Java Program to Find the Frequency of Elements in an Array

Introduction

Finding the frequency of each element in an array is a common task that involves counting how many times each element appears in the array. This guide will walk you through writing a Java program that counts the frequency of elements in a given array.

Problem Statement

Create a Java program that:

• Prompts the user to enter the size of an array and its elements.
• Calculates the frequency of each element in the array.
• Displays the element along with its frequency.

Example:

• Input: [1, 2, 2, 3, 4, 4, 4, 5]
• Output:

  Element 1 occurs 1 time(s) Element 2 occurs 2 time(s) Element 3 occurs 1 time(s) Element 4 occurs 3 time(s) Element 5 occurs 1 time(s) 

Solution Steps

1. Read the Array Size and Elements: Use the Scanner class to take the size and elements of the array as input from the user.
2. Use a Data Structure to Track Frequencies: Utilize a HashMap to store each element as a key and its frequency as the value.
3. Traverse the Array: Iterate through the array and update the frequency of each element in the HashMap.
4. Display the Frequencies: Print each element along with its frequency.

Java Program

// Java Program to Find the Frequency of Elements in an Array // Author: https://www.rameshfadatare.com/ import java.util.HashMap; import java.util.Map; import java.util.Scanner; public class FrequencyOfElements { public static void main(String[] args) { // Step 1: Read the size and elements of the array from the user Scanner scanner = new Scanner(System.in); System.out.print("Enter the size of the array: "); int size = scanner.nextInt(); int[] array = new int[size]; System.out.println("Enter the elements of the array:"); for (int i = 0; i < size; i++) { array[i] = scanner.nextInt(); } // Step 2: Find the frequency of elements using a HashMap Map<Integer, Integer> frequencyMap = findFrequency(array); // Step 3: Display the frequency of each element System.out.println("Frequency of elements in the array:"); for (Map.Entry<Integer, Integer> entry : frequencyMap.entrySet()) { System.out.println("Element " + entry.getKey() + " occurs " + entry.getValue() + " time(s)"); } } // Method to find the frequency of elements in the array public static Map<Integer, Integer> findFrequency(int[] array) { Map<Integer, Integer> frequencyMap = new HashMap<>(); // Traverse the array and update the frequency map for (int element : array) { if (frequencyMap.containsKey(element)) { frequencyMap.put(element, frequencyMap.get(element) + 1); } else { frequencyMap.put(element, 1); } } return frequencyMap; } } 

Explanation

Step 1: Read the Array Size and Elements

• The Scanner class is used to read the size of the array and its elements. The nextInt() method captures the size and each element.

Step 2: Use a Data Structure to Track Frequencies

• A HashMap is used to store each element as a key and its frequency as the value.

Step 3: Traverse the Array

• A for loop is used to iterate through each element of the array.
• During each iteration:
  • If the element already exists in the HashMap, its frequency is incremented by 1.
  • If the element does not exist in the HashMap, it is added with a frequency of 1.

Step 4: Display the Frequencies

• The program prints each element along with its frequency using a for-each loop that iterates over the HashMap‘s entry set.

Output Example

Example 1:

Enter the size of the array: 8 Enter the elements of the array: 1 2 2 3 4 4 4 5 Frequency of elements in the array: Element 1 occurs 1 time(s) Element 2 occurs 2 time(s) Element 3 occurs 1 time(s) Element 4 occurs 3 time(s) Element 5 occurs 1 time(s) 

Example 2:

Enter the size of the array: 5 Enter the elements of the array: 10 20 20 30 10 Frequency of elements in the array: Element 10 occurs 2 time(s) Element 20 occurs 2 time(s) Element 30 occurs 1 time(s) 

Example 3:

Enter the size of the array: 6 Enter the elements of the array: 7 7 7 7 7 7 Frequency of elements in the array: Element 7 occurs 6 time(s) 

Conclusion

This Java program demonstrates how to find the frequency of elements in an array using a HashMap. The HashMap efficiently tracks the frequency of each element, making this approach both simple and effective. This exercise is valuable for understanding how to use data structures like HashMap in Java programming for counting occurrences and managing key-value pairs.