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

Csharp program for Find the frequency of each element in an array. Here more information.

// Include namespace system
using System;
using System.Collections.Generic;
using System.Collections;
public class Occurrence
{
	// Function which is display array elements
	public static void display(int[] arr)
	{
		for (var i = 0; i < arr.Length; ++i)
		{
			Console.Write("   " + arr[i]);
		}
		Console.Write("\n");
	}
	// Count occurrence of given array
	public static void frequency(int[] arr)
	{
		// Display given array
		Occurrence.display(arr);
		// Create a empty dictionary
		var map = new Dictionary < int , int > ();
		for (var i = 0; i < arr.Length; i++)
		{
			if (map.ContainsKey(arr[i]))
			{
				// When key exists then update value
				map[arr[i]] = map[arr[i]] + 1;
			}
			else
			{
				// Add new element
				map[arr[i]] = 1;
			}
		}
		Console.WriteLine("  Occurrence ");
		//  Display calculated result
		foreach(KeyValuePair < int, int > entry in map)
		{
			Console.WriteLine("  " + entry.Key + "  :  " +  entry.Value);
		}
	}
	public static void Main(String[] args)
	{
		// Array element
		int[] arr = {
			1 , 3 , 2 , 1 , 4 , 2 , 7 , 9 , 1 , 3 , 3 , 4 , 7
		};
		Occurrence.frequency(arr);
	}
}

Output

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