Sorting of elements by frequency in Java

May 2, 2023

Sorting of elements by frequency in Java

Here, in this page we will discuss the program for sorting of elements by frequency frequency in Java programming language. We are given with an integer array and need to sort the array on the basis of there occurence.

Sort Elements by Frequency of Occurrences in Java

To solve sort elements by frequency of occurrences in Java, we can use an array to count the occurrences of each element , and then use a sorting algorithm to sort the elements by their counts . Lets see this in detail .

Method Discussed :

Method 1 : Naive Approach
Method 2 : Using Hash-map and then sorting.

Let’s discuss both methods one by one in brief.

Method 1 :

First we declare an array.
Declare two 2d array arr[MAX][2] and brr[MAX][2].
Store 1d array in 0^th index of arr array.
Set arr[][1] = 0for all indexes upto n.
Now, count the frequency of elements of the array.
If element is unique the push it at brr[][0] array, and its frequency will represent by brr[][1].
Now, sort the brr on the basis of frequency.
Print the brr array on basis of their frequency.

Method 1 :

Run

import java.util.*;

public class Main {
    public static void main(String[] args) {
        int[] a = {10, 20, 10, 10, 20, 30, 30, 30, 30, 0};
        int n = a.length;
        int[][] arr = new int[256][2];
        int[][] brr = new int[256][2];
        int k = 0, temp, count;

        for (int i = 0; i < n; i++) {
            arr[i][0] = a[i];
            arr[i][1] = 0;
        }

        // Unique elements and its frequency are stored in another array
        for (int i = 0; i < n; i++) {
            if (arr[i][1] != 0) {
                continue;
            }
            count = 1;
            for (int j = i + 1; j < n; j++) {
                if (arr[i][0] == arr[j][0]) {
                    arr[j][1] = 1;
                    count++;
                }
            }
            brr[k][0] = arr[i][0];
            brr[k][1] = count;
            k++;
        }
        n = k;

        // Store the array and its frequency in sorted form
        for (int i = 0; i < n - 1; i++) {
            temp = brr[i][1];
            for (int j = i + 1; j < n; j++) {
                if (temp < brr[j][1]) {
                    temp = brr[j][1];
                    brr[j][1] = brr[i][1];
                    brr[i][1] = temp;

                    temp = brr[j][0];
                    brr[j][0] = brr[i][0];
                    brr[i][0] = temp;
                }
            }
        }

        System.out.println("Sorted Array based on its frequency:");
        for (int i = 0; i < n; i++) {
            while (brr[i][1] != 0) {
                System.out.print(brr[i][0] + " ");
                brr[i][1]--;
            }
        }
    }
}

Output

30 30 30 30 10 10 10 20 20 0

Sort the elements of an array

Finding the frequency of elements in an array

Finding the Longest Palindrome in an Array

Counting Distinct Elements in an Array

Finding Repeating elements in an Array

Method 2 :

In this method we will use the concept of hashing. We first declare the hash map and insert all the elements in the map, in which key represents the array elements and value represents the count of their occurrence.

Method 2 :

Run

import java.util.*;

public class Main {
    // Compare function
    static int compare(Map.Entry< Integer, Map.Entry< Integer, Integer>> p, Map.Entry< Integer, Map.Entry< Integer, Integer>> p1) {
        if (p.getValue().getValue() != p1.getValue().getValue())
            return Integer.compare(p1.getValue().getValue(), p.getValue().getValue());
        else
            return Integer.compare(p.getValue().getKey(), p1.getValue().getKey());
    }
    static void sortByFrequency(int[] arr, int n) {
        Map< Integer, Map.Entry< Integer, Integer>> mp = new HashMap<>();
        for (int i = 0; i < n; i++) {
            if (mp.containsKey(arr[i]))
                mp.get(arr[i]).setValue(mp.get(arr[i]).getValue() + 1);
            else
                mp.put(arr[i], new AbstractMap.SimpleEntry< Integer, Integer>(i, 1));
        }
        List< Map.Entry< Integer, Map.Entry< Integer, Integer>>> b = new ArrayList<>(mp.entrySet());
        Collections.sort(b, (p, p1) -> compare(p, p1));

        for (Map.Entry< Integer, Map.Entry< Integer, Integer>> it : b) {
            int count = it.getValue().getValue();
            while (count-- > 0)
                System.out.print(it.getKey() + " ");
        }
    }

    // Driver Function
    public static void main(String[] args) {
        int[] arr = {10, 20, 10, 10, 20, 30, 30, 30, 30, 0};
        int n = arr.length;

        sortByFrequency(arr, n);
    }
}

Output

30 30 30 30 10 10 10 20 20 0

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Sorting of elements by frequency in Java