How can I get to any item in a list at random?

Problem

I have an ArrayList, and I need to be able to press a button, then pick a string at random from the list and show it in a messagebox.

What would be the best way for me to go about it?

Asked by jay_t55

Solution #1

Answered by mmx

Solution #2

This small selection of extension techniques is what I generally use:

public static class EnumerableExtension
{
    public static T PickRandom<T>(this IEnumerable<T> source)
    {
        return source.PickRandom(1).Single();
    }

    public static IEnumerable<T> PickRandom<T>(this IEnumerable<T> source, int count)
    {
        return source.Shuffle().Take(count);
    }

    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source)
    {
        return source.OrderBy(x => Guid.NewGuid());
    }
}

This would allow you to write: for a strongly typed list:

var strings = new List<string>();
var randomString = strings.PickRandom();

You can cast an ArrayList if that is all you have:

var strings = myArrayList.Cast<string>();

Answered by Mark Seemann

Solution #3

You can do:

list.OrderBy(x => Guid.NewGuid()).FirstOrDefault()

Answered by Felipe Pessoto

Solution #4

Alternatively, you could use a basic extension class like this:

public static class CollectionExtension
{
    private static Random rng = new Random();

    public static T RandomElement<T>(this IList<T> list)
    {
        return list[rng.Next(list.Count)];
    }

    public static T RandomElement<T>(this T[] array)
    {
        return array[rng.Next(array.Length)];
    }
}

Then just call:

myList.RandomElement();

It’ll also work with arrays.

I would avoid using OrderBy() because it can be costly when dealing with huge datasets. For this, use indexed collections like ListT> or arrays.

Answered by Dave_cz

Solution #5

Make a haphazard instance of yourself:

Random rnd = new Random();

To get a random string, perform the following:

string s = arraylist[rnd.Next(arraylist.Count)];

Keep in mind that if you’re going to do this a lot, you should reuse the Random object. Put it in the class as a static field so it’s only initialized once and then accessed.

Answered by Joey

Post is based on https://stackoverflow.com/questions/2019417/how-to-access-random-item-in-list