How do I check if an array contains only one distinct element?

Question

How do I check if my array contains only one distinct element? `

var arr = [1, 1, 1];
var arr2 = [1, 2, 3];
checkIfThereIsOnlyOneElement(arr); //should return true
checkIfThereIsOnlyOneElement(arr2) //should return false;`

Possible duplicate of [PHP array: count or sizeof?](http://stackoverflow.com/questions/3974385/php-array-count-or-sizeof) — , Apr 12 '17 at 12:46
For checking if your array contains a value: http://stackoverflow.com/questions/1181575/determine-whether-an-array-contains-a-value . If you just want to know if the array 'is populated', as @Saravana said, use `Array.length`. — SrAxi, Apr 12 '17 at 12:47
Loop over the array, starting with the second iteration of the loop (basically start the loop at 1 instead of 0) compare the element to the previous element. If the comparison is ever false, return false. After the loop, return true. — David, Apr 12 '17 at 12:47
@stephenpassero So you want to count unique values. I did not get that from reading your question. My bad. — Saravana, Apr 12 '17 at 12:50

RomanPerekhrest · Answer 1 · 2017-04-12T12:53:23.000

6

Use Set object:

var arr = [1, 1, 1],
    arr2 = [1, 2, 3],
    checkForOne = function(arr){
        return (new Set(arr)).size === 1;
    }
    
console.log(checkForOne(arr));
console.log(checkForOne(arr2));

edited Apr 12 '17 at 12:53

answered Apr 12 '17 at 12:49

RomanPerekhrest

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`Array.length` wouldn't be enough? What's the point to complicate the problem? – kind user Apr 12 '17 at 12:50
@Kinduser: How *would* `Array.length` be enough? – David Apr 12 '17 at 12:51
@Kinduser, the question seems to me about "one unique element". That's why the OP wanted `true` for `[1, 1, 1]` – RomanPerekhrest Apr 12 '17 at 12:51
@David Fine, if OP is looking for just unique elements, this is the way to go. – kind user Apr 12 '17 at 12:52

TobiasR. · Accepted Answer · 2017-04-12T13:00:27.363

3

You could implement it this way:

var arr = [1, 1, 1];
var arr2 = [1, 2, 3];

console.log(checkIfThereIsOnlyOneElement(arr));
console.log(checkIfThereIsOnlyOneElement(arr2));

function checkIfThereIsOnlyOneElement(arr) {
    arr.sort();
    return arr[0] == arr[arr.length -1]
}

edited Apr 12 '17 at 13:00

answered Apr 12 '17 at 12:49

TobiasR.

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Very nice. Sort the array and then check to see if the first and last item are the same. The other answers complicate things. – castis Apr 12 '17 at 12:55
1

@castis What if the array is one thousand elements long? I just guess the sorting function will be an overkill. The `Set` method seems to be much better, imo. – kind user Apr 12 '17 at 12:56
Well the OP had an example 3 elements long. No sense in over-engineering something just for that. If OP has a huge performance issue I assume they'll be back. – castis Apr 12 '17 at 12:59

gurvinder372 · Answer 3 · 2017-04-12T12:50:28.203

0

try this simple implementation

function checkIfThereIsOnlyOneElement(arr)
{
  var map = {};//create a map
  for( var counter = 0; counter < arr.length; counter++)
  {
    map[ arr[ counter ] ] = true;
    if ( Object.keys ( map ).length > 1 ) //check if this map has only one key
    {
      return false;
    }
  }
  return true;
}

edited Apr 12 '17 at 12:50

answered Apr 12 '17 at 12:47

gurvinder372

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@Kinduser it is checking if there is only one unique element in the array or not. – gurvinder372 Apr 12 '17 at 12:49
@Kinduser didn't get you. `it's detonating opened door.` ?? – gurvinder372 Apr 12 '17 at 12:50
@Kinduser i know, i meant to ask - in what way i am doing so. – gurvinder372 Apr 12 '17 at 12:52

score 0 · Answer 4 · answered Apr 12 '17 at 12:56

0

simply check if all elements contain the same value, one way is to use "uniq" function of "lodash" module:

checkIfThereIsOnlyOneElement(arr){
     return _.uniq(arr).length == 1; 
}

answered Apr 12 '17 at 12:56

Mohiransh

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score 0 · Answer 5 · answered Apr 12 '17 at 12:57

You can check unique values in array. Based on this term we can implement your function:

var arr = [1, 1, 1];
var arr2 = [1, 2, 3];

function checkIfThereIsOnlyOneElement(arr) {
    return arr.filter(function (item, i, ar) {
            return ar.indexOf(item) === i;
        }).length === 1;
}

console.log(checkIfThereIsOnlyOneElement(arr));
console.log(checkIfThereIsOnlyOneElement(arr2));

jsfiddle

score 0 · Answer 6 · answered Apr 12 '17 at 13:07

There are plenty of pretty easy implementations for this function. Let me give you two, one is a functional one-liner, the other is an iterative (probably better performing) snippet.

By the way, your naming is very bad, it should be called: isArrayOfTheSameElements

  // Iterative:
  function isArrayOfTheSameElements(arr){
    var firstElement = arr[0];
    for(var i=1; i < arr.length; i++){
      if (arr[i]!==firstElement) {
        return false;
      }
    }
    return true;
  }

  // Functional:
  function isArrayOfTheSameElements(arr){
    return Object.keys(arr.reduce(function(current, next){
      return current[next] = next && current;
    }, {})).length <= 1;
  }

score 0 · Answer 7 · answered Apr 12 '17 at 13:30

You could use Array#every and check only agsindt the firsr element.

function checkDistinct(array) {
    return array.every(function (a, _, aa) { return a === aa[0]; });
}

var arr = [1, 1, 1],
    arr2 = [1, 2, 3];

console.log(checkDistinct(arr));  // true
console.log(checkDistinct(arr2)); // false

score 0 · Answer 8 · edited May 23 '17 at 12:10

0

You can always properly do like .every() as pointed out by @Nina Scholz but for a variety you may also do like;

var all = a => a.length-1 ? a[0] === a[1] && all(a.slice(1)) : true;
console.log(all([1,1,1,1]));
console.log(all([1,1,1,2]));

It has the same benefit to cut the iteration once it meets a false value.

edited May 23 '17 at 12:10

Community

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answered Apr 12 '17 at 16:23

Redu

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How do I check if an array contains only one distinct element?

8 Answers8