Published at Jul 13 2018
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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
```

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

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

```
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
SQUARE = 2
private_constant :SQUARE
def initialize(number)
@sum = (1..number).method(:sum)
end
def square_of_sum
sum.call**SQUARE
end
def sum_of_squares
sum.call { |number| number**SQUARE }
end
def difference
square_of_sum - sum_of_squares
end
private
attr_reader :sum
end
```

A huge amount can be learned 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 you could read more about to improve your understanding?

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