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Published at Nov 24 2020
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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)Â² = 55Â² = 3025.

The sum of the squares of the first ten natural numbers is 1Â² + 2Â² + ... + 10Â² = 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.

Go through the project setup instructions for Xcode using Swift:

http://exercism.io/languages/swift

http://exercism.io/languages/swift/tests

Notably from the source directory:

`swift test`

runs tests

`swift package generate-xcodeproj`

creates an Xcode project

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.

```
import XCTest
@testable import DifferenceOfSquares
class DifferenceOfSquaresTests: XCTestCase {
func testSquareOfSumTo5() {
XCTAssertEqual(225, Squares(5).squareOfSum)
}
func testSumOfSquaresTo5() {
XCTAssertEqual(55, Squares(5).sumOfSquares)
}
func testDifferenceOfSquaresOfSumTo5() {
XCTAssertEqual(170, Squares(5).differenceOfSquares)
}
func testSquareOfSumTo10() {
XCTAssertEqual(3025, Squares(10).squareOfSum)
}
func testSumOfSquaresTo10() {
XCTAssertEqual(385, Squares(10).sumOfSquares)
}
func testDifferenceOfSquaresOfSumTo10() {
XCTAssertEqual(2640, Squares(10).differenceOfSquares)
}
func testSquareOfSumTo100() {
XCTAssertEqual(25_502_500, Squares(100).squareOfSum)
}
func testSumOfSquaresTo100() {
XCTAssertEqual(338_350, Squares(100).sumOfSquares)
}
func testDifferenceOfSquaresOfSumTo100() {
XCTAssertEqual(25_164_150, Squares(100).differenceOfSquares)
}
static var allTests: [(String, (DifferenceOfSquaresTests) -> () throws -> Void)] {
return [
("testSquareOfSumTo5", testSquareOfSumTo5),
("testSumOfSquaresTo5", testSumOfSquaresTo5),
("testDifferenceOfSquaresOfSumTo5", testDifferenceOfSquaresOfSumTo5),
("testSquareOfSumTo10", testSquareOfSumTo10),
("testSumOfSquaresTo10", testSumOfSquaresTo10),
("testDifferenceOfSquaresOfSumTo10", testDifferenceOfSquaresOfSumTo10),
("testSquareOfSumTo100", testSquareOfSumTo100),
("testSumOfSquaresTo100", testSumOfSquaresTo100),
("testDifferenceOfSquaresOfSumTo100", testDifferenceOfSquaresOfSumTo100),
]
}
}
```

```
import XCTest
@testable import DifferenceOfSquaresTests
XCTMain([
testCase(DifferenceOfSquaresTests.allTests),
])
```

```
//Solution goes in Sources
func square(_ n: Int) -> Int {
n * n
}
public class Squares {
let squareOfSum: Int
let sumOfSquares: Int
let differenceOfSquares: Int
init(_ n: Int) {
let numbers = (1...n).lazy
sumOfSquares = numbers.map(square).reduce(0, +)
squareOfSum = square(numbers.reduce(0, +))
differenceOfSquares = squareOfSum - sumOfSquares
}
}
```

A huge amount can be learned from reading other peopleâ€™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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