xnaFan's Blog

This is an extensionmethod for shuffling generic lists.

I absolutely LOVE generics!

Back in the days of .net 1.0 ("yaaawn" - I know, but bear with me...) there was a class called CollectionBase which had an ArrayList (like a "List<Object>") inside, and to make your own strongly typed collection you would inherit from CollectionBase and implement a typed Setter and Getter. No more of that - phew!!  Thank you Microsoft!!

The sample project  gives a demonstration of shuffling four different List types:

List<int> _numbers;
List<string> _names;
List<Color> _colors;
List<Card> _cards;

How to use it

When you've added a reference to the ListShufflerExtensionMethods class, all Lists get a Shuffle() method. So you can call Shuffle on any type of List.

_numbers.Shuffle();
_names.Shuffle();
_colors.Shuffle();
_cards.Shuffle();

The Shuffle() method has an optional parameter numberOfTimesToShuffle where you can specify how many times to shuffle. The default value is 5.

Demonstration

Complete code

Here you can see the implementation. Extensionmethods have to be public, static methods on a public static class. Using the "this" keyword on the first parameter passed to a method you effectively add the method to that type (see line 16 below).

/// <summary>
/// Class for shuffling lists
/// </summary>
/// <typeparam name="T">The type of list to shuffle</typeparam>
public static class ListShufflerExtensionMethods
{
    //for getting random values
    private static Random _rnd = new Random();

    /// <summary>
    /// Shuffles the contents of a list
    /// </summary>
    /// <typeparam name="T">The type of the list to sort</typeparam>
    /// <param name="listToShuffle">The list to shuffle</param>
    /// <param name="numberOfTimesToShuffle">How many times to shuffle the list
    /// by default this is 5 times</param>
    public static void Shuffle<T>(this List<T> listToShuffle, int numberOfTimesToShuffle = 5)
    {
        //make a new list of the wanted type
        List<T> newList = new List<T>();

        //for each time we want to shuffle
        for (int i = 0; i < numberOfTimesToShuffle; i++)
        {
            //while there are still items in our list
            while (listToShuffle.Count > 0)
            {
                //get a random number within the list
                int index = _rnd.Next(listToShuffle.Count);

                //add the item at that position to the new list
                newList.Add(listToShuffle[index]);

                //and remove it from the old list
                listToShuffle.RemoveAt(index);
            }

            //then copy all the items back in the old list again
            listToShuffle.AddRange(newList);

            //and clear the new list
            //to make ready for next shuffling
            newList.Clear();
        }
    }
}

Source code

Solution file as ZIP

ListShufflerExtensions.cs (right click > save as...)

ListShufflerExtensions.zip

This entry was posted on Friday, January 4th, 2013 at 22:22 and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.