How to get index based value from Set

Question

I am looking to store a list of unique string (hence Set) and wanted to retrieve the value based on the index. I used get(index) But it turned out it returns undefined. So it seems I did not understand Set well.

In case value needed to be retrieved do we have to convert it back to the array and then only read it or with "get(index)" it can be achieved?

Also, I have checked Set tests to understand get(index) but still not clear.

const { Set } = require('immutable');

const set = Set(["ab", "cd", "ef"])
console.log(set.get(1)) //logs undefined
console.log(set.toJS()[1]) //logs "cd"

First you need to clarify for yourself if you are using ES6 `Set`, or Immutable.js `Set` - they are different. For one thing, the former doesn't have `get`. And Immutable.js provides `get` for all collections, but with sets it just returns the item itself: `new Immutable.Set().add("foo").get("foo")` returns `"foo"` (and `new Immutable.Set().add("foo").get("bar")` returns `undefined`). Sets are inherently unordered, "set index" makes no sense. If you want indices, you want an array (or at least `Immutable.IndexedSeq`). — Amadan, Jun 23 '17 at 07:48
@Amadan thanks for the input, Set's Item is not strict ordered made me use List() and it looks promising. It seems I just scratched surface of Immutable :) — Anil Namde, Jun 23 '17 at 14:42

score 14 · Answer 1 · answered May 05 '19 at 21:06

14

Another way to do that would be to use Array.from(set)[0].

answered May 05 '19 at 21:06

user1514042

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score 2 · Answer 2 · answered Jun 23 '17 at 07:34

2

Here I'm trying to using the Set in es2015 directly without ImmutableJS:

You could write customized function like this:

Set.prototype.getByIndex = function(index) { return [...this][index]; }

var set = new Set(['a', 'b', 'c'])

console.log(set.getByIndex(0))  // 'a'

Note that the spread operator converts a set to an array so that you can use index to get access to the element

answered Jun 23 '17 at 07:34

Chang

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It is bad practice to modify objects that one does not own; this paradigm has already caused problems when [MooTools created their own Array.prototype.flatten](https://developers.google.com/web/updates/2018/03/smooshgate). – Alexander Turinske Jul 15 '20 at 22:25
[Here](https://humanwhocodes.com/blog/2010/03/02/maintainable-javascript-dont-modify-objects-you-down-own/) is another good article on the matter. It really makes code less maintainable when you modify objects you don't own. – jens1101 Apr 23 '21 at 14:01

score 2 · Answer 3 · answered Jul 16 '19 at 15:09

2

let data = new Set();
data.add('Value 1');
data.add('Value 2');
data.add('Value 3');

function getSetValueByIndex(setObj, index) {
    return [...setObj][index]
}
console.log(getSetValueByIndex(data, 1))

answered Jul 16 '19 at 15:09

Emin Adiloğlu

91
7

score 1 · Answer 4 · answered Jun 23 '17 at 08:13

the way to use Immutable's get is by "key" not by index

console.log(set.get("cd")) // logs cd, at index 1

if you want to get an element from the Set's iterator, you have to extend Immutable's Set

Set.prototype.getByIdx = function(idx){
  if(typeof idx !== 'number') throw new TypeError(`Argument idx must be a Number. Got [${idx}]`);

  let i = 0;
  for( let iter = this.keys(), curs = iter.next(); !curs.done; curs = iter.next(), i++ )
    if(idx === i) return curs.value;

  throw new RangeError(`Index [${idx}] is out of range [0-${i-1}]`);
}

const set = Set(["ab", "cd", "ef"]);

console.log(set.getByIdx(1)) //logs cd

Ron S. · Answer 5 · 2020-06-24T05:33:59.277

Using Array.from() or spread requires iterating the entire set before returning an example. This is not ideal from a performance perspective.

A better solution would be:

function getSetValueByIndex(target, index) { 
  if (typeof index !== 'number') throw new Error(`Index must be a number!`);

  let i = 0;
  for (const item of target) if (i++ === index) return item;

  throw new Error(`Index ${index} out of bounds!`);
}

score 0 · Answer 6 · answered Aug 30 '23 at 04:11

Here's an improved version of @Ron S. 's answer. Works with negative indexes, just like Array.at

Set.prototype.at = function(index) {
    if (Math.abs(index) > this.size){
        return null;
    }

    let idx = index;
    if (idx < 0){
        idx = this.size + index;
    }
    let counter = 0;
    for (const elem of this){
        if (counter == idx){
            return elem
        }
        counter += 1;
    }
}

pery mimon · Answer 7 · 2022-05-25T09:38:51.210

-2

I find myself using an Array with another Set to check if I should put the new Value into the Array

example:

let myArray = [1,2,3,4]
let check = new Set(myArray)

function PushToArray( newValue ){
  if( !check.has(newValue) )
   myArray.push(newValue)
   check.add(newValue)
}

edited May 25 '22 at 09:38

answered Apr 15 '22 at 10:34

pery mimon

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How to get index based value from Set

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