How to find the number of equal elements in javascript array

Question

I'm new to JavaScript so please help me on this problem: I have array and I want to get the number of same values. My array is:

 var arr = ["red", "blue", "green", "red", "red", "gray"];

I want to output 3 because I have 3 elements red.

This is what I have done so far:

var numberOfSameElements = 0;
var arr = ["red", "blue", "green", "red", "red", "gray"];
for(var i = 1 ; i <arr.length;i++){
if(arr[i] === arr[i-1]){
   numberOfSameElements++;  
 }
}

  console.log(numberOfSameElements);

I keep getting 1. Please tell me what I am doing wrong. Thank you so much!

Have a look at the accepted answer here: http://stackoverflow.com/questions/840781/easiest-way-to-find-duplicate-values-in-a-javascript-array — , Aug 16 '15 at 05:10
You keep getting one because you're always comparing items against the previous one... — Maria Ines Parnisari, Aug 16 '15 at 05:11
but how to not compare items against the previous one :(.... (Any hints ). Thank you — HenryDev, Aug 16 '15 at 05:12

rrowland · Answer 1 · 2015-08-16T05:26:41.390

3

You can reduce the array to a hash map to figure out how many of each word there are:

var words = ["red", "blue", "green", "red", "red", "gray"];
var wordCounts = words.reduce(function(counts, word) {
  counts[word] = (counts[word] || 0)++;
}, { });

console.log(wordCounts);

// >> {
//      red: 3,
//      blue: 1,
//      green: 1,
//      gray: 1
//    }

If you want the name of any words that are duplicated, you can filter the original array down:

var repeatedWords = words.filter(function(word) {
  return wordCounts[word] > 1;
});

console.log(repeatedWords);

// >> ['red']

edited Aug 16 '15 at 05:26

answered Aug 16 '15 at 05:22

rrowland

2,734
2
17
32

OP is new to JavaScript and you suggest the `reduce` function? LOL – Maria Ines Parnisari Aug 16 '15 at 05:24
Yes, I'll suggest the correct way to everybody. Why would I suggest the wrong way just because somebody is learning? – rrowland Aug 16 '15 at 05:28
Not the wrong way obviously, but something more digestible – Maria Ines Parnisari Aug 16 '15 at 05:30
I wish somebody had told me how to program in a readable, functional way when I was learning. – rrowland Aug 16 '15 at 05:31
*reduce* is really handy, it works a lot like *forEach* except that it usually saves an extra initialisation and returns something useful (whereas *forEach* returns *undefined*). – RobG Aug 16 '15 at 06:26

score 2 · Accepted Answer · answered Aug 16 '15 at 05:21

2

Here you've used one loop and checked your current value with it's previous value. But you need actually two loops.

First one will pick a value. And second one will match the picked value with all others. I've used a found flag that checks if a duplicate value has already been found or not. Try this way,

var numberOfSameElements = 0;
var found = false;
var arr = ["red", "blue", "green", "red", "red", "gray"];
for(var i = 0 ; i <arr.length;i++){
    for(var j = 0; j < arr.length; j++){
        if(arr[i] === arr[j] && i != j){
            if(!found){
                numberOfSameElements++;
                found = true;
            }
        }
    }
    found = false;
}

console.log(numberOfSameElements);

jsFiddle

answered Aug 16 '15 at 05:21

Md Ashaduzzaman

4,032
2
18
34

I'm not saying it's the best way to find duplicate values in array. But it's one of the simplest ways you can go with. – Md Ashaduzzaman Aug 16 '15 at 05:27
There's no need for two loops. This is fairly inefficient as it loops over the array `(arrayLength - 1)^2` times. – RobG Aug 16 '15 at 06:01
why don't you start j at i + 1? – Uma Kanth Aug 17 '15 at 01:42

score 2 · Answer 3 · answered Aug 16 '15 at 05:22

If you want to get the max number of duplicates find in the array, you first have to count them all. Then find the maximum repetition number (fiddle):

function getLargestNumberOfSame(arr) {
    var counterMap = arr.reduce(function (sameMap, item) { // create a map of same values
        if (!sameMap[item]) {
            sameMap[item] = 1;
        } else {
            sameMap[item]++;
        }

        return sameMap;
    }, {});

    var maxValues = Object.keys(counterMap).map(function(key) {  // turn the map into array of max values
        return counterMap[key];
    });

    return Math.max.apply(window, maxValues); // return the maximum value
}

score 1 · Answer 4 · answered Aug 16 '15 at 05:24

1

 var numberOfSameElements = 1;
var arr = ["red", "blue", "green", "red", "red", "gray"];
arr = arr.sort();
var last;
for(var i = 0 ; i <arr.length -1 ;i++) {
    //console.log(arr[i],arr[i+1]);
if(arr[i] == arr[i+1]){
   numberOfSameElements++;
   last = arr[i];

 } else { 
    console.log(arr[i],"repert",numberOfSameElements,"times");
    numberOfSameElements=1;
   }

}
console.log(last,"repert",numberOfSameElements,"times");

answered Aug 16 '15 at 05:24

Nitesh singh

915
11
21

i Think this will solve your problem easily and easy to understand. – Nitesh singh Aug 16 '15 at 05:26

How to find the number of equal elements in javascript array

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