CODEWITHSUNDEEP
javascriptnode.js
hannah
hannah

Reputation: 195

print number of duplicates in javascript array

I am trying to print the total number of duplicates in the array. Instead the total number of elements is printing from the array instead of the duplicates that are there

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];

function pairs(a) {

  var pairCount = 0;

  for (var i = 0; i < ar.length; i++) {
    for (var j = i + 1; j < ar.length; j++) {
      if (ar[i] === ar[j]) {
        pairCount++;
      }

    }
  }
  return pairCount;
}
console.log(pairs(ar));

Upvotes: 1

Views: 1232

Answers (8)

Shidersz
Shidersz

Reputation: 17190

If you want to count the number of values that have duplicated you can use filter() this way:

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];

let res = ar.filter((x, i) => ar.some((y, j) => y === x && i !== j));
console.log(res.length);

If you need detailed information about duplicated values, then you can do something like this:

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];

let res = ar.reduce((data, curr) =>
{
    data[curr] = data[curr] ? ++data[curr] : 1;
    return data;
}, {});

Object.entries(res).forEach(([val, numTimes]) =>
{
    if (numTimes > 1)
        console.log(`Value ${val} appears ${numTimes} times`);
});

Upvotes: 1

Ele
Ele

Reputation: 33726

An alternative is using the function reduce with a special accumulator which provides a dupes property as the counter and a dict as a breadcrumb of previously counted numbers.

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20],
      dupes = ar.reduce((a, c) => 
                  (a.dupes += a.dict[c] || 0, a.dict[c] = 1, a), {dupes: 0, dict: {}}).dupes;
      
console.log(dupes);

Upvotes: 1

jo_va
jo_va

Reputation: 13963

Here is a way to return the duplicates by value in an object:

const values = [10, 20, 20, 10, 10, 30, 50, 10, 20];

function duplicates(arr) {
  return Object.values(arr).reduce((acc, val) => {
    acc.count[val] = (acc.count[val] || 0) + 1;
    if (acc.count[val] > 1) acc.duplicates[val] = acc.count[val];
    return acc;
  }, { count: {}, duplicates: {} }).duplicates;
}

console.log(duplicates(values));

console.log(Object.values(duplicates(values)).reduce((sum, x) => sum + x, 0));

Upvotes: 0

Nina Scholz
Nina Scholz

Reputation: 386604

You could take a Set for already counted values.

function pairs(array) {
    var pairCount = 0,
        visited = new Set;

    for (var i = 0; i < array.length; i++) {
        if (visited.has(array[i])) pairCount++;
        visited.add(array[i]);
    }
    return pairCount;
}

console.log(pairs([10, 20, 20, 10, 10, 30, 50, 10, 20]));

Upvotes: 0

Ikbel
Ikbel

Reputation: 7851

You can use this function instead to get the total number of duplicates in an array.

const getDupCount = array => {
    const uniqueItems = new Set(array)
    const duplicates = array.length - uniqueItems.size

    return duplicates
}

Upvotes: 0

Karen Grigoryan
Karen Grigoryan

Reputation: 5432

If you want a total number of just the duplicates, meaning [1,1,2,2,3] // 2, then the solution can be pretty short and sweet with es6

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];

const duplicateCount = (a) => a.length - new Set(a).size;

console.log(duplicateCount(ar));

Upvotes: 3

tymeJV
tymeJV

Reputation: 104775

If you want to just get the total count of duplicates, you can create another array with them filtered out, then just get the difference:

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];
const duplicatesFilteredOut = ar.filter((a, i, self) => i === self.indexOf(a))
const totalDuplicates = ar.length - duplicatesFilteredOut.length

Upvotes: 1

Olian04
Olian04

Reputation: 6872

You could use an object to keept track of each kind of element. Like this: 

const ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];

function pairs(arr) {

  const pairCount = {};
  
  for (let a of ar) {
    if (a in pairCount) {
      pairCount[a]++;
    } else {
      pairCount[a] = 1;
    }
  }
  
  return pairCount;
}
console.log(pairs(ar));

Upvotes: 0

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