Picking a random string from a list with no replacement C#

Question

I have a list of strings and I want to randomly pick from this list. When a string is picked, it must be removed from the list. Only when all the strings have been picked then the list is repopulated. How do I achieve this?

Answer 1

First, make a random number generator.

Random rand = new Random();

Then, generate a number and remove the item from the list. I'm assuming you're using System.Collections.Generic.List.

int num = Random.Next(list.Count);
list.RemoveAt(num);

Answer 2

You can implement it quite straightforward:

  public static partial class EnumerableExtensions {
    public static IEnumerable<T> RandomItemNoReplacement<T>(this IEnumerable<T> source, 
      Random random) {

      if (null == source)
        throw new ArgumentNullException("source");
      else if (null == random)
        throw new ArgumentNullException("random");

      List<T> buffer = new List<T>(source);

      if (buffer.Count <= 0)
        throw new ArgumentOutOfRangeException("source");

      List<T> urn = new List<T>(buffer.Count);

      while (true) {
        urn.AddRange(buffer);

        while (urn.Any()) {
          int index = random.Next(urn.Count);

          yield return urn[index];

          urn.RemoveAt(index);
        }
      }
    }
  }

And then use it:

private static Random gen = new Random();    

...

var result = new List<string>() {"A", "B", "C", "D"}
  .RandomItemNoReplacement(gen)
  .Take(10);

// D, C, B, A, C, A, B, D, A, C (seed == 123)  
Console.Write(string.Join(", ", result));

Answer 3

I think, you need somethink like next class:

public class RandomlyPickedWord
    {
        public IList<string> SourceWords{ get; set; }

        protected IList<string> Words{ get; set; }

        public RandomlyPickedWord(IList<string> sourceWords)
        {
            SourceWords = sourceWords;
        }

        protected IList<string> RepopulateWords(IList<string> sources)
        {
            Random randomizer = new Random ();
            IList<string> returnedValue;
            returnedValue = new List<string> ();

            for (int i = 0; i != sources.Count; i++)
            {
                returnedValue.Add (sources [randomizer.Next (sources.Count - 1)]);
            }

            return returnedValue;
        }

        public string GetPickedWord()
        {
            Random randomizer = new Random ();
            int curIndex;
            string returnedValue;

            if ((Words == null) || (Words.Any () == false))
            {
                Words = RepopulateWords (SourceWords);
            }

            curIndex = randomizer.Next (Words.Count);
            returnedValue = Words [curIndex];
            Words.RemoveAt (curIndex);

            return returnedValue;
        }
    }

Which you should use by the next way:

IList<string> source = new List<string> ();
            source.Add ("aaa");
            source.Add ("bbb");
            source.Add ("ccc");
            RandomlyPickedWord rndPickedWord = new RandomlyPickedWord (source);

            for (int i = 0; i != 6; i++)
            {
                Console.WriteLine (rndPickedWord.GetPickedWord ());
            }