Obtain a listing of values that appear multiple times within an array

Question

Obtain a listing of values that appear multiple times within an array

I need a solution to find values that appear more than once in an array. The current code I have is quite complex.

var arr = [1, 2, 3, 4, 2, 3];
var flag = {}
var exist2arr = [];

for(var i = 0; i < arr.length; i++){
  for(var j = 0 ; j < arr.length; j ++){
     if(i !=j && arr[i] == arr[j]){
       if(!flag[arr[i]])
         exist2arr.push(arr[i]);
       flag[arr[i]] = 1;
     }
  }
}
console.log(exist2arr);

Is there a simpler way (using a built-in JavaScript function) to achieve this? Any suggestions would be greatly appreciated.

javascript arrays loops

Answer 1

Answer №1

To eliminate duplicate values in an array, you can filter the array to include only values where the first occurrence is not at the current index. Then, you can convert the filtered array into a Set to get unique values.

const arr = [1, 2, 3, 4, 2, 3, 2, 3, 2, 3, 2, 3]; // added in some extras

const filtered = arr.filter((v, i) => arr.indexOf(v) !== i)
const unique = new Set(filtered)

console.info(Array.from(unique)) // using Array.from so it can be logged

Answer 2

To eliminate duplicate values in an array, you can filter the array to include only values where the first occurrence is not at the current index. Then, you can convert the filtered array into a Set to get unique values.

const arr = [1, 2, 3, 4, 2, 3, 2, 3, 2, 3, 2, 3]; // added in some extras

const filtered = arr.filter((v, i) => arr.indexOf(v) !== i)
const unique = new Set(filtered)

console.info(Array.from(unique)) // using Array.from so it can be logged

Answer 3

Answer №2

let numbers = [1, 2, 3, 4, 2, 3];

let obj = numbers.reduce((obj, num) => {
  num in obj ? obj[num] += 1 : obj[num] = 1;
  return obj;
}, {});

let result = Object.keys(obj).filter(key => obj[key] > 1);

console.log(result);

Answer 4

let numbers = [1, 2, 3, 4, 2, 3];

let obj = numbers.reduce((obj, num) => {
  num in obj ? obj[num] += 1 : obj[num] = 1;
  return obj;
}, {});

let result = Object.keys(obj).filter(key => obj[key] > 1);

console.log(result);

Answer 5

Answer №3

This solution might be considered a bit unconventional, but it gets the job done efficiently and in linear time complexity:

var arr = [1, 2, 3, 4, 2, 3, 2, 2]

var a = arr.reduce((r, v) => ((r[v + .1] = r[v + .1] + 1 || 1) - 2 || r.push(v), r), [])

console.log( a )              // [2,3]
console.log({ ...a })         // to display the "hidden" items
console.log({ ...a.slice() }) // using .slice() to eliminate the excess items

Answer 6

This solution might be considered a bit unconventional, but it gets the job done efficiently and in linear time complexity:

var arr = [1, 2, 3, 4, 2, 3, 2, 2]

var a = arr.reduce((r, v) => ((r[v + .1] = r[v + .1] + 1 || 1) - 2 || r.push(v), r), [])

console.log( a )              // [2,3]
console.log({ ...a })         // to display the "hidden" items
console.log({ ...a.slice() }) // using .slice() to eliminate the excess items

Answer 7

Answer №4

One possible solution is as follows:

function countOccurrences(value, array) {
  var count = 0;
  for(var i = 0; i < array.length; i++){
    if(array[i] === value){
      count++;
    }
  }
  return count;
}
function findDuplicates(dupArray, num) {
  var minOccurrences = num === undefined ? 2 : num;
  var result = [];
  for(var i = 0, currentElement, length = dupArray.length; i < length; i++){
    currentElement = dupArray[i];
    if(!countOccurrences(currentElement, result) && countOccurrences(currentElement, dupArray) >= minOccurrences){
      result.push(currentElement);
    }
  }
  return result;
}
var testArray = [4, 5, 2, 5, 7, 7, 2, 1, 3, 7, 7, 7, 25, 77, 4, 2];
console.log(findDuplicates(testArray)); console.log(findDuplicates(testArray, 3));

Answer 8

One possible solution is as follows:

function countOccurrences(value, array) {
  var count = 0;
  for(var i = 0; i < array.length; i++){
    if(array[i] === value){
      count++;
    }
  }
  return count;
}
function findDuplicates(dupArray, num) {
  var minOccurrences = num === undefined ? 2 : num;
  var result = [];
  for(var i = 0, currentElement, length = dupArray.length; i < length; i++){
    currentElement = dupArray[i];
    if(!countOccurrences(currentElement, result) && countOccurrences(currentElement, dupArray) >= minOccurrences){
      result.push(currentElement);
    }
  }
  return result;
}
var testArray = [4, 5, 2, 5, 7, 7, 2, 1, 3, 7, 7, 7, 25, 77, 4, 2];
console.log(findDuplicates(testArray)); console.log(findDuplicates(testArray, 3));

Obtain a listing of values that appear multiple times within an array

Answer №1

Answer №2

Answer №3

Answer №4

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