List BinarySearch() Method in C# (original) (raw)

Last Updated : 19 Jul, 2024

List.BinarySearch(T) Method uses a binary search algorithm to locate a specific element in the sorted List or a portion of it. There are 3 methods in the overload list of this method as follows:

BinarySearch(T) Method

This method searches for an element in the entire sorted **List using the _default comparer and returns the zero-based index of the searched element.

**Syntax:

public int BinarySearch (T item);

Here, item is the object which is to be locate and the value of _item can be **null or **reference type.

**Return Type: If the _item is found, then this method returns the _zero-based index of the element to be searched for and if not found, then a negative number that is the _bitwise complement of the index of the next element will be return and the complement is larger than that item. If there is no larger element, the _bitwise complement of _Count will be return.

**Exception: This method will give **InvalidOperationException if the default comparer _Default cannot find an implementation of the IComparable generic interface or the IComparable interface for type T.

Below programs illustrate the use of above-discussed method:

**Example 1:

C# `

// C# program to illustrate the // List.BinarySearch(T) Method using System; using System.Collections.Generic;

class GFG {

// Main Method
public static void Main()
{
    // List creation
    List<string> Geek = new List<string>();

    // List elements
    Geek.Add("ABCD");
    Geek.Add("QRST");
    Geek.Add("XYZ");
    Geek.Add("IJKL");


    Console.WriteLine("The Original List is:");
    foreach(string g in Geek)
    {
        
        // prints original List
        Console.WriteLine(g);
        
    }

    Console.WriteLine("\nThe List in Sorted form");
    
    // sort the List
    Geek.Sort();


    Console.WriteLine();
    foreach(string g in Geek)
    {
        // prints the sorted List
        Console.WriteLine(g);
        
    }

    Console.WriteLine("\nInsert EFGH :");
    
    // insert "EFGH" in the List
    //"EFGH" insert into its original 
    // position when the List is sorted
    int index = Geek.BinarySearch("EFGH");
    

    if (index < 0) 
    {
        
        Geek.Insert(~index, "EFGH");
    }

    Console.WriteLine();
    
    foreach(string g in Geek)
    {
        
        // prints the sorted list
        // after inserting "EFGH"
        Console.WriteLine(g);
    }
}

}

`

Output

The Original List is: ABCD QRST XYZ IJKL

The List in Sorted form

ABCD IJKL QRST XYZ

Insert EFGH :

ABCD EFGH IJKL QRST XYZ

**Example 2: In this example, the List is created with some integer values and to insert a new integer using _BinarySearch(T) method in the List by using a user define function.

C# `

// C# program to illustrate the // List.BinarySearch(T) Method using System; using System.Collections.Generic;

class GFG {

// method for inserting "3" public void binarySearch(List Geek) {

// insert "3" in the List
Console.WriteLine("\nInsert 3 :");

// "3" insert into its original 
// position when the List is 
// sorted
int index = Geek.BinarySearch(3);


if (index < 0) 
{
    Geek.Insert(~index, 3);
}

foreach(int g in Geek)
{
    
    // prints the sorted list
    // after inserting "3"
    Console.WriteLine(g);
    
}

} }

// Driver Class public class search {

public static void Main()
{
    
    // List creation
    GFG gg = new GFG();
    
    List<int> Geek = new List<int>() {
                          5, 6, 1, 9};
                          
    Console.WriteLine("Original List");

    foreach(int g in Geek)
    {
        Console.WriteLine(g);
        // prints original List
    }

    Console.WriteLine("\nList in Sorted form");
    Geek.Sort();

    foreach(int g in Geek)
    {
        Console.WriteLine(g);
        // prints the sorted List
    }

    // calling the method "binarySearch"
    gg.binarySearch(Geek);
}

}

`

Output

Original List 5 6 1 9

List in Sorted form 1 5 6 9

Insert 3 : 1 3 5 6 9

BinarySearch(T) Method

This method searches for an element in the entire sorted List using the specified comparer and returns the zero-based index of the searched element.

**Syntax:

public int BinarySearch (T item, System.Collections.Generic.IComparer comparer);

**Parameters:

**Return Value: If the item founds, then this method returns the zero-based index of the element to be searched for and if not found, then 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.

**Exception: This method will give _InvalidOperationException if the comparer is null, and the default comparer Default cannot find an implementation of the IComparable generic interface or the IComparable interface for type _T.

Below programs illustrate the use of the above-discussed method:

**Example 1:

C# `

// C# program to demonstrate the // List.BinarySearch(T, // IComparer) Method using System; using System.Collections.Generic;

class GFG : IComparer {

public int Compare(string x, string y)
{
    if (x == null || y == null) 
    {
        return 0;
    }
    return x.CompareTo(y);
    //"CompareTo()" method
}

}

// Driver Class class geek {

// Main Method
public static void Main()
{
    // list creation
    List<string> list1 = new List<string>();

    // list elements
    list1.Add("B");
    list1.Add("C");
    list1.Add("E");
    list1.Add("A");

    // prints Original list
    Console.WriteLine("Original string");

    foreach(string g in list1)
    {
        Console.WriteLine(g);
    }

    GFG gg = new GFG();

     // sort the list
    list1.Sort(gg);
   
    // prints the sorted form of original list
    Console.WriteLine("\nList in sorted form");

    foreach(string g in list1)
    {
        Console.WriteLine(g);
    }

    //"D" is going to insert
    //"gg" is the IComparer
    int index = list1.BinarySearch("D", gg);
   
    if (index < 0) 
    {
        list1.Insert(~index, "D");
    }

    // prints the final List
    Console.WriteLine("\nAfter inserting \"D\" in the List");
    
    foreach(string g in list1)
    {
        Console.WriteLine(g);
    }
}

}

`

**Output:

Original string
B
C
E
A

List in sorted form
A
B
C
E

After inserting "D" in the List
A
B
C
D
E

**Example 2: In this example, the List is created with some integer values and to insert a new integer using BinarySearch(T, Comparer ) method in the List by using a user defined function.

C# `

// C# program to demonstrate the // List.BinarySearch(T, // IComparer) Method using System; using System.Collections.Generic;

class GFG : IComparer {

public int Compare(int x, int y)
{
    if (x == 0 || y == 0) 
    {
        return 0;
    }
    return x.CompareTo(y);
}

}

// Driver Class class geek {

// Main Method
public static void Main()
{
    // list creation
    List<int> list1 = new List<int>() {
                           5, 6, 1, 9};

    // prints Original list
    Console.WriteLine("Original string");

    foreach(int g in list1)
    {
        Console.WriteLine(g);
    }

     // creating object of class GFG
    GFG gg = new GFG();
 
    // sort the list
    list1.Sort(gg);
    
    // prints the sorted form 
    // of original list
    Console.WriteLine("\nList in sorted form");

    foreach(int g in list1)
    {
        Console.WriteLine(g);
    }

    bSearch b = new bSearch();
    b.binarySearch(list1);
}

}

class bSearch {

public void binarySearch(List<int> list1)
{

    // creating object of class GFG
    GFG gg = new GFG();

    // "3" is going to insert
    // "gg" is the IComparer
    int index = list1.BinarySearch(3, gg);
    
    if (index < 0) 
    {
        list1.Insert(~index, 3);
    }

    // prints the final List
    Console.WriteLine("\nAfter inserting \"3\" in the List");
    foreach(int g in list1)
    {
        Console.WriteLine(g);
    }
}

}

`

Output

Original string 5 6 1 9

List in sorted form 1 5 6 9

After inserting "3" in the List 1 3 5 6 9

BinarySearch(Int32, Int32, T, IComparer)

This method is used to search a range of elements in the sorted List for an element using the specified comparer and returns the zero-based index of the element.

**Syntax:

public int BinarySearch (int index, int count, T item, System.Collections.Generic.IComparer comparer);

**Parameters:

**index: It is the zero-based starting index of the range to search.
**count: It is the length of the range to search.
**item: It is the object to locate. The value can be null for the reference type.
**comparer: It is the IComparer implementation to use when comparing elements, or null to use the default comparer Default.

**Return Value: It returns the zero-based index of item in the sorted List, if the 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.

**Exceptions:

**Example:

C# `

// C# program to demonstrate the // List.BinarySearch(Int32, // Int32, T, Comparer ) method using System; using System.Collections.Generic;

class GFG : IComparer { public int Compare(int x, int y) { if (x == 0 || y == 0) { return 0; } return x.CompareTo(y); } }

class search {

// "binarySearch" function public void binarySearch(List list1, int i) { Console.WriteLine("\nBinarySearch a "+ "range and Insert 3");

// "gg" is the object of class GFG
GFG gg = new GFG();

// binary search
int index = list1.BinarySearch(0, i,
                             3, gg);


if (index < 0)
{
    
    // insert "3"
    list1.Insert(~index, 3);
    i++;
}

Display(list1);

}

// "Display" function public void Display(List list) {

foreach( int g in list )
{
    Console.WriteLine(g);
}

} }

// Driver Class class geek {

// Main Method
public static void Main()
{
    List<int> list1 = new List<int>()
    {
        // list elements
        15,4,2,9,5,7,6,8,10
        
    };

    int i = 7;
    Console.WriteLine("Original List");
    
    // "d" is the object of 
    // the class "search"
    search d = new search();
    
    // prints Original list
    d.Display(list1);

    // "gg" is the object 
    // of class GFG 
    GFG gg = new GFG();
    
    Console.WriteLine("\nSort a range with "+
                   "the alternate comparer");
    
    // sort is happens between
    // index 1 to 7            
    list1.Sort(1, i, gg);
    
    // prints sorted list
    d.Display(list1);
    
    // call "binarySearch" function
    d.binarySearch(list1,i);
    
}

}

`

Output

Original List 15 4 2 9 5 7 6 8 10

Sort a range with the alternate comparer 15 2 4 5 6 7 8 9 10

BinarySearch a range and Insert 3 15 2 3 4 5 6 7 8 9 10

**Note:

**Reference: