C++ Program to Find the Frequency of Elements in an Array

In C++, arrays are a type of data structure that can store a fixed-size sequential collection of elements of the same type. In this article, we will learn how to find the frequency of elements in an array in C++.

Example:

Input:
Array: {1, 2, 3, 4, 2, 1, 3, 2, 4, 5}

Output:
Element: 1, Frequency: 2
Element: 2, Frequency: 3
Element: 3, Frequency: 2
Element: 4, Frequency: 2
Element: 5, Frequency: 1

Frequency of Elements in an Array in C++

To find the frequency of all the elements in an array in C++, we can use the unordered_map with the array elements as the key and its frequency as its value. We can then traverse the array and increment the frequency of each element.

Approach

• Create an unordered_map to store the frequency of each element. The keys of the map will be the elements from the array, and the corresponding values will be their frequencies.
• Iterate through each element of the array.
• For each element encountered in the array, increment its frequency count in the hash map. If the element is encountered for the first time, initialize its frequency count to 1.
• After traversing the entire array, iterate through the hash map. For each key-value pair in the hash map, print the element and its corresponding frequency.

C++ Program to Find the Frequency of Elements in an Array

// C++ Program to illustrate how to find the frequency of
// elements in an array
#include <iostream>
#include <unordered_map>
using namespace std;

// Driver Code
int main()
{
    // Declaring an array of integers
    int arr[] = { 10, 20, 20, 10, 30, 40, 10, 20 };
    int n = sizeof(arr) / sizeof(arr[0]);

    // Declaring an unordered_map to store the frequency of
    // elements
    unordered_map<int, int> frequencyMap;

    // Counting the frequency of elements
    for (int i = 0; i < n; i++) {
        frequencyMap[arr[i]]++;
    }

    // Printing the frequency of elements
    cout << "Frequency of elements in the array is: "
         << endl;
    for (auto& i : frequencyMap) {
        cout << i.first << " -> " << i.second << endl;
    }

    return 0;
}

Frequency of elements in the array is: 
40 -> 1
30 -> 1
20 -> 3
10 -> 3

Time Complexity: O(N), where N is the number of array elements.
Auxiliary Space: O(N)

Sample Problem: Find frequencies in array

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