Array.BinarySearch(Of T) Method (T(), T, IComparer(Of T))
Searches an entire one-dimensional sorted array for a value using the specified IComparer(Of T) generic interface.
Assembly: mscorlib (in mscorlib.dll)
Public Shared Function BinarySearch(Of T) ( array As T(), value As T, comparer As IComparer(Of T) ) As Integer
Parameters
- array
-
Type:
T()
The sorted one-dimensional, zero-based Array to search.
- value
-
Type:
T
The object to search for.
- comparer
-
Type:
System.Collections.Generic.IComparer(Of T)
The IComparer(Of T) implementation to use when comparing elements.
-or-
null to use the IComparable(Of T) implementation of each element.
Return Value
Type: System.Int32The index of the specified value in the specified array, if value is found; otherwise, a negative number. If value is not found and value is less than one or more elements in array, the negative number returned 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 all elements in array, the negative number returned is the bitwise complement of (the index of the last element plus 1). If this method is called with a non-sorted array, the return value can be incorrect and a negative number could be returned, even if value is present in array.
Type Parameters
- T
The type of the elements of the array.
| Exception | Condition |
|---|---|
| ArgumentNullException | array is null. |
| ArgumentException | comparer is null, and value is of a type that is not compatible with the elements of array. |
| InvalidOperationException | comparer is null, and T does not implement the IComparable(Of T) generic interface |
This method does not support searching arrays that contain negative indexes. array must be sorted before calling this method.
If the Array does not contain the specified value, the method returns a negative integer. You can apply the bitwise complement operator (~ in C#, Not in Visual Basic) to the negative result to produce an index. If this index is 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.
The comparer customizes how the elements are compared. For example, you can use a System.Collections.CaseInsensitiveComparer as the comparer to perform case-insensitive string searches.
If comparer is not null, the elements of array are compared to the specified value using the specified IComparer(Of T) generic interface implementation. The elements of array must already be sorted in increasing value according to the sort order defined by comparer; otherwise, the result might be incorrect.
If comparer is null, the comparison is done using the IComparable(Of T) generic interface implementation provided by T. 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.
Note |
|---|
If comparer is null and 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.
null can always be compared with any other reference type; therefore, comparisons with null do not generate an exception.
Note |
|---|
For every element tested, value is passed to the appropriate IComparable(Of T) implementation, even if value is null. That is, the IComparable(Of T) implementation determines how a given element compares to null. |
This method is an O(log n) operation, where n is the Length of array.
The following example demonstrates the Sort(Of T)(T(), IComparer(Of T)) generic method overload and the BinarySearch(Of T)(T(), T, IComparer(Of T)) generic method overload.
The code example defines an alternative comparer for strings, named ReverseCompare, which implements the IComparer<string> (IComparer(Of String) in Visual Basic, IComparer<String^> in Visual C++) generic interface. The comparer calls the CompareTo(String) method, reversing the order of the comparands so that the strings sort high-to-low instead of low-to-high.
The array is displayed, sorted, and displayed again. Arrays must be sorted in order to use theBinarySearch(Of T) method.
Note |
|---|
The calls to the Sort(Of T)(T(), IComparer(Of T)) and BinarySearch(Of T)(T(), T, IComparer(Of T)) 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 Ildasm.exe (IL Disassembler) to examine the Microsoft intermediate language (MSIL), you can see that the generic methods are being called. |
The BinarySearch(Of T)(T(), T, IComparer(Of 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(Of T)(T(), T, IComparer(Of T)) 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 ShowWheremethod 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 ReverseComparer Implements IComparer(Of String) Public Function Compare(ByVal x As String, _ ByVal y As String) As Integer _ Implements IComparer(Of String).Compare ' Compare y and x in reverse order. Return y.CompareTo(x) End Function End Class 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 Dim rc As New ReverseComparer() Console.WriteLine(vbLf & "Sort") Array.Sort(dinosaurs, rc) 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", rc) ShowWhere(dinosaurs, index) Console.WriteLine(vbLf & _ "BinarySearch for 'Tyrannosaurus':") index = Array.BinarySearch(dinosaurs, "Tyrannosaurus", rc) 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 ' 'Tyrannosaurus 'Pachycephalosaurus 'Mamenchisaurus 'Edmontosaurus 'Deinonychus 'Amargasaurus ' 'BinarySearch for 'Coelophysis': 'Not found. Sorts between: Deinonychus and Amargasaurus. ' 'BinarySearch for 'Tyrannosaurus': 'Found at index 0.
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
