Randomize files output in folder

Question

How do I randomize the order of the files I get out of:

string[] files = Directory.GetFiles("folder");

Thank you! :-)

Answer 1

One option is to use Random:

Random rng = new Random();

and then:

var randomOrderFiles = files.OrderBy(f => rng.Next());

This isn't the most efficient method as it takes O(nlogn). If this is a problem for you, better algorithms exist.

Answer 2

If you can't use Linq the following method should work:

static Random rand = new Random();
static void Randomize<T>(IList<T> list)
{
    for (int i = list.Count - 1; i > 0; i--)
    {
        int i2 = rand.Next(i + 1);
        if (i2 != i)
        {
            T tmp = list[i2];
            list[i2] = list[i];
            list[i] = tmp;
        }
    }
}

Answer 3

A Fisher-Yates-Durstenfeld shuffle is O(n) and should give unbiased distribution.

Create a helper/extension method to perform an in-place shuffle on the array returned from GetFiles:

// uses ShuffleInPlace extension from https://stackoverflow.com/a/5589250/55847
var arrayOfFiles = Directory.GetFiles("folder");
arrayOfFiles.ShuffleInPlace();

If you prefer to return a new sequence -- à la LINQ -- you could create a suitable Shuffle extension method instead:

// uses Shuffle extension from https://stackoverflow.com/a/1653204/55847
var sequenceOfFiles = Directory.EnumerateFiles("folder").Shuffle();

Answer 4

1. Iterate over the source list.
2. Remove a random source item from the source list
3. Append removed item to a result list
4. Repeat until the source list is empty

List<string> files = Directory.GetFiles("folder");
List<string> result = new List<string>();

while (files.Count > 0)
{
  int n = IntegerUtility.Random(files.Count);
  string file = files.Remove(n);
  result.Add(file);
} 
return result;