Question

How to select N reservoir items randomly from an array in C#

Answer 1

using System;
using System.Collections.Generic;

public class Program
{
	private static IList<int> selectReservoir(int[] arr, int N) {
		int size = arr.Length;
		IList<int> reservoir = new List<int>();
		
		Random rand = new Random(); 

		int items = 0;
		while (items < N) {
			int i = rand.Next(0, size); // index between 0 to size
			bool found = false;

			// Check if value present in reservoir
			for (int j = 0; j < items; j++) {
				if (reservoir.Contains(arr[i])) {
					found = true;
					break;
				}
			}

			// If not present add value to the reservoir 
			if (!found) {
				reservoir.Add(arr[i]);
				items++;
			}
		}

		return reservoir;
	}
	
	public static void Main(string[] args)
	{
		int[] arr = new int[] {4, 9, 14, 96, 13, 0, 3, 99, 19, 2, 80, 1, 7};
		int N = 5;

		IList<int> reservoir = selectReservoir(arr, N);

		Console.WriteLine(string.Join(' ', reservoir));
	}
}




/*
run:
  
13 96 0 1 80
  
*/

 

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Answer 2

using System;
using System.Linq;

public class Program
{
	public static void Main(string[] args)
	{
		int[] arr = new int[] {4, 9, 14, 96, 13, 0, 3, 99, 19, 2, 80, 1, 7};
		int N = 5;
		
		Random rand = new Random(); 
 
		var reservoir = arr.OrderBy(x => rand.Next()).Take(N);

		Console.WriteLine(string.Join(' ', reservoir));
	}
}




/*
run:
  
9 80 4 19 2
  
*/

 

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