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Array.BinarySearch<T> Method (T[], T, IComparer<T>)

Searches an entire one-dimensional sorted Array for a value using the specified IComparer<T> generic interface.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
public static int BinarySearch<T>(
	T[] array,
	T value,
	IComparer<T> comparer
)

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.

comparer
Type: System.Collections.Generic.IComparer<T>

The IComparer<T> implementation to use when comparing elements.

-or-

null to use the IComparable<T> implementation of each element.

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 null.

ArgumentException

comparer is null, and value is of a type that is not compatible with the elements of array.

InvalidOperationException

comparer is null, value does not implement the IComparable<T> generic interface, and the search encounters an element that does not implement the IComparable<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.

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<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<T> generic interface implementation provided by the element itself or by the specified value. The elements of array must already be sorted in increasing value according to the sort order defined by the IComparable<T> implementation; otherwise, the result might be incorrect.

NoteNote:

If comparer is null and value does not implement the IComparable<T> generic interface, the elements of array are not tested for IComparable<T> before the search begins. An exception is thrown if the search encounters an element that does not implement IComparable<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. When sorting, null is considered to be less than any other object.

NoteNote:

For every element tested, value is passed to the appropriate IComparable<T> implementation, even if value is null. That is, the IComparable<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 code example demonstrates the Sort<T>(T[], IComparer<T>) generic method overload and the BinarySearch<T>(T[], T, IComparer<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 the BinarySearch method.

NoteNote:

The calls to the Sort<T>(T[], IComparer<T>) and BinarySearch<T>(T[], T, IComparer<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 MSIL Disassembler (Ildasm.exe) to examine the Microsoft intermediate language (MSIL), you can see that the generic methods are being called.

The BinarySearch<T>(T[], T, IComparer<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<T>(T[], T, IComparer<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 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.

using System;
using System.Collections.Generic;

public class ReverseComparer: IComparer<string>
{
    public int Compare(string x, string y)
    {
        // Compare y and x in reverse order. 
        return y.CompareTo(x);
    }
}

public class Example
{
    public static void Main()
    {
        string[] dinosaurs = {"Pachycephalosaurus", 
                              "Amargasaurus", 
                              "Tyrannosaurus", 
                              "Mamenchisaurus", 
                              "Deinonychus", 
                              "Edmontosaurus"};

        Console.WriteLine();
        foreach( string dinosaur in dinosaurs )
        {
            Console.WriteLine(dinosaur);
        }

        ReverseComparer rc = new ReverseComparer();

        Console.WriteLine("\nSort");
        Array.Sort(dinosaurs, rc);

        Console.WriteLine();
        foreach( string dinosaur in dinosaurs )
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\nBinarySearch for 'Coelophysis':");
        int index = Array.BinarySearch(dinosaurs, "Coelophysis", rc);
        ShowWhere(dinosaurs, index);

        Console.WriteLine("\nBinarySearch for 'Tyrannosaurus':");
        index = Array.BinarySearch(dinosaurs, "Tyrannosaurus", rc);
        ShowWhere(dinosaurs, index);
    }

    private static void ShowWhere<T>(T[] array, int index)
    {
        if (index<0)
        {
            // If the index is negative, it represents the bitwise 
            // complement of the next larger element in the array. 
            //
            index = ~index;

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

            if (index == 0)
                Console.Write("beginning of array and ");
            else
                Console.Write("{0} and ", array[index-1]);

            if (index == array.Length)
                Console.WriteLine("end of array.");
            else
                Console.WriteLine("{0}.", array[index]);
        }
        else
        {
            Console.WriteLine("Found at index {0}.", index);
        }
    }
}

/* 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.
 */

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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