 # w1zeman1p's solution

## to Difference Of Squares in the Ruby Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

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.

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 exercism help 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: 1.1.0 7a1108b
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

# Problems in exercism evolve over time, as we find better ways to ask
# questions.
# The version number refers to the version of the problem you solved,
#
# Define a constant named VERSION inside of the top level BookKeeping
# module, which may be placed near the end of your file.
#
# In your file, it will look like this:
#
# module BookKeeping
#   VERSION = 1 # Where the version number matches the one in the test.
# end
#
# http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

def test_bookkeeping
skip
assert_equal 4, BookKeeping::VERSION
end
end``````
``````class Squares
def initialize(number)
@number = number
end

def difference
(sum_of_squares - square_of_sum).abs
end

def square_of_sum
sum ** 2
end

def sum_of_squares
(number * (number + 1) * ((2 * number) + 1)) / 6
end

private

def sum
(number * (number + 1)) / 2
end
end

module BookKeeping
VERSION = 3
end``````