Find the frequency of each element in an array in c++

C++ program for Find the frequency of each element in an array. Here problem description and explanation.

// Include header file
#include <iostream>
#include <unordered_map>
using namespace std;
class Occurrence
{
	public:
		// Function which is display array elements
		void display(int arr[], int n)
		{
			for (int i = 0; i < n; ++i)
			{
				cout << "   " << arr[i];
			}
			cout << "\n";
		}
	// Count occurrence of given array
	void frequency(int arr[], int n)
	{
		// Display given array
		this->display(arr, n);
		// Create a empty map
		unordered_map < int, int > map;
		for (int i = 0; i < n; i++)
		{
			if (map.find(arr[i]) != map.end())
			{
				// When key exists then update value
				map[arr[i]] = map[arr[i]] + 1;
			}
			else
			{
				// Add new element
				map[arr[i]] = 1;
			}
		}
		cout << "  Occurrence " << endl;
		//  Display calculated result
		for (auto &key: map)
		{
			cout << "  " << key.first << "  :  " 
                << map[key.first] << endl;
		}
	}
};
int main()
{
	// Array element
	int arr[] = {
		1 , 3 , 2 , 1 , 4 , 2 , 7 , 9 , 1 , 3 , 3 , 4 , 7
	};
  	int n = sizeof(arr) / sizeof(arr[0]);
  	Occurrence task;
	task.frequency(arr, n);
	return 0;
}

Output

   1   3   2   1   4   2   7   9   1   3   3   4   7
  Occurrence
  9  :  1
  7  :  2
  1  :  3
  3  :  3
  2  :  2
  4  :  2