Avatar of chynh

chynh's solution

to Difference Of Squares in the TypeScript Track

Published at Oct 07 2019 · 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)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.

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)
    })

  })

})
class Squares {
  public squareOfSum: number;
  public sumOfSquares: number;
  public difference: number;
  constructor(n: number) {
    this.squareOfSum = (n * (n + 1) / 2) ** 2;
    this.sumOfSquares = new Array(n).fill('_').reduce((a, _, i) => a + ((i + 1) ** 2), 0)
    this.difference = this.squareOfSum - this.sumOfSquares;
  }
}

export default Squares;

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

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?