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JesseSingleton's solution

to Difference Of Squares in the TypeScript Track

Published at Jul 13 2018 · 0 comments
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.

You are not expected to discover an efficient solution to this yourself from first principles; research is allowed, indeed, encouraged. Finding the best algorithm for the problem is a key skill in software engineering.

Setup

Go through the setup instructions for TypeScript to install the necessary dependencies:

https://exercism.io/tracks/typescript/installation

Requirements

Install assignment dependencies:

``````\$ yarn install
``````

Making the test suite pass

Execute the tests with:

``````\$ yarn test
``````

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing `xit` to `it`.

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.ts

``````import Squares from './difference-of-squares'

describe('Squares', () => {

describe('up to 5', () => {
const squares = new Squares(5)

it('gets the square of sum', () => {
expect(squares.squareOfSum).toBe(225)
})

xit('gets the sum of squares', () => {
expect(squares.sumOfSquares).toBe(55)
})

xit('gets the difference', () => {
expect(squares.difference).toBe(170)
})

})

describe('up to 10', () => {
const squares = new Squares(10)

xit('gets the square of sum', () => {
expect(squares.squareOfSum).toBe(3025)
})

xit('gets the sum of squares', () => {
expect(squares.sumOfSquares).toBe(385)
})

xit('gets the difference', () => {
expect(squares.difference).toBe(2640)
})

})

describe('up to 100', () => {
const squares = new Squares(100)

xit('gets the square of sum', () => {
expect(squares.squareOfSum).toBe(25502500)
})

xit('gets the sum of squares', () => {
expect(squares.sumOfSquares).toBe(338350)
})

xit('gets the difference', () => {
expect(squares.difference).toBe(25164150)
})

})

})``````
``````export default class Squares {
public squareOfSum: number;
public sumOfSquares: number;
public difference: number;

public constructor(n: number) {
this.squareOfSum = this._squareOfSum(n);
this.sumOfSquares = this._sumOfSquares(n);
this.difference = this.squareOfSum - this.sumOfSquares;
}

private _squareOfSum(n: number) {
const sum = this._range(n).reduce(
(acc: number, curr: number) => (acc += curr)
);

return sum * sum;
}

private _sumOfSquares(n: number) {
const range = this._range(n);

return range.reduce((acc: number, curr: number) => (acc += curr ** 2));
}

private _range = (count: number): number[] =>
Array(count)
.fill(1)
.map((n: number, i: number) => n + i);
}``````