Check if an array contains duplicate values

Question

I wanted to write a javascript function which checks if array contains duplicate values or not.

I have written the following code but its giving answer as "true" always.

Can anybody please tell me what am I missing.

function checkIfArrayIsUnique(myArray) 
    {
        for (var i = 0; i < myArray.length; i++) 
        {
            for (var j = 0; j < myArray.length; j++) 
            {
                if (i != j) 
                {
                    if (myArray[i] == myArray[j]) 
                    {
                        return true; // means there are duplicate values
                    }
                }
            }
        }
        return false; // means there are no duplicate values.
    }

You're comparing the same index ( when i == j ). Of course they are the same. — Raptor, Oct 29 '13 at 10:47
You want to check if each of the values in the array are unique (check for duplicates) or if the array itself is unique compared to another array? — David Thomas, Oct 29 '13 at 10:48
Edited the question to be more specific. Hope it clear the doubts. — milan m, Oct 29 '13 at 10:49
checkIfArrayIsUnique([1,2,3]) its false buddy , ur function is working — Sarath, Oct 29 '13 at 10:49
Poor naming. "Check if unique" → "True" means "there are duplicates"... — deceze, Oct 29 '13 at 10:50
You could also change the second loop to `for (var j = i; j < myArray.length; j++) ` as you have already checked the earlier indexes for uniqueness so there is no need to compare them again. — Richard Dalton, Oct 29 '13 at 10:53

score 117 · Answer 1 · edited Dec 05 '19 at 16:58

117

An easy solution, if you've got ES6, uses Set:

function checkIfArrayIsUnique(myArray) {
  return myArray.length === new Set(myArray).size;
}

let uniqueArray = [1, 2, 3, 4, 5];
console.log(`${uniqueArray} is unique : ${checkIfArrayIsUnique(uniqueArray)}`);

let nonUniqueArray = [1, 1, 2, 3, 4, 5];
console.log(`${nonUniqueArray} is unique : ${checkIfArrayIsUnique(nonUniqueArray)}`);

edited Dec 05 '19 at 16:58

Zameer Ansari

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answered Dec 10 '15 at 01:13

natetitterton

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This does not work for an array of arrays: ```[['x','y'], ['a','b'], ['42', 'awerq'], ['12'], ['x', 'y']]``` – J.E.C. Oct 05 '20 at 08:08
@J.E.C. Sure it does. The first and second `['x','y']` elements are different objects in memory. – General Grievance May 06 '22 at 01:26
@J.E.C. ...but if you want something that can handle array equality, I made an [answer](https://stackoverflow.com/a/72137598/4294399) for that. – General Grievance May 09 '22 at 14:31

ofir_aghai · Answer 2 · 2020-03-11T21:14:56.277

25

let arr = [11,22,11,22];

let hasDuplicate = arr.some((val, i) => arr.indexOf(val) !== i);
// hasDuplicate = true

True -> array has duplicates

False -> uniqe array

edited Mar 11 '20 at 21:14

answered Mar 15 '17 at 15:04

ofir_aghai

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you could write as a one-liner (with some strict equality): `a.some((val, i) => a.indexOf(val) !== i);` – William Myers May 25 '17 at 13:26
if you want the array without the duplicates: `const filtered = a.filter((val, i) => a.indexOf(val) === i);` NB: these only work for arrays of primitive values – William Myers May 25 '17 at 13:44
If you use findIndex() instead of indexOf(), it will stop when it finds a match instead of continuing with the rest of the array. – bobpal Mar 01 '20 at 14:56
1

@bobpal both findIndex() and indexOf() will stop after find the first value and will not continue till the rest of the array. – ofir_aghai Mar 03 '20 at 15:43

score 23 · Answer 3 · answered Oct 29 '13 at 10:51

This should work with only one loop:

function checkIfArrayIsUnique(arr) {
    var map = {}, i, size;

    for (i = 0, size = arr.length; i < size; i++){
        if (map[arr[i]]){
            return false;
        }

        map[arr[i]] = true;
    }

    return true;
}

score 21 · Accepted Answer · answered Oct 29 '13 at 10:49

21

You got the return values the wrong way round:

As soon as you find two values that are equal, you can conclude that the array is not unique and return false.
At the very end, after you've checked all the pairs, you can return true.

If you do this a lot, and the arrays are large, you might want to investigate the possibility of sorting the array and then only comparing adjacent elements. This will have better asymptotic complexity than your current method.

answered Oct 29 '13 at 10:49

NPE

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Thanks for your answer. I think my function was correct but the return values were being send the wrong way. – milan m Oct 29 '13 at 10:55
Also I was passing null values in the array and hence I was getting wrong answer. Anyways, my problem is solved. – milan m Oct 29 '13 at 10:56
@NPE how will sorting way have "better asymptotic complexity"? it increases the time complexity to O(nlogn) from O(n). the way does drop the space complexity from O(n) to O(1). Were you talking about space complexity? – Vishal Jan 14 '21 at 08:19

score 7 · Answer 5 · edited Jun 12 '14 at 11:41

7

Assuming you're targeting browsers that aren't IE8,

this would work as well:

function checkIfArrayIsUnique(myArray) 
{
    for (var i = 0; i < myArray.length; i++) 
    {
        if (myArray.indexOf(myArray[i]) !== myArray.lastIndexOf(myArray[i])) { 
            return false; 
        } 
    } 
    return true;   // this means not unique
}

edited Jun 12 '14 at 11:41

IanMac

3
3

answered Oct 29 '13 at 10:53

MildlySerious

8,750
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score 2 · Answer 6 · edited May 06 '22 at 05:09

2

Another solution:

 Array.prototype.checkIfArrayIsUnique = function() {
    this.sort();    
    for ( var i = 1; i < this.length; i++ ){
        if(this[i-1] == this[i])
            return false;
    }
    return true;
    }

edited May 06 '22 at 05:09

Cristik

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answered Jan 20 '15 at 19:05

sharad-garg

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I'm not sure if this is the best solution, it's frowned upon to directly add onto a prototype of a native in javaScript, this should just be a regular function to prevent possible conflicts. – Michael Ryan Soileau Aug 14 '15 at 18:38
Note that this doesn't work if there are more than 2 duplicate values. – aarjithn Oct 20 '15 at 12:48
1

Why is this the best solution? You're adding to the prototype chain seemingly for no good reason and it is not as performant as the answers that do it within one loop (since you sort the array first). Perhaps explain why you think this is the best solution? – GrayedFox Aug 16 '19 at 13:30
Sorting costs 0(n*log(n)) while you can tell whether duplicates exist in O(n) - Far from best solution. – Gil Epshtain Sep 12 '19 at 08:54
Also, wouldn't `sort` mutate the array? That might be a big problem. – General Grievance May 06 '22 at 01:49

score 2 · Answer 7 · answered Jan 06 '16 at 20:18

Here's an O(n) solution:

function hasDupes(arr) {
  /* temporary object */
  var uniqOb = {};
  /* create object attribute with name=value in array, this will not keep dupes*/
  for (var i in arr)
    uniqOb[arr[i]] = "";
  /* if object's attributes match array, then no dupes! */
  if (arr.length == Object.keys(uniqOb).length)
    alert('NO dupes');
  else
    alert('HAS dupes');


}
var arr = ["1/1/2016", "1/1/2016", "2/1/2016"];
hasDupes(arr);

https://jsfiddle.net/7kkgy1j3/

score 2 · Answer 8 · edited May 06 '22 at 05:09

2

function hasNoDuplicates(arr) {
    return arr.every(num => arr.indexOf(num) === arr.lastIndexOf(num));
}

hasNoDuplicates accepts an array and returns true if there are no duplicate values. If there are any duplicates, the function returns false.

edited May 06 '22 at 05:09

Cristik

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answered Mar 02 '18 at 00:39

Alex Ticovschi

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Morris S · Answer 9 · 2018-07-31T18:57:22.450

Without a for loop, only using Map().

You can also return the duplicates.

(function(a){
  let map = new Map();

  a.forEach(e => {
    if(map.has(e)) {
      let count = map.get(e);
      console.log(count)
      map.set(e, count + 1);
    } else {
      map.set(e, 1);
    }
  });

  let hasDup = false;
  let dups = [];
  map.forEach((value, key) => {
    if(value > 1) {
      hasDup = true;
      dups.push(key);
    }
  });
   console.log(dups);
   return hasDup;
 })([2,4,6,2,1,4]);

score 1 · Answer 10 · answered Jul 28 '19 at 13:18

Late answer but can be helpful

function areThereDuplicates(args) {

    let count = {};
    for(let i = 0; i < args.length; i++){
         count[args[i]] = 1 + (count[args[i]] || 0);
    }
    let found = Object.keys(count).filter(function(key) {
        return count[key] > 1;
    });
    return found.length ? true : false; 
}

areThereDuplicates([1,2,5]);

score 1 · Answer 11 · answered May 06 '22 at 07:21

If your array nests other arrays/objects, using the Set approach may not be what you want since comparing two objects compares their references. If you want to check that their contained values are equal, something else is needed. Here are a couple different approaches.

Approach 1: Map using `JSON.stringify` for keys

If you want to consider objects with the same contained values as equal, here's one simple way to do it using a Map object. It uses JSON.stringify to make a unique id for each element in the array.

I believe the runtime of this would be O(n * m) on arrays, assuming JSON.stringify serializes in linear time. n is the length of the outer array, m is size of the arrays. If the objects get very large, however, this may slow down since the keys will be very long. Not a very space-efficient implementation, but it is simple and works for many data types.

function checkArrayDupeFree(myArray, idFunc) {
    const dupeMap = new Map();
    for (const el of myArray) {
        const id = idFunc(el);
        if (dupeMap.has(id))
            return false;
        dupeMap.set(id, el);
    }
    return true;
}

const notUnique = [ [1, 2], [1, 3], [1, 2] ];
console.log(`${JSON.stringify(notUnique)} has no duplicates? ${checkArrayDupeFree(notUnique, JSON.stringify)}`);

const unique = [ [2, 1], [1, 3], [1, 2] ];
console.log(`${JSON.stringify(unique)} has no duplicates? ${checkArrayDupeFree(unique, JSON.stringify)}`);

Of course, you could also write your own id-generator function, though I'm not sure you can do much better than JSON.stringify.

Approach 2: Custom HashMap, Hashcode, and Equality implementations

If you have a lot of big arrays, it may be better performance-wise to implement your own hash/equality functions and use a Map as a HashMap.

In the following implementation, we hash the array. If there is a collision, map a key to an array of collided values, and check to see if any of the array values match according to the equality function.

The downside of this approach is that you may have to consider a wide range of types for which to make hashcode/equality functions, depending on what's in the array.

function checkArrayDupeFreeWHashes(myArray, hashFunc, eqFunc) {
    const hashMap = new Map();
    
    for (const el of myArray) {
        const hash = hashFunc(el);
        const hit = hashMap.get(hash);
    
        if (hit == null)
            hashMap.set(hash, [el]);
        else if (hit.some(v => eqFunc(v, el)))
            return false;
        else
            hit.push(el);
    }

    return true;
}

Here's a demo of the custom HashMap in action. I implemented a hashing function and an equality function for arrays of arrays.

function checkArrayDupeFreeWHashes(myArray, hashFunc, eqFunc) {
    const hashMap = new Map();
    
    for (const el of myArray) {
        const hash = hashFunc(el);
        const hit = hashMap.get(hash);
    
        if (hit == null)
            hashMap.set(hash, [el]);
        else if (hit.some(v => eqFunc(v, el)))
            return false;
        else
            hit.push(el);
    }

    return true;
}

function arrayHasher(arr) {
    let hash = 19;
    for (let i = 0; i < arr.length; i++) {
        const el = arr[i];
        const toHash = Array.isArray(el)
            ? arrayHasher(el)
            : el * 23;
        hash = hash * 31 + toHash;
    }

    return hash;
}

function arrayEq(a, b) {
    if (a.length != b.length)
        return false;

    for (let i = 0; i < a.length; i++) {
        if ((Array.isArray(a) || Array.isArray(b)) && !arrayEq(a[i], b[i]))
            return false;
        else if (a[i] !== b[i])
            return false;
    }

    return true;
}

const notUnique = [ [1, 2], [1, 3], [1, 2] ];
const unique = [ [2, 1], [1, 3], [1, 2] ];
console.log(`${JSON.stringify(notUnique)} has no duplicates? ${checkArrayDupeFreeWHashes(notUnique, arrayHasher, arrayEq)}`);
console.log(`${JSON.stringify(unique)} has no duplicates? ${checkArrayDupeFreeWHashes(unique, arrayHasher, arrayEq)}`);

score 0 · Answer 12 · answered Nov 07 '16 at 22:07

The code given in the question can be better written as follows

function checkIfArrayIsUnique(myArray) 
    {
        for (var i = 0; i < myArray.length; i++) 
        {
            for (var j = i+1; j < myArray.length; j++) 
            {                  
                    if (myArray[i] == myArray[j]) 
                    {
                        return true; // means there are duplicate values
                    }

            }
        }
        return false; // means there are no duplicate values.
    }

This seems to only improve the performance of that function, without fixing the problem the asked has/had. I might be wrong, in this case, can you update your answer with more details about how your code addresses the problem? — Cristik, May 08 '22 at 06:48

score 0 · Answer 13 · edited Dec 04 '17 at 07:29

Returns the duplicate item in array and creates a new array with no duplicates:

 var a = ["hello", "hi", "hi", "juice", "juice", "test"];
    var b = ["ding", "dong", "hi", "juice", "juice", "test"];
    var c = a.concat(b);
    var dupClearArr = [];

    function dupArray(arr) {

        for (i = 0; i < arr.length; i++) {
            if (arr.indexOf(arr[i]) != i && arr.indexOf(arr[i]) != -1) {
                console.log('duplicate item ' + arr[i]);
            } else {
                dupClearArr.push(arr[i])
            }

        }
        console.log('actual array \n' + arr + ' \nno duplicate items array \n' + dupClearArr)
    }

    dupArray(c);

score 0 · Answer 14 · answered May 05 '22 at 07:26

0

const containsMatches = (a1, a2) => a1.some((v) => a2.includes(v));

answered May 05 '22 at 07:26

readikus

369
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I see `a1` and `a2`. Which one is being checked for duplicates? – General Grievance May 06 '22 at 01:45
The ‘some’ method on a1 is being used to iterate over all the values, and checks if that value (v) is in a2. – readikus May 06 '22 at 06:18
1

Still, there's not clear how you can use the code to check for duplicates. Can you add a usage example? – Cristik May 06 '22 at 07:06

score -1 · Answer 15 · answered Oct 29 '13 at 10:59

function checkIfArrayIsUnique(myArray) 
    {
      isUnique=true

        for (var i = 0; i < myArray.length; i++) 
        {
            for (var j = 0; j < myArray.length; j++) 
            {
                if (i != j) 
                {
                    if (myArray[i] == myArray[j]) 
                    {
                        isUnique=false
                    }
                }
            }
        }
        return isUnique;
    }

This assume that the array is unique at the start.

If find two equals values, then change to false

Reg.A.Khalil · Answer 16 · 2021-03-07T08:30:26.033

i think this is the simple way

$(document).ready(function() {

  var arr = [1,2,3,9,6,5,6];
  
  console.log( "result =>"+ if_duplicate_value (arr));
  
});


function if_duplicate_value (arr){
    
    for(i=0;i<arr.length-1;i++){
        
        for(j=i+1;j<arr.length;j++){
            
            if(arr[i]==arr[j]){
                
                return true;
                
            }
            
        }
        
    }
    
    return false;
    
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

score -1 · Answer 17 · answered Mar 04 '22 at 19:21

-1

var c=[2,2,3,3,5,5,4,4,8,8];

for(var i=0; i<b.length; i++){
    for(var j=i+1; j<b.length; j++){
        if(c[i]==c[j]){
            console.log(c[j]);
        }
    }
}

answered Mar 04 '22 at 19:21

Tomal

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`Uncaught ReferenceError: b is not defined` – General Grievance Aug 16 '23 at 16:53

Check if an array contains duplicate values

17 Answers17

Approach 1: Map using `JSON.stringify` for keys

Approach 2: Custom HashMap, Hashcode, and Equality implementations

Linked

Related

Check if an array contains duplicate values

17 Answers17

Approach 1: Map using JSON.stringify for keys

Approach 2: Custom HashMap, Hashcode, and Equality implementations

Linked

Related

Approach 1: Map using `JSON.stringify` for keys