using System;
namespace RandomSamplingDemo
{
internal static class Program
{
/*
* GetRandomSample
*
* Returns k unique random elements from the source array.
*
* Parameters:
* source : Input array. Its size is determined at runtime.
* k : Number of random elements to retrieve.
*
* Returns:
* An array containing k randomly selected elements.
*
* Approach:
* We create a copy of the original array and perform a
* partial Fisher-Yates shuffle.
*
* During iteration i:
* 1. Select a random position between i and the end.
* 2. Swap the elements.
* 3. Position i now contains one randomly selected item.
*
* After k iterations, the first k elements form a valid
* random sample without duplicates.
*
* Benefits:
* - No duplicate selections.
* - Original array is not modified.
* - Uniform random distribution.
* - Simple and efficient.
*/
public static int[] GetRandomSample(int[] source, int k)
{
if (source == null) {
throw new ArgumentNullException(nameof(source));
}
if (k < 0 || k > source.Length) {
throw new ArgumentOutOfRangeException(
nameof(k),
"k must be between 0 and the number of elements.");
}
// Make a copy so the caller's array remains unchanged.
int[] workingCopy = (int[])source.Clone();
// Use a single Random instance.
Random random = new Random();
// Partial Fisher-Yates shuffle.
for (int i = 0; i < k; i++)
{
int randomIndex = random.Next(i, workingCopy.Length);
(workingCopy[i], workingCopy[randomIndex]) =
(workingCopy[randomIndex], workingCopy[i]);
}
int[] result = new int[k];
Array.Copy(
workingCopy,
sourceIndex: 0,
destinationArray: result,
destinationIndex: 0,
length: k);
return result;
}
private static void Main()
{
try
{
// Example array.
// The size is known only at runtime.
int[] numbers =
{
10, 20, 30, 40, 50,
60, 70, 80, 90, 100
};
int k = 4;
int[] sample = GetRandomSample(numbers, k);
Console.WriteLine("Original array:");
Console.WriteLine(string.Join(", ", numbers));
Console.WriteLine();
Console.WriteLine($"Random sample ({k} elements):");
Console.WriteLine(string.Join(", ", sample));
}
catch (Exception ex)
{
Console.WriteLine($"Error: {ex.Message}");
}
}
}
}
/*
run:
Original array:
10, 20, 30, 40, 50, 60, 70, 80, 90, 100
Random sample (4 elements):
10, 60, 90, 40
*/
