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tikaro's solution

to Difference Of Squares in the TypeScript Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

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.

Setup

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

http://exercism.io/languages/typescript

Requirements

Install assignment dependencies:

$ yarn install

Making the test suite pass

Execute the tests with:

$ yarn test

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 sums', () => {
      expect(squares.squareOfSums).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 sums', () => {
      expect(squares.squareOfSums).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 sums', () => {
      expect(squares.squareOfSums).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 {
    squareOfSums: number
    sumOfSquares: number
    difference: number

    upto: number = 0

    constructor(input: number) {
        this.squareOfSums = this._squareOfSums(input)
        this.sumOfSquares = this._sumOfSquares(input)
        this.difference = this._difference()
    }

    private _squareOfSums(upto: number): number {
        // the numbers are consecutive starting from 1,
        // so I can do this without a loop.
        var sum: number = (upto * ( upto +1 ))/2
        var squareofsums: number = sum * sum //could also be sum ** 2
        return squareofsums
    }

    private _sumOfSquares(upto: number): number {
        var sumofsquares: number = 0
        for (var index = 1; index <= upto; index++) {
            sumofsquares += (index * index) //could also be index ** 2
        }
        return sumofsquares
    }

    private _difference(): number {
        return this.squareOfSums - this.sumOfSquares
    }
}

Community comments

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Avatar of tikaro

I decided to use the simplest expression of "squared": number * number

...rather than the ES 2016 exponentiation operator I Googled up: number ** 2

...because I think this way is more readable.

I pinched the whole structure here from the example exercise:

Class declares properties like squareOfSums

Each property has a matching private method named the same as the property, with an underbar prefix: private _squareOfSums(upto: number)

The class constructor matches the public properties to the underbar-prefixed private methods: this._squareOfSums(input)

That seems like a lot of tooling, but it makes sense to me.

What can you learn from this solution?

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Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
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