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4d47's solution

to Difference Of Squares in the PHP 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.

Running the tests

  1. Go to the root of your PHP exercise directory, which is <EXERCISM_WORKSPACE>/php. To find the Exercism workspace run

     % exercism debug | grep Workspace
    
  2. Get PHPUnit if you don't have it already.

     % wget --no-check-certificate https://phar.phpunit.de/phpunit.phar
     % chmod +x phpunit.phar
    
  3. Execute the tests:

     % ./phpunit.phar difference-of-squares/difference-of-squares_test.php
    

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.php

<?php

require_once "difference-of-squares.php";

class SquaresTest extends PHPUnit\Framework\TestCase
{
    public function testSquareOfSumTo5()
    {
        $this->assertEquals(225, squareOfSum(5));
    }

    public function testSumOfSquaresTo5()
    {
        $this->assertEquals(55, sumOfSquares(5));
    }

    public function testDifferenceOfSumTo5()
    {
        $this->assertEquals(170, difference(5));
    }

    public function testSquareOfSumTo10()
    {
        $this->assertEquals(3025, squareOfSum(10));
    }

    public function testSumOfSquaresTo10()
    {
        $this->assertEquals(385, sumOfSquares(10));
    }

    public function testDifferenceOfSumTo10()
    {
        $this->assertEquals(2640, difference(10));
    }

    public function testSquareOfSumTo100()
    {
        $this->assertEquals(25502500, squareOfSum(100));
    }

    public function testSumOfSquaresTo100()
    {
        $this->assertEquals(338350, sumOfSquares(100));
    }

    public function testDifferenceOfSumTo100()
    {
        $this->assertEquals(25164150, difference(100));
    }
}
<?php

function squareOfSums($num) {
    return array_sum(range(1, $num)) ** 2;
}

function sumOfSquares($num) {
    return array_sum(array_map(square, range(1, $num)));
}

function difference($num) {
    return squareOfSums($num) - sumOfSquares($num);
}

function square($num) {
    return $num ** 2;
}

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?

Community comments

See what others have said about this solution
almost 2 years ago
davidsilvasmith says

I didn't know about array_sum! thanks!