multiples_of_3_or_5_up_to_10
    skip
    sum_of_multiples = SumOfMultiples.new(3, 5)
    assert_equal 23, sum_of_multiples.to(10)
  end

  def test_multiples_of_3_or_5_up_to_100
    skip
    sum_of_multiples = SumOfMultiples.new(3, 5)
    assert_equal 2_318, sum_of_multiples.to(100)
  end

  def test_multiples_of_3_or_5_up_to_1000
    skip
    sum_of_multiples = SumOfMultiples.new(3, 5)
    assert_equal 233_168, sum_of_multiples.to(1_000)
  end

  def test_multiples_of_7_13_or_17_up_to_20
    skip
    sum_of_multiples = SumOfMultiples.new(7, 13, 17)
    assert_equal 51, sum_of_multiples.to(20)
  end

  def test_multiples_of_4_or_6_up_to_15
    skip
    sum_of_multiples = SumOfMultiples.new(4, 6)
    assert_equal 30, sum_of_multiples.to(15)
  end

  def test_multiples_of_5_6_or_8_up_to_150
    skip
    sum_of_multiples = SumOfMultiples.new(5, 6, 8)
    assert_equal 4_419, sum_of_multiples.to(150)
  end

  def test_multiples_of_5_or_25_up_to_51
    skip
    sum_of_multiples = SumOfMultiples.new(5, 25)
    assert_equal 275, sum_of_multiples.to(51)
  end

  def test_multiples_of_43_or_47_up_to_10000
    skip
    sum_of_multiples = SumOfMultiples.new(43, 47)
    assert_equal 2_203_160, sum_of_multiples.to(10_000)
  end

  def test_multiples_of_1_up_to_100
    skip
    sum_of_multiples = SumOfMultiples.new(1)
    assert_equal 4_950, sum_of_multiples.to(100)
  end

  def test_multiples_of_an_empty_list_up_to_10000
    skip
    sum_of_multiples = SumOfMultiples.new()
    assert_equal 0, sum_of_multiples.to(10_000)
  end
end

class SumOfMultiples
  def initialize(*numbers)
    @numbers = numbers
  end

  def to(limit)
    @limit = limit
    natural_numbers.sum
  end

  private

  attr_reader :numbers, :limit

  def range
    (1...limit)
  end

  def natural_numbers
    range.select do |number|
      numbers.any? do |n|
        (number % n).zero?
      end
    end
  end
end

What can you learn from this solution?

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

What compromises have been made?
Are there new concepts here that I could read more about to develop my understanding?

PatrickMcSweeny's solution

to Sum Of Multiples in the Ruby Track

Source

Submitting Incomplete Solutions

sum_of_multiples_test.rb

What can you learn from this solution?