Javascript Arrays のまとめ – Javascript Fundamentals – JSchallenger

はじめに

JSchallenger の Javascript Fundamentals – Javascript Arrays のまとめです。
まとめページのトップは、JSchallenger まとめです。

問題一覧と回答

Get nth element of array

// Write a function that takes an array (a) and a value (n) as argument
// Return the nth element of ‘a’

function myFunction(a, n) {
   return a[n - 1];
}

Remove first n elements of an array

// Write a function that takes an array (a) as argument
// Remove the first 3 elements of ‘a’
// Return the result

function myFunction(a) {
   return a.slice(3);
}

Get last n elements of an array

// Write a function that takes an array (a) as argument
// Extract the last 3 elements of ‘a’
// Return the resulting array

function myFunction(a) {
   return a.slice(-3);
}

Get first n elements of an array

// Write a function that takes an array (a) as argument
// Extract the first 3 elements of a
// Return the resulting array

function myFunction(a) {
   return a.slice(0, 3);
}

Return last n array elements

// Write a function that takes an array (a) and a number (n) as arguments
// It should return the last n elements of a

function myFunction(a, n) {
  return a.slice(-n);
}

Remove a specific array element

// Write a function that takes an array (a) and a value (b) as argument
// The function should remove all elements equal to ‘b’ from the array
// Return the filtered array

function myFunction( a, b ) {
  return a.filter(cur => cur !== b)
}

Count number of elements in JavaScript array

// Write a function that takes an array (a) as argument
// Return the number of elements in a

function myFunction(a) {
   return a.length;
}

Count number of negative values in array

// Write a function that takes an array of numbers as argument
// Return the number of negative values in the array

括弧はいるのか？

function myFunction(a) {
   return a.filter( v => v < 0 ).length;
}

function myFunction(a) {
   return a.filter((el) => el < 0).length;
}

Sort an array of strings alphabetically

// Write a function that takes an array of strings as argument
// Sort the array elements alphabetically
// Return the result

function myFunction( arr ) {
    return arr.sort()
}

Sort an array of numbers in descending order

// Write a function that takes an array of numbers as argument
// It should return an array with the numbers sorted in descending order

function myFunction( arr ) {
  return arr.sort((a, b) => b - a)
}

Calculate the sum of an array of numbers

// Write a function that takes an array of numbers as argument
// It should return the sum of the numbers

function myFunction(a) {
   return a.reduce((acc, cur) => acc + cur, 0);
}

Return the average of an array of numbers

// Write a function that takes an array of numbers as argument
// It should return the average of the numbers

function myFunction( arr ) {
    return arr.reduce((acc, cur) => acc + cur, 0) / arr.length
}

Return the longest string from an array of strings

// Write a function that takes an array of strings as argument
// Return the longest string

function myFunction( arr ) {
    return arr.reduce((a, b) => a.length <= b.length ? b : a)
}

Check if all array elements are equal

// Write a function that takes an array as argument
// It should return true if all elements in the array are equal
// It should return false otherwise

function myFunction( arr ) {
    returnnewSet(arr).size === 1
}

Merge an arbitrary number of arrays

// Write a function that takes arguments an arbitrary number of arrays
// It should return an array containing the values of all arrays

flat 知らんかった。

function myFunction(...arrays) {
   return arrays.reduce((sum, cur) => [...sum, ...cur], []);
}

function myFunction( ...arrays ) {
    return arrays.flat()
}

Sort array by object property

// Write a function that takes an array of objects as argument
// Sort the array by property b in ascending order
// Return the sorted array

これは連結した方が分かりやすいだろう

function myFunction(arr) {
   return arr.sort((a, b) => a.b - b.b);
}

function myFunction(arr) {
   constsort = (x, y) => x.b - y.b;
   return arr.sort(sort);
}

Merge two arrays with duplicate values

// Write a function that takes two arrays as arguments
// Merge both arrays and remove duplicate values
// Sort the merge result in ascending order
// Return the resulting array

Set で array ２つで初期化する方法が思いつかんかった(/_＼*)

function myFunction(a, b) {
   s = newSet(a);
   b.map(v => s.add(v));
   return [...s].sort((a, b) => a - b);
}

function myFunction(a, b) {
  return [...newSet([...a, ...b])].sort((x, y) => x - y);
}

Sum up all array elements with values greater than

// Write a function that takes an array (a) and a number (b) as arguments
// Sum up all array elements with a value greater than b
// Return the sum

function myFunction(a, b) {
   return a.reduce((sum, cur) => cur > b ? sum + cur : sum, 0);
}

function myFunction(a, b) {
  return a.reduce((sum, cur) => {
    if (cur > b) return sum + cur;
    return sum;
  }, 0);
}

Create a range of numbers

// Write a function that takes two numbers (min and max) as arguments
// Return an array of numbers in the range min to max

これ、author’s solution と同じ方法しか思いつかなかったけども、いい方法ないのかな。
Perl なら min…max で終わりなのだが。

function myFunction(min, max) {
    let arr = [];
    for (let i = min; i <= max; i++) {
      arr.push(i);
    }
    return arr;
}

Group array of strings by first letter

// Write a function that takes an array of strings as argument
// Group those strings by their first letter
// Return an object that contains properties with keys representing first letters
// The values should be arrays of strings containing only the corresponding strings
// For example, the array [‘Alf’, ‘Alice’, ‘Ben’] should be transformed to
// { a: [‘Alf’, ‘Alice’], b: [‘Ben’]}

上手いやり方作れず泥臭い方法しか思いつかなかった。。
author’s solution のように return できるとキレイだけど、わかりにくい気も。

function myFunction(arr) {
  return arr.reduce((sum, cur) => {
    const k = cur[0].toLowerCase();
    if (sum[k]) {
        sum[k].push(cur);
    }
    else {
        sum[k] = [cur];
    }
    return sum;
  }, {}); 
}

function myFunction(arr) {
  return arr.reduce((acc, cur) => {
    const firstLetter = cur.toLowerCase().charAt(0);
    return { ...acc, [firstLetter]: [...(acc[firstLetter] || []), cur] };
  }, {});
}

Define an array with conditional elements

// Write a function that takes an array with arbitrary elements and a number as arguments
// Return a new array, the first element should be either the given number itself
// or zero if the number is smaller than 6
// The other elements should be the elements of the original array
// Try not to mutate the original array

問題文が “smaller than 6” なのに num > 5 とコードを書くのは違和感がある。

function myFunction(arr, num) {
   return [...[num < 6 ? 0 : num], ...arr];
}

function myFunction(arr, num) {
  return [...(num > 5 ? [num] : [0]), ...arr];
}

Get every nth element of array

// Write a function that takes an array (a) and a value (n) as arguments
// Save every nth element in a new array
// Return the new array

これも愚直な方法で解いたけども、author’s solution はもっと愚直だったｗ
もっと美しい解法ないのかな。

function myFunction(a, n) {
   r = [];
   for (i = n - 1; i <= a.length; i += n) {
      r.push(a[i]);
   }
   return r;
}

function myFunction(a, n) {
   let rest = [...a];
   let result = [];
   for (let i = 0; i < a.length; i++) {
      if (rest.length < n) break;
      result.push(rest[n - 1]);
      rest = rest.slice(n);
   }
   return result;
}

Javascript Arrays のまとめ - Javascript Fundamentals - JSchallenger