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PatrickMcSweeny's solution

to Difference Of Squares in the Ruby Track

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Instructions
Test suite
Solution

Find the difference between the square of the sum and the sum of the squares of the first N natural numbers.

The square of the sum of the first ten natural numbers is (1 + 2 + ... + 10)² = 55² = 3025.

The sum of the squares of the first ten natural numbers is 1² + 2² + ... + 10² = 385.

Hence the difference between the square of the sum of the first ten natural numbers and the sum of the squares of the first ten natural numbers is 3025 - 385 = 2640.


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 difference_of_squares_test.rb

To include color from the command line:

ruby -r minitest/pride difference_of_squares_test.rb

Source

Problem 6 at Project Euler http://projecteuler.net/problem=6

Submitting Incomplete Solutions

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

difference_of_squares_test.rb

require 'minitest/autorun'
require_relative 'difference_of_squares'

# Common test data version: 1.2.0 1b6851d
class DifferenceOfSquaresTest < Minitest::Test
  def test_square_of_sum_1
    # skip
    assert_equal 1, Squares.new(1).square_of_sum
  end

  def test_square_of_sum_5
    skip
    assert_equal 225, Squares.new(5).square_of_sum
  end

  def test_square_of_sum_100
    skip
    assert_equal 25_502_500, Squares.new(100).square_of_sum
  end

  def test_sum_of_squares_1
    skip
    assert_equal 1, Squares.new(1).sum_of_squares
  end

  def test_sum_of_squares_5
    skip
    assert_equal 55, Squares.new(5).sum_of_squares
  end

  def test_sum_of_squares_100
    skip
    assert_equal 338_350, Squares.new(100).sum_of_squares
  end

  def test_difference_of_squares_1
    skip
    assert_equal 0, Squares.new(1).difference
  end

  def test_difference_of_squares_5
    skip
    assert_equal 170, Squares.new(5).difference
  end

  def test_difference_of_squares_100
    skip
    assert_equal 25_164_150, Squares.new(100).difference
  end
end
class Squares
  def initialize(number)
    @range = (1..number)
  end

  def square_of_sum
    @range.reduce(:+)**2
  end

  def sum_of_squares
    @range.reduce { |sum, n| sum + n**2 }
  end

  def difference
    square_of_sum - sum_of_squares
  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?