Randomizing elements of the List<>

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Randomize a List<T> in C#

Let's suppose that I have

List<SomeClass> _myList;

and I populate _myList with ordered collection of objects. Now, I would like to shuffle them. How?

I was thinking of using

_myList.Sort( (a,b) => r.GetNext() - 0.5 );

but I suppose that can work only on SOME implementations of Sort().

BTW, r = new Random();

BTW2, my lists are thousands elements large...

See http://stackoverflow.com/questions/273313/randomize-a-listt-in-c-sharp and http://stackoverflow.com/questions/375351/most-efficient-way-to-randomly-sort-shuffle-a-list-of-integers-in-c-sharp — flipchart, Oct 27 '11 at 07:12
I think you need [this][1] [1]: http://stackoverflow.com/questions/273313/randomize-a-listt-in-c-sharp — luqi, Oct 27 '11 at 07:14
See also http://stackoverflow.com/questions/1287567/c-is-using-random-and-orderby-a-good-shuffle-algorithm — MartinStettner, Oct 27 '11 at 07:44

vc 74 · Answer 1 · 2011-10-27T08:13:54.180

3

_myList.OrderBy(item => r.Next());

Should return you the list's items in random order.

edited Oct 27 '11 at 08:13

answered Oct 27 '11 at 07:20

vc 74

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@DanielMošmondor, nope but I that that should do it – vc 74 Oct 27 '11 at 07:31
It's `r.Next()` by the way... – Yannick Motton Oct 27 '11 at 07:33
@Yannick, true, I've fixed the code (un bete copier/coller...) – vc 74 Oct 27 '11 at 08:14
This does work, but keep in mind, it does not change the order of the existing list, it only returns a new sequence randomly ordered. – Chris Dunaway Oct 27 '11 at 15:14

score 0 · Answer 2 · answered Oct 27 '11 at 07:17

u look some thing like this.

private List<E> ShuffleList<E>(List<E> inputList)
{
     List<E> randomList = new List<E>();

     Random r = new Random();
     int randomIndex = 0;
     while (inputList.Count > 0)
     {
          randomIndex = r.Next(0, inputList.Count); //Choose a random object in the list
          randomList.Add(inputList[randomIndex]); //add it to the new, random list
          inputList.RemoveAt(randomIndex); //remove to avoid duplicates
     }

     return randomList; //return the new random list
}

score 0 · Answer 3 · answered Oct 27 '11 at 08:40

Here's an idea, extend IList in a (hopefully) efficient way.

public static IEnumerable<T> Shuffle<T>(this IList<T> list)
{
    var choices = Enumerable.Range(0, list.Count).ToList();
    var rng = new Random();
    for(int n = choices.Count; n > 1; n--)
    {
        int k = rng.Next(n);
        yield return list[choices[k]];
        choices.RemoveAt(k);
    }
    yield return list[choices[0]];
}

Randomizing elements of the List<>

3 Answers3