Most efficient way to randomly “sort” (Shuffle) a list of integers in C#

Question

I need to randomly \'sort\' a list of integers (0-1999) in the most efficient way possible. Any ideas?

Currently, I am doing something like this:

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Answer 1

Here is what I used. This is surely not the fastest one, but it is probably good enough for most cases and most importantly, it is very simple.

IEnumerable<ListItem> list = ...;
Random random = new Random(); // important to not initialize a new random in the OrderBy() function
return list.OrderBy(i => random.Next());

Answer 2

Wouldn't something like this work?

var list = new[]{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
var random = new Random();
list.Sort((a,b)=>random.Next(-1,1));

Answer 3

itemList.OrderBy(x=>Guid.NewGuid()).Take(amount).ToList()

Answer 4

I am not sure of the efficiency factor, but I have used something similar to the following, if you aren't opposed to using an ArrayList:

private ArrayList ShuffleArrayList(ArrayList source)
{
    ArrayList sortedList = new ArrayList();
    Random generator = new Random();

    while (source.Count > 0)
    {
        int position = generator.Next(source.Count);
        sortedList.Add(source[position]);
        source.RemoveAt(position);
    }

    return sortedList;
}

Using this, you do not have to worry about the intermediate swapping.

Answer 5

To improve your efficiency you can keep a set of values/indices that have been swapped rather than a boolean for indicating they were swapped. Pick your randomized swap index from the remaining pool. When the pool is 0, or when you made it through the initial list then you are done. You don't have the potential to try to select a random swap index value.

When you do a swap, just remove them from the pool.

For the size of data you are looking at it is no big deal.

Answer 6

what about :

System.Array.Sort(arrayinstance, RandomizerMethod);
...
//any evoluated random class could do it !
private static readonly System.Random Randomizer = new System.Random();

private static int RandomizerMethod<T>(T x, T y)
    where T : IComparable<T>
{
    if (x.CompareTo(y) == 0)
        return 0;

    return Randomizer.Next().CompareTo(Randomizer.Next());
}

voila!