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Array.BinarySearch(Of T) Method (T(), T)

Searches an entire one-dimensional sorted Array for a specific element, using the IComparable(Of T) generic interface implemented by each element of the Array and by the specified object.

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

'Declaration
Public Shared Function BinarySearch(Of T) ( _
	array As T(), _
	value As T _
) As Integer
'Usage
Dim array As T()
Dim value As T
Dim returnValue As Integer 

returnValue = Array.BinarySearch(array, _
	value)

Type Parameters

T

The type of the elements of the array.

Parameters

array
Type: T()

The sorted one-dimensional, zero-based Array to search.

value
Type: T

The object to search for.

Return Value

Type: System.Int32
The index of the specified value in the specified array, if value is found. If value is not found and value is less than one or more elements in array, a negative number which is the bitwise complement of the index of the first element that is larger than value. If value is not found and value is greater than any of the elements in array, a negative number which is the bitwise complement of (the index of the last element plus 1).

ExceptionCondition
ArgumentNullException

array is Nothing.

InvalidOperationException

value does not implement the IComparable(Of T) generic interface, and the search encounters an element that does not implement the IComparable(Of T) generic interface.

If the Array does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operator (~) to the negative result (in Visual Basic, Xor the negative result with -1) to produce an index. If this index is greater than or equal to the size of the array, there are no elements larger than value in the array. Otherwise, it is the index of the first element that is larger than value.

Either value or every element of array must implement the IComparable(Of T) generic interface, which is used for comparisons. The elements of array must already be sorted in increasing value according to the sort order defined by the IComparable(Of T) implementation; otherwise, the result might be incorrect.

NoteNote:

If value does not implement the IComparable(Of T) generic interface, the elements of array are not tested for IComparable(Of T) before the search begins. An exception is thrown if the search encounters an element that does not implement IComparable(Of T).

Duplicate elements are allowed. If the Array contains more than one element equal to value, the method returns the index of only one of the occurrences, and not necessarily the first one.

Nothing can always be compared with any other reference type; therefore, comparisons with Nothing do not generate an exception. When sorting, Nothing is considered to be less than any other object.

NoteNote:

For every element tested, value is passed to the appropriate IComparable(Of T) implementation, even if value is Nothing. That is, the IComparable(Of T) implementation determines how a given element compares to Nothing.

This method is an O(log n) operation, where n is the Length of array.

The following code example demonstrates the Sort(Of T)(T()) generic method overload and the BinarySearch(Of T)(T(), T) generic method overload. An array of strings is created, in no particular order.

The array is displayed, sorted, and displayed again. Arrays must be sorted in order to use the BinarySearch method.

NoteNote:

The calls to the Sort and BinarySearch generic methods do not look any different from calls to their nongeneric counterparts, because Visual Basic, C#, and C++ infer the type of the generic type parameter from the type of the first argument. If you use the MSIL Disassembler (Ildasm.exe) to examine the Microsoft intermediate language (MSIL), you can see that the generic methods are being called.

The BinarySearch(Of T)(T(), T) generic method overload is then used to search for two strings, one that is not in the array and one that is. The array and the return value of the BinarySearch method are passed to the ShowWhere generic method, which displays the index value if the string is found, and otherwise the elements the search string would fall between if it were in the array. The index is negative if the string is not n the array, so the ShowWhere method takes the bitwise complement (the ~ operator in C# and Visual C++, Xor -1 in Visual Basic) to obtain the index of the first element in the list that is larger than the search string.

Imports System
Imports System.Collections.Generic

Public Class Example

    Public Shared Sub Main()

        Dim dinosaurs() As String = { _
            "Pachycephalosaurus", _
            "Amargasaurus", _
            "Tyrannosaurus", _
            "Mamenchisaurus", _
            "Deinonychus", _
            "Edmontosaurus"  }

        Console.WriteLine()
        For Each dinosaur As String In dinosaurs
            Console.WriteLine(dinosaur)
        Next

        Console.WriteLine(vbLf & "Sort")
        Array.Sort(dinosaurs)

        Console.WriteLine()
        For Each dinosaur As String In dinosaurs
            Console.WriteLine(dinosaur)
        Next

        Console.WriteLine(vbLf & _
            "BinarySearch for 'Coelophysis':")
        Dim index As Integer = _
            Array.BinarySearch(dinosaurs, "Coelophysis")
        ShowWhere(dinosaurs, index)

        Console.WriteLine(vbLf & _
            "BinarySearch for 'Tyrannosaurus':")
        index = Array.BinarySearch(dinosaurs, "Tyrannosaurus")
        ShowWhere(dinosaurs, index)

    End Sub 

    Private Shared Sub ShowWhere(Of T) _
        (ByVal array() As T, ByVal index As Integer) 

        If index < 0 Then 
            ' If the index is negative, it represents the bitwise 
            ' complement of the next larger element in the array. 
            '
            index = index Xor -1

            Console.Write("Not found. Sorts between: ")

            If index = 0 Then
                Console.Write("beginning of array and ")
            Else
                Console.Write("{0} and ", array(index - 1))
            End If  

            If index = array.Length Then
                Console.WriteLine("end of array.")
            Else
                Console.WriteLine("{0}.", array(index))
            End If  
        Else
            Console.WriteLine("Found at index {0}.", index)
        End If 

    End Sub 

End Class 

' This code example produces the following output: 

'Pachycephalosaurus 
'Amargasaurus 
'Tyrannosaurus 
'Mamenchisaurus 
'Deinonychus 
'Edmontosaurus 

'Sort 

'Amargasaurus 
'Deinonychus 
'Edmontosaurus 
'Mamenchisaurus 
'Pachycephalosaurus 
'Tyrannosaurus 

'BinarySearch for 'Coelophysis': 
'Not found. Sorts between: Amargasaurus and Deinonychus. 

'BinarySearch for 'Tyrannosaurus': 
'Found at index 5.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0
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