Finding the Frequency of Elements in an Array

Understanding Frequency Calculation

Finding the frequency of elements in an array involves counting the number of times each element appears.

We will explore three different methods to determine the frequency using Java.

Method 1: Using a Loop

This method iterates through the array and counts occurrences of each element.

import java.util.Arrays;

public class FrequencyFinder {
    public static void findFrequency(int[] arr) {
        boolean[] visited = new boolean[arr.length];
        Arrays.fill(visited, false);
        
        for (int i = 0; i < arr.length; i++) {
            if (visited[i]) continue;
            int count = 1;
            for (int j = i + 1; j < arr.length; j++) {
                if (arr[i] == arr[j]) {
                    count++;
                    visited[j] = true;
                }
            }
            System.out.println(arr[i] + " appears " + count + " times.");
        }
    }
    
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 2, 1, 3, 3};
        findFrequency(arr);
    }
}
            

Method 2: Using a HashMap

This method utilizes a HashMap to store frequencies.

import java.util.HashMap;

public class FrequencyFinder {
    public static void findFrequency(int[] arr) {
        HashMap freqMap = new HashMap<>();
        
        for (int num : arr) {
            freqMap.put(num, freqMap.getOrDefault(num, 0) + 1);
        }
        
        for (var entry : freqMap.entrySet()) {
            System.out.println(entry.getKey() + " appears " + entry.getValue() + " times.");
        }
    }
    
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 2, 1, 3, 3};
        findFrequency(arr);
    }
}
            

Method 3: Using Sorting

This method sorts the array first and then counts occurrences.

import java.util.Arrays;

public class FrequencyFinder {
    public static void findFrequency(int[] arr) {
        Arrays.sort(arr);
        
        for (int i = 0; i < arr.length; i++) {
            int count = 1;
            while (i < arr.length - 1 && arr[i] == arr[i + 1]) {
                count++;
                i++;
            }
            System.out.println(arr[i] + " appears " + count + " times.");
        }
    }
    
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 2, 1, 3, 3};
        findFrequency(arr);
    }
}