Javascript: Count duplicates in an array

array_dupplicate_counter.js

/** 
Problem:

You have a javascript array that likely has some duplicate values and you would like a count of those values.
Solution:

Try this schnippet out.
*/


function compressArray(original) {
 
	var compressed = [];
	// make a copy of the input array
	var copy = original.slice(0);
 
	// first loop goes over every element
	for (var i = 0; i < original.length; i++) {
 
		var myCount = 0;	
		// loop over every element in the copy and see if it's the same
		for (var w = 0; w < copy.length; w++) {
			if (original[i] == copy[w]) {
				// increase amount of times duplicate is found
				myCount++;
				// sets item to undefined
				delete copy[w];
			}
		}
 
		if (myCount > 0) {
			var a = new Object();
			a.value = original[i];
			a.count = myCount;
			compressed.push(a);
		}
	}
 
	return compressed;
};

// It should go something like this:

var testArray = new Array("dog", "dog", "cat", "buffalo", "wolf", "cat", "tiger", "cat");
var newArray = compressArray(testArray);
 
/*
console: [
	Object { value="dog", count=2}, 
	Object { value="cat", count=3}, 
	Object { value="buffalo", count=1}, 
	Object { value="wolf", count=1}, 
	Object { value="tiger", count=1}
]
*/

Also, after some further testing and analysis, I found that both of my ideas are significantly faster than the original gist.

I ran all three on an array of 100,000 random numbers between 0-7

my map idea: 1.974 ms
my original idea: 9.172 ms
gist idea: 16.987644 SECONDS!!!

The performance difference was staggering...

@davidnagli ty man.

//THIS IS JUST FOR FUN, ALTHOUGH IT IS WORKING AWESOMELY!!
let bag={};
["NA","NA","NA","NA","NA","NA","NA","NA","NA","BATMAN"].forEach((item)=>{
    bag[item] = bag[item]?++bag[item]:1;
})
return bag;

Hi,

I'm using the original counter up there: array_dupplicate_counter.js

The script produce an object not an array.
I'd like to sort the object based on the value property not on the count property.
I tried many things with no avail.

Is someone know how to sort that kind of object?

Thanks

François

great work

How to use this code in typescript ?? I am unable to use as it is.
Please help me....

Thanks for the work :D ! Handy and works great!

Typescript version:

let compressArray = (original: Array) => {

    let compressed = [];
    // make a copy of the input array
    let copy = original.slice(0);

    // first loop goes over every element
    for (let i = 0; i < original.length; i++) {

        let myCount = 0;
        // loop over every element in the copy and see if it's the same
        for (let w = 0; w < copy.length; w++) {
            if (original[i] == copy[w]) {
                // increase amount of times duplicate is found
                myCount++;
                // sets item to undefined
                delete copy[w];
            }
        }

        if (myCount > 0) {
            let a = {};
            a['value'] = original[i];
            a['count'] = myCount;
            compressed.push(a);
        }
    }
    return compressed;
};

@davidnagli thanks a bunch man! I learned quite a few things from you today.

thanks for both author and @davidnagli

God bless you :)

Very useful!
Thanks both of you @ralphcrisostomo
and @davidnagli for the gain-performance implementation and the explanation bonus!

function countDup(arr = []) {
  const set = new Set(arr);
  const uniqueArr = Array.from(set);
  if (uniqueArr.length === arr.length) return;

  const map = new Map();
  uniqueArr.forEach((uniqueVal) => {
    let n = 0;
    arr.forEach((val) => {
      if (val === uniqueVal) {
        n++;
      }
    });
    map.set(uniqueVal, n);
  });
  console.log('newMap ', map);
  for (const [key, val] of map) {
    console.log(key, val);
  }
}

array = ['Apples', 'Oranges', 'Apples', 'Strawberries', 'Apples'];

For a one liner to get unique values and their counts and put them in a dictionary :

reducedArray = array.reduce((acc, datum) => { 
                              let count = acc[datum];
                              acc[datum] = count === undefined ? 1 : ++count;
                              return acc;
                           }, {});

acc = accumulator that indexes according to datum
and returns a key value pair with key as datum and count as the number of times the datum appears in the array.

o/p: (try console.log(reducedArray))
Apples: 3
Oranges: 1
Strawberries: 1

array = ['abc', '123', 'abc', 'xyz', 'abc'];

For a one liner using reduce to count unique distinct values and put them in a dictionary:

reducedArray= array.reduce((acc, datum) => {
      let count= acc[datum];
      acc[datum] = count === undefined ? 1 : ++count;
      return acc;
    }, {});

acc: indexes the dictionary using datum and increments the count if it finds a duplicate

console: 
abc: 3
123: 1
xyz: 1

@davidnagli Thanks Buddy.. it really helped me with just one line code change.
appreciated

Thanks for this @ralphcrisostomo! I implemented one change, though. I needed to use data repeatedly in different ways. The "delete copy[w]" on line 27 line really threw my data off. I changed it for "i++" and was successful in keeping my data intact. Thanks again!!

Had to search high and low for this solution, every tutorial and answer before was only outputting in the console. You are a legend! Thanks you!

@williamlsh those first three lines can be reduced to one via ES6 spread syntax!

let uniqueArr = [...new Set(arr)];

Thank you!