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How to select random objects from a list using C #?

I have an IQueryable containing over 300 objects:

public class Detail
{
    public int Id { get; set; }
    public int CityId { get; set; }
    public bool Chosen { get; set; }
}

IQueryable<Detail> details = ...

How can I go against this, randomly select 50 objects? I assume that I will need to convert this using .ToList (), but I'm not sure how I can select random elements.

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5 answers

300 is not very much, so yes, do this List:

IQueryable<Detail> details = ...
IList<Detail> detailList = details.ToList();

And now you can select a random item:

var randomItem = detailList[rand.Next(detailList.Count)];

and you can repeat it 50 times. This, however, will lead to duplication, and the process of eliminating them will become erratic.

Therefore, use the standard shuffle algorithm, and then select the first 50:

Shuffle(detailList);
var selection = detailList.Take(50);
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, , .

:

/// <summary>Randomly selects items from a sequence.</summary>
/// <typeparam name="T">The type of the items in the sequence.</typeparam>
/// <param name="sequence">The sequence from which to randomly select items.</param>
/// <param name="count">The number of items to randomly select from the sequence.</param>
/// <param name="sequenceLength">The number of items in the sequence among which to randomly select.</param>
/// <param name="rng">The random number generator to use.</param>
/// <returns>A sequence of randomly selected items.</returns>
/// <remarks>This is an O(N) algorithm (N is the sequence length).</remarks>

public static IEnumerable<T> RandomlySelectedItems<T>(IEnumerable<T> sequence, int count, int sequenceLength, System.Random rng)
{
    if (sequence == null)
    {
        throw new ArgumentNullException("sequence");
    }

    if (count < 0 || count > sequenceLength)
    {
        throw new ArgumentOutOfRangeException("count", count, "count must be between 0 and sequenceLength");
    }

    if (rng == null)
    {
        throw new ArgumentNullException("rng");
    }

    int available = sequenceLength;
    int remaining = count;
    var iterator  = sequence.GetEnumerator();

    for (int current = 0; current < sequenceLength; ++current)
    {
        iterator.MoveNext();

        if (rng.NextDouble() < remaining/(double)available)
        {
            yield return iterator.Current;
            --remaining;
        }

        --available;
    }
}

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, ( ).

, , :

// n is the number of items to randomly choose.

public static List<T> RandomlyChooseItems<T>(IEnumerable<T> items, int n, Random rng)
{
    var result = new List<T>(n);
    int index = 0;

    foreach (var item in items)
    {
        if (index < n)
        {
            result.Add(item);
        }
        else
        {
            int r = rng.Next(0, index + 1);

            if (r < n)
                result[r] = item;
        }

        ++index;
    }

    return result;
}

, Shuffle, . _rng Random:

/// <summary>Shuffles the specified array.</summary>
/// <typeparam name="T">The type of the array elements.</typeparam>
/// <param name="array">The array to shuffle.</param>

public void Shuffle<T>(IList<T> array)
{
    for (int n = array.Count; n > 1;)
    {
        int k = _rng.Next(n);
        --n;
        T temp = array[n];
        array[n] = array[k];
        array[k] = temp;
    }
}
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Random rnd = new Random();
IQueryable<Detail> details = myList.OrderBy(x => rnd.Next()).Take(50);
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IQueryable<Detail> details = myList.OrderBy(x => Guid.NewGuid()).ToList();

:

var item1 = details[0];

.

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var l = new List<string>();
l.Add("A");
l.Add("B");
l.Add("C");
l.Add("D");
l.Add("E");
l.Add("F");
l.Add("G");
l.Add("H");
l.Add("I");

var random = new Random();
var nl = l.Select(i=> new {Value=i,Index = random.Next()});

var finalList = nl.OrderBy(i=>i.Index).Take(3);
foreach(var i in finalList)
{
    Console.WriteLine(i.Value);
}
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