Is there an easy way to randomize a list in VB.NET?

Solution 1:

Check out the Fisher-Yates shuffle algorithm here: http://en.wikipedia.org/wiki/Knuth_shuffle

with a more concise discussion by this site's chief overlord here: http://www.codinghorror.com/blog/archives/001015.html

There is a simple C# implementation in the blog entry that should be real easy to change to VB.NET

Solution 2:

I've extended the List class with the following Randomize() function to use the Fisher-Yates shuffle algorithm:

''' <summary>
''' Randomizes the contents of the list using Fisher–Yates shuffle (a.k.a. Knuth shuffle).
''' </summary>
''' <typeparam name="T"></typeparam>
''' <param name="list"></param>
''' <returns>Randomized result</returns>
''' <remarks></remarks>
<Extension()>
Function Randomize(Of T)(ByVal list As List(Of T)) As List(Of T)
    Dim rand As New Random()
    Dim temp As T
    Dim indexRand As Integer
    Dim indexLast As Integer = list.Count - 1
    For index As Integer = 0 To indexLast
        indexRand = rand.Next(index, indexLast)
        temp = list(indexRand)
        list(indexRand) = list(index)
        list(index) = temp
    Next index
    Return list
End Function

Solution 3:

Build a Comparer:

Public Class Randomizer(Of T)
    Implements IComparer(Of T)

    ''// Ensures different instances are sorted in different orders
    Private Shared Salter As New Random() ''// only as random as your seed
    Private Salt As Integer
    Public Sub New()
        Salt = Salter.Next(Integer.MinValue, Integer.MaxValue)
    End Sub

    Private Shared sha As New SHA1CryptoServiceProvider()
    Private Function HashNSalt(ByVal x As Integer) As Integer
      Dim b() As Byte = sha.ComputeHash(BitConverter.GetBytes(x))
      Dim r As Integer = 0
      For i As Integer = 0 To b.Length - 1 Step 4
          r = r Xor BitConverter.ToInt32(b, i)
      Next

      Return r Xor Salt
    End Function

    Public Function Compare(x As T, y As T) As Integer _
        Implements IComparer(Of T).Compare

        Return HashNSalt(x.GetHashCode()).CompareTo(HashNSalt(y.GetHashCode()))
    End Function
End Class

Use it like this, assuming you mean a generic List(Of FileInfo):

list.Sort(New Randomizer(Of IO.FileInfo)())

You can also use a closure to make the random value 'sticky' and then just use linq's .OrderBy() on that (C# this time, because the VB lambda syntax is ugly):

list = list.OrderBy(a => Guid.NewGuid()).ToList();

Explained here, along with why it might not even be as fast as real shuffle:
http://www.codinghorror.com/blog/archives/001008.html?r=31644

Solution 4:

There are several reasonable methods of shuffling.

One has already been mentioned. (The Knuth Shuffle.)

Another method would be to assign a "weight" to each element and sort the list according to that "weight." This method is possible but would be unweildy because you cannot inherit from FileInfo.

One final method would be to randomly select an element in the original list and add it to a new list. Of course, that is, if you don't mind creating a new list. (Haven't tested this code...)

        Dim rnd As New Random
        Dim lstOriginal As New List(Of FileInfo)
        Dim lstNew As New List(Of FileInfo)

        While lstOriginal.Count > 0
            Dim idx As Integer = rnd.Next(0, lstOriginal.Count - 1)
            lstNew.Add(lstOriginal(idx))
            lstOriginal.RemoveAt(idx)
        End While