How Can I Efficiently Get a Random Subset from a Collection Using LINQ?

Optimizing LINQ: Obtaining a Random Shuffled Sub-Collection

Determining efficient methods for retrieving random subsets of a larger collection is a common challenge in programming. For LINQ queries, one approach is to leverage the Shuffle() extension method.

The Shuffle Method:

The Shuffle() method takes an IEnumerable collection as its input and returns a shuffled version of it. This method is particularly useful when one needs to obtain a random order of the elements in a collection.

Fisher-Yates-Durstenfeld Shuffle:

The Shuffle() method employs the Fisher-Yates-Durstenfeld shuffle algorithm, which provides a robust and efficient means of shuffling a collection. This algorithm involves iterating through the collection while randomly swapping elements with subsequent elements.

LINQ Extension Method:

Below is a LINQ-friendly extension method that implements the Fisher-Yates-Durstenfeld shuffle for use with IEnumerable collections:

public static class EnumerableExtensions
{
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source)
    {
        return source.Shuffle(new Random());
    }

    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source, Random rng)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (rng == null) throw new ArgumentNullException(nameof(rng));

        return source.ShuffleIterator(rng);
    }

    private static IEnumerable<T> ShuffleIterator<T>(
        this IEnumerable<T> source, Random rng)
    {
        var buffer = source.ToList();
        for (int i = 0; i < buffer.Count; i++)
        {
            int j = rng.Next(i, buffer.Count);
            yield return buffer[j];

            buffer[j] = buffer[i];
        }
    }
}

Usage:

To utilize the Shuffle() method, you can simply call it on your desired collection:

var yourCollection = new List<int> { 1, 2, 3, 4, 5 };
var randomItems = yourCollection.Shuffle().Take(3);

This will yield a shuffled collection of 3 random items from yourCollection. By default, the Shuffle() method uses a new Random() instance for randomization. You can also specify a custom Random instance for custom shuffling behavior.

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