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lydiacupery's solution

to List Ops in the TypeScript Track

Published at Apr 01 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Implement basic list operations.

In functional languages list operations like length, map, and reduce are very common. Implement a series of basic list operations, without using existing functions.

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

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

list-ops.test.ts

import List from './list-ops'

describe('append entries to a list and return the new list', () => {
    it('empty lists', () => {
        const list1 = new List()
        const list2 = new List()
        expect(list1.append(list2)).toEqual(new List())
    })

    xit('empty list to list', () => {
        const list1 = new List([1, 2, 3, 4])
        const list2 = new List<number>()
        expect(list1.append(list2)).toEqual(list1)
    })

    xit('non-empty lists', () => {
        const list1 = new List([1, 2])
        const list2 = new List([2, 3, 4, 5])
        expect(list1.append(list2).values).toEqual([1, 2, 2, 3, 4, 5])
    })
})

describe('concat lists and lists of lists into new list', () => {
    xit('empty list', () => {
        const list1 = new List()
        const list2 = new List([])
        expect(list1.concat(list2).values).toEqual([])
    })

    xit('list of lists', () => {
        const list1 = new List([1, 2])
        const list2 = new List([3])
        const list3 = new List([])
        const list4 = new List([4, 5, 6])
        const listOfLists = new List([list2, list3, list4])
        expect(list1.concat(listOfLists).values).toEqual([1, 2, 3, 4, 5, 6])
    })
})

describe('filter list returning only values that satisfy the filter function', () => {
    xit('empty list', () => {
        const list1 = new List([])
        expect(list1.filter((el: number) => el % 2 === 1).values).toEqual([])
    })

    xit('non empty list', () => {
        const list1 = new List([1, 2, 3, 5])
        expect(list1.filter((el: number) => el % 2 === 1).values).toEqual([1, 3, 5])
    })
})

describe('returns the length of a list', () => {
    xit('empty list', () => {
        const list1 = new List()
        expect(list1.length()).toEqual(0)
    })

    xit('non-empty list', () => {
        const list1 = new List([1, 2, 3, 4])
        expect(list1.length()).toEqual(4)
    })
})

describe('returns a list of elements whose values equal the list value transformed by the mapping function', () => {
    xit('empty list', () => {
        const list1 = new List()
        expect(list1.map((el: number) => ++el).values).toEqual([])
    })

    xit('non-empty list', () => {
        const list1 = new List([1, 3, 5, 7])
        expect(list1.map((el: number) => ++el).values).toEqual([2, 4, 6, 8])
    })
})

describe('folds (reduces) the given list from the left with a function', () => {
    xit('empty list', () => {
        const list1 = new List()
        expect(list1.foldl((acc: number, el: number) => el / acc, 2)).toEqual(2)
    })

    xit('division of integers', () => {
        const list1 = new List([1, 2, 3, 4])
        expect(list1.foldl((acc: number, el: number) => el / acc, 24)).toEqual(64)
    })
})

describe('folds (reduces) the given list from the right with a function', () => {
    xit('empty list', () => {
        const list1 = new List()
        expect(list1.foldr((acc: number, el: number) => el / acc, 2)).toEqual(2)
    })

    xit('division of integers', () => {
        const list1 = new List([1, 2, 3, 4])
        expect(list1.foldr((acc: number, el: number) => el / acc, 24)).toEqual(9)
    })
})

describe('reverse the elements of a list', () => {
    xit('empty list', () => {
        const list1 = new List()
        expect(list1.reverse().values).toEqual([])
    })

    xit('non-empty list', () => {
        const list1 = new List([1, 3, 5, 7])
        expect(list1.reverse().values).toEqual([7, 5, 3, 1])
    })
})
export default class List<T> {
  public values: T[] = []

  constructor(items?: T[]){
    if(items){
      this.values = items 
    }
  }

   append(list: List<T>): List<T>{
     for(const i in list.values){
       this.values.push(list.values[i])
     }
    return this
  }

  concat(listOfLists: List<List<T>>): List<T>{
    for(const i in listOfLists.values){
      this.append(listOfLists.values[i])
    }
    return this
  }

  filter(fn: (el: T) => boolean): List<T>{
    let newValues: T[] = []
    for(const i in this.values){
      if(fn(this.values[i])) {
        newValues.push(this.values[i])
      }
    }
    this.values = newValues
    return this;
  }

  length(): number{
    let count = -1
    for(const i in this.values){
      count = Number(i)
    }
    return count + 1
  }

  map(fn: (el:T) => T): List<T> {
    for(const i in this.values){
      this.values[i] = fn(this.values[i])
    }
    return this;
  }

  foldl(fn: (acc: T, el: T) => T, initial: T): T {
    return this.leftFoldHelper(fn, initial, this)
  }

  leftFoldHelper(fn: (acc: T, el: T) => T, initial: T, list: List<T>): T {
    if(list.length() == 0 ){
      return initial
    }
    return fn(this.leftFoldHelper(fn,initial, new List(list.values.slice(0, list.length()-1))), list.values[list.length()-1])
  }

  foldr(fn: (acc: T, el: T) => T, initial: T): T {
    return this.rightFoldHelper(fn, initial, this)
  }

  rightFoldHelper(fn: (acc: T, el: T) => T, initial: T, list: List<T>): T{
    if(list.length() == 0 ){
      return initial
    }
    return fn(this.rightFoldHelper(fn,initial, new List(list.values.slice(1, list.length()))), list.values[0])
  }

  reverse(): List<T> {
    const reversedList: T[] = []
    for(const i in this.values){
      reversedList.push(this.values[this.length()-1-Number(i)])
    }
    this.values = reversedList
    return this
  }

}

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