check for duplicates in array code example

Example 1: java find duplicates in array

package dto;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

/**
 * Java Program to find duplicate elements in an array. There are two straight
 * forward solution of this problem first, brute force way and second by using
 * HashSet data structure. A third solution, similar to second one is by using
 * hash table data structure e.g. HashMap to store count of each element and
 * print element with count 1.
 * 
 * @author java67
 */

public class DuplicatesInArray{

    public static void main(String args[]) {

        String[] names = { "Java", "JavaScript", "Python", "C", "Ruby", "Java" };

        // First solution : finding duplicates using brute force method
        System.out.println("Finding duplicate elements in array using brute force method");
        for (int i = 0; i < names.length; i++) {
            for (int j = i + 1; j < names.length; j++) {
                if (names[i].equals(names[j]) ) {
                   // got the duplicate element
                }
            }
        }

        // Second solution : use HashSet data structure to find duplicates
        System.out.println("Duplicate elements from array using HashSet data structure");
        Set<String> store = new HashSet<>();

        for (String name : names) {
            if (store.add(name) == false) {
                System.out.println("found a duplicate element in array : "
                        + name);
            }
        }

        // Third solution : using Hash table data structure to find duplicates
        System.out.println("Duplicate elements from array using hash table");
        Map<String, Integer> nameAndCount = new HashMap<>();

        // build hash table with count
        for (String name : names) {
            Integer count = nameAndCount.get(name);
            if (count == null) {
                nameAndCount.put(name, 1);
            } else {
                nameAndCount.put(name, ++count);
            }
        }

        // Print duplicate elements from array in Java
        Set<Entry<String, Integer>> entrySet = nameAndCount.entrySet();
        for (Entry<String, Integer> entry : entrySet) {

            if (entry.getValue() > 1) {
                System.out.println("Duplicate element from array : "
                        + entry.getKey());
            }
        }
    }
}
Output :
Finding duplicate elements in array using brute force method
Duplicate elements from array using HashSet data structure
found a duplicate element in array : Java
Duplicate elements from array using hash table
Duplicate element from array : Java

Example 2: js find duplicates in array

const names = ['Mike', 'Matt', 'Nancy', 'Adam', 'Jenny', 'Nancy', 'Carl']

const count = names =>
  names.reduce((a, b) => ({ ...a,
    [b]: (a[b] || 0) + 1
  }), {}) // don't forget to initialize the accumulator

const duplicates = dict =>
  Object.keys(dict).filter((a) => dict[a] > 1)

console.log(count(names)) // { Mike: 1, Matt: 1, Nancy: 2, Adam: 1, Jenny: 1, Carl: 1 }
console.log(duplicates(count(names))) // [ 'Nancy' ]

Example 3: find duplicate values in array javascript

const findDuplicates = (arr) => {
  let sorted_arr = arr.slice().sort(); // You can define the comparing function here. 
  // JS by default uses a crappy string compare.
  // (we use slice to clone the array so the
  // original array won't be modified)
  let results = [];
  for (let i = 0; i < sorted_arr.length - 1; i++) {
    if (sorted_arr[i + 1] == sorted_arr[i]) {
      results.push(sorted_arr[i]);
    }
  }
  return results;
}

let duplicatedArray = [9, 4, 111, 2, 3, 4, 9, 5, 7];
console.log(`The duplicates in ${duplicatedArray} are ${findDuplicates(duplicatedArray)}`);

Example 4: javascript get duplicates in array

function getDuplicateArrayElements(arr){
    var sorted_arr = arr.slice().sort();
    var results = [];
    for (var i = 0; i < sorted_arr.length - 1; i++) {
        if (sorted_arr[i + 1] === sorted_arr[i]) {
            results.push(sorted_arr[i]);
        }
    }
    return results;
}

var colors = ["red","orange","blue","green","red","blue"];
var duplicateColors= getDuplicateArrayElements(colors);//["blue", "red"]

Example 5: check for duplicates in array javascript

[1, 2, 3].every((e, i, a) => a.indexOf(e) === i) // true

[1, 2, 1].every((e, i, a) => a.indexOf(e) === i) // false

Example 6: check for duplicates in array javascript

[1, 2, 2, 4, 3, 4].filter((e, i, a) => a.indexOf(e) !== i) // [2, 4]