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List(Of T).BinarySearch Method (Int32, Int32, T, IComparer(Of T))

Searches a range of elements in the sorted List(Of T) for an element using the specified comparer and returns the zero-based index of the element.

Namespace:  System.Collections.Generic
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Public Function BinarySearch ( _
	index As Integer, _
	count As Integer, _
	item As T, _
	comparer As IComparer(Of T) _
) As Integer

Parameters

index
Type: System.Int32
The zero-based starting index of the range to search.
count
Type: System.Int32
The length of the range to search.
item
Type: T
The object to locate. The value can be Nothing for reference types.
comparer
Type: System.Collections.Generic.IComparer(Of T)
The IComparer(Of T) implementation to use when comparing elements, or Nothing to use the default comparer Comparer(Of T).Default.

Return Value

Type: System.Int32
The zero-based index of item in the sorted List(Of T), if item is found; otherwise, a negative number that is the bitwise complement of the index of the next element that is larger than item or, if there is no larger element, the bitwise complement of Count.

ExceptionCondition
ArgumentOutOfRangeException

index is less than 0.

-or-

count is less than 0.

ArgumentException

index and count do not denote a valid range in the List(Of T).

InvalidOperationException

comparer is Nothing, and the default comparer Comparer(Of T).Default cannot find an implementation of the IComparable(Of T) generic interface or the IComparable interface for type T.

The comparer customizes how the elements are compared.

If comparer is provided, the elements of the List(Of T) are compared to the specified value using the specified IComparer(Of T) implementation.

If comparer is Nothing, the default comparer Comparer(Of T).Default checks whether type T implements the IComparable(Of T) generic interface and uses that implementation, if available. If not, Comparer(Of T).Default checks whether type T implements the IComparable interface. If type T does not implement either interface, Comparer(Of T).Default throws InvalidOperationException.

The List(Of T) must already be sorted according to the comparer implementation; otherwise, the result is incorrect.

Comparing Nothing with any reference type is allowed and does not generate an exception when using the IComparable(Of T) generic interface. When sorting, Nothing is considered to be less than any other object.

If the List(Of T) contains more than one element with the same value, the method returns only one of the occurrences, and it might return any one of the occurrences, not necessarily the first one.

If the List(Of T) does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operation (~) to this negative integer to get the index of the first element that is larger than the search value. When inserting the value into the List(Of T), this index should be used as the insertion point to maintain the sort order.

This method is an O(log n) operation, where n is the number of elements in the range.

The following code example demonstrates the Sort(Int32, Int32, IComparer(Of T)) method overload and the BinarySearch(Int32, Int32, T, IComparer(Of T)) method overload.

The code example defines an alternative comparer for strings named DinoCompare, which implements the IComparer<string> (IComparer(Of String) in Visual Basic, IComparer<String^> in Visual C++) generic interface. The comparer works as follows: First, the comparands are tested for Nothing, and a null reference is treated as less than a non-null. Second, the string lengths are compared, and the longer string is deemed to be greater. Third, if the lengths are equal, ordinary string comparison is used.

A List(Of T) of strings is created and populated with the names of five herbivorous dinosaurs and three carnivorous dinosaurs. Within each of the two groups, the names are not in any particular sort order. The list is displayed, the range of herbivores is sorted using the alternate comparer, and the list is displayed again.

The BinarySearch(Int32, Int32, T, IComparer(Of T)) method overload is then used to search only the range of herbivores for "Brachiosaurus". The string is not found, and the bitwise complement (the ~ operator in C# and Visual C++, Xor -1 in Visual Basic) of the negative number returned by the BinarySearch(Int32, Int32, T, IComparer(Of T)) method is used as an index for inserting the new string.


Imports System.Collections.Generic

Public Class DinoComparer
   Implements IComparer(Of String)

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

      If x Is Nothing Then
         If y Is Nothing Then
            ' If x is Nothing and y is Nothing, they're
            ' equal. 
            Return 0
         Else
            ' If x is Nothing and y is not Nothing, y
            ' is greater. 
            Return -1
         End If
      Else
         ' If x is not Nothing...
         '
         If y Is Nothing Then
            ' ...and y is Nothing, x is greater.
            Return 1
         Else
            ' ...and y is not Nothing, compare the 
            ' lengths of the two strings.
            '
            Dim retval As Integer = _
                x.Length.CompareTo(y.Length)

            If retval <> 0 Then
               ' If the strings are not of equal length,
               ' the longer string is greater.
               '
               Return retval
            Else
               ' If the strings are of equal length,
               ' sort them with ordinary string comparison.
               '
               Return x.CompareTo(y)
            End If
         End If
      End If
   End Function
End Class

Public Class Example

   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      Dim dinosaurs As New List(Of String)

      dinosaurs.Add("Pachycephalosaurus")
      dinosaurs.Add("Parasauralophus")
      dinosaurs.Add("Amargasaurus")
      dinosaurs.Add("Galimimus")
      dinosaurs.Add("Mamenchisaurus")
      dinosaurs.Add("Deinonychus")
      dinosaurs.Add("Oviraptor")
      dinosaurs.Add("Tyrannosaurus")

      Dim herbivores As Integer = 5
      Display(outputBlock, dinosaurs)

      Dim dc As New DinoComparer

      outputBlock.Text &= vbLf & _
          "Sort a range with the alternate comparer:" & vbCrLf
      dinosaurs.Sort(0, herbivores, dc)
      Display(outputBlock, dinosaurs)

      outputBlock.Text &= String.Format(vbLf & _
          "BinarySearch a range and Insert ""{0}"":", _
          "Brachiosaurus") & vbCrLf

      Dim index As Integer = _
          dinosaurs.BinarySearch(0, herbivores, "Brachiosaurus", dc)

      If index < 0 Then
         index = index Xor -1
         dinosaurs.Insert(index, "Brachiosaurus")
         herbivores += 1
      End If

      Display(outputBlock, dinosaurs)

   End Sub

   Private Shared Sub Display(ByVal outputBlock As System.Windows.Controls.TextBlock, ByVal lis As List(Of String))
      outputBlock.Text &= vbCrLf
      For Each s As String In lis
         outputBlock.Text &= s & vbCrLf
      Next
   End Sub
End Class

' This code example produces the following output:
'
'Pachycephalosaurus
'Parasauralophus
'Amargasaurus
'Galimimus
'Mamenchisaurus
'Deinonychus
'Oviraptor
'Tyrannosaurus
'
'Sort a range with the alternate comparer:
'
'Galimimus
'Amargasaurus
'Mamenchisaurus
'Parasauralophus
'Pachycephalosaurus
'Deinonychus
'Oviraptor
'Tyrannosaurus
'
'BinarySearch a range and Insert "Brachiosaurus":
'
'Galimimus
'Amargasaurus
'Brachiosaurus
'Mamenchisaurus
'Parasauralophus
'Pachycephalosaurus
'Deinonychus
'Oviraptor
'Tyrannosaurus


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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