How to sort an array in javascript, but save results in separate array

Question

I want to sort an array of objects based upon any of those object properties. But I want the original array left unchanged. Instead, I want to save the sort order of indexes in a separate array.

var source = [
  {"a": 2, "b": 8, "c": 9},
  {"a": 4, "b": 3, "c": 7},  
  {"a": 1, "b": 0, "c": 6}
]

var sortedIndexes;

SomeSortOfSortMethod("a", "asc");

// result of sortedIndexes containing indexes to source array:
// [2, 0, 1]

Any ideas how to do this? I can't use the built in javascript sort method because it changes source. I need to just capture what the sort would be and save that order as indexes to the source arry.

Copy initial array and operate copied array ;)

Rayon
– Rayon

2016-02-06 03:08:02 +00:00
Commented Feb 6, 2016 at 3:08 — Rayon
– Rayon, Commented Feb 6, 2016 at 3:08

ronIDX · Accepted Answer · 2016-02-07 21:44:54Z

1

var source = [
      {"a": 2, "b": 8, "c": 9},
      {"a": 4, "b": 3, "c": 7},  
      {"a": 1, "b": 0, "c": 6}
    ];

var orderedCopyArray = _.sortBy(source, "a");

// Defualt ascending
console.log(JSON.stringify(orderedCopyArray));

// Descending
console.log(JSON.stringify(orderedCopyArray.reverse()));

var indexesArray = [], leng = source.length;

// Descending array ordered
var reverse = orderedCopyArray.reverse();

// Get index
for(var i=0; i < leng; i++){
  var obj1 = reverse[i]; 
  for(var j=0; j < leng; j++){
    var obj2 = source[j];
    if(_.isEqual(obj1, obj2)){
      indexesArray.push(j);
      break;
    }
  }
}

console.log(indexesArray); //[2, 0, 1]

edited Feb 7, 2016 at 21:44

answered Feb 6, 2016 at 3:44

ronIDX

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11 Comments

RobG Over a year ago

That should be a comment, not an answer.

ronIDX Over a year ago

Mmmmm honestly in my opinion this is a valid alternative answer. Using external library is most of the times better than implement on your own a new one.

RobG Over a year ago

Because otherwise questions will be "answered" with suggestions to use libraries and not real answers. Imagine asking a DOM question and just getting answers like "use jQuery", "use Dojo", "use prototype.js"…

ronIDX Over a year ago

Ok, I added an example how do it wi UnderscoreJS

risingtiger Over a year ago

I agree RobG that questions can't be answered by just saying use such and such library. But I do appreciate ronIDX taking the time to explain the process through.

|

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

Make deep copy of initial array using array.map() and clone and apply sorting function over copied array.

The map() method creates a new array with the results of calling a provided function on every element in this array.

The sort() method sorts the elements of an array in place and returns the array.

Try this:

var source = [{
  "a": 2,
  "b": 8,
  "c": 9
}, {
  "a": 4,
  "b": 3,
  "c": 7
}, {
  "a": 1,
  "b": 0,
  "c": 6
}];

function clone(obj) {
  if (null == obj || "object" != typeof obj) return obj;
  var copy = obj.constructor();
  for (var attr in obj) {
    if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
  }
  return copy;
}
var temp = source.map(function(arr) {
  return clone(arr); //clone will make deep copy of the object
});
source[0].a = 50; //update the value from source object, it will not update `temp` array 
temp.sort(function(a, b) {
  return a.a - b.a; // `.a` will be the `key` to be sorted
});
snippet.log(JSON.stringify(temp));
snippet.log(JSON.stringify(source));

<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Feb 6, 2016 at 3:36

Rayon

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2 Comments

RobG Over a year ago

map doesn't make a "deep" copy, var temp = source.slice() gives the same result and is less to type.

Rayon Over a year ago

@RobG, I got that wrong ;) Have updated the code with helper clone and added some comments too! I hope I am correct this time. Thank you for correcting me..

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