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JavaScript: Perform stable sorting of an array, preserving the initial indexes of items when their values are the same

JavaScript fundamental (ES6 Syntax): Exercise-137 with Solution

Write a JavaScript program to perform stable sorting of an array, preserving the initial indexes of items when their values are the same. Do not mutate the original array, but returns a new array instead.

  • Use Array.prototype.map() to pair each element of the input array with its corresponding index.
  • Use Array.prototype.sort() and a compare function to sort the list, preserving their initial order if the items compared are equal.
  • Use Array.prototype.map() to convert back to the initial array items.
  • Does not mutate the original array, but returns a new array instead.

Sample Solution:

JavaScript Code:

//#Source https://bit.ly/2neWfJ2 
const stableSort = (arr, compare) =>
  arr
    .map((item, index) => ({ item, index }))
    .sort((a, b) => compare(a.item, b.item) || a.index - b.index)
    .map(({ item }) => item);

const arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
console.log(stableSort(arr, () => 0));

Sample Output:

[0,1,2,3,4,5,6,7,8,9,10]

Flowchart:

flowchart: Perform stable sorting of an array, preserving the initial indexes of items when their values are the same

Live Demo:

See the Pen javascript-basic-exercise-137-1 by w3resource (@w3resource) on CodePen.


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JavaScript: Tips of the Day

How to remove a specific item from an array?

Find the index of the array element you want to remove using indexOf, and then remove that index with splice.

The splice() method changes the contents of an array by removing existing elements and/or adding new elements.

const array = [2, 5, 9];

console.log(array);

const index = array.indexOf(5);
if (index > -1) {
  array.splice(index, 1);
}

// array = [2, 9]
console.log(array);

The second parameter of splice is the number of elements to remove. Note that splice modifies the array in place and returns a new array containing the elements that have been removed.

For the reason of completeness, here are functions. The first function removes only a single occurrence (i.e. removing the first match of 5 from [2,5,9,1,5,8,5]), while the second function removes all occurrences:

function removeItemOnce(arr, value) {
  var index = arr.indexOf(value);
  if (index > -1) {
    arr.splice(index, 1);
  }
  return arr;
}

function removeItemAll(arr, value) {
  var i = 0;
  while (i < arr.length) {
    if (arr[i] === value) {
      arr.splice(i, 1);
    } else {
      ++i;
    }
  }
  return arr;
}
//Usage
console.log(removeItemOnce([2,5,9,1,5,8,5], 5))
console.log(removeItemAll([2,5,9,1,5,8,5], 5))

Ref: https://bit.ly/2N9nKRp