How to use Array.BinarySearch() Method in C# | Set -1

Array.BinarySearch() method is used to search a value in a sorted one dimensional array. The binary search algorithm is used by this method. This algorithm searches a sorted array by repeatedly dividing the search interval in half. Begin with an interval covering the whole array. If the value of the search key is less than the item in the middle of the interval, narrow the interval to the lower half. Otherwise, narrow it to the upper half. Repeatedly check until the value is found or the interval is empty.

Important Points:

• Before calling this method, the array must be sorted.
• This method will return the negative integer if the array doesn’t contain the specified value.
• The array must be one-dimensional otherwise this method can’t be used.
• The Icomparable interface must be implemented by the value or every element of the array.
• The method will return the index of only one of the occurrences if more than one matched elements found in the array and it is not necessary that index will be of the first occurrence.

There are total 8 methods in the overload list of this method as follows:

• BinarySearch(Array, Object)
• BinarySearch(Array, Object, IComparer)
• BinarySearch(Array, Int32, Int32, Object)
• BinarySearch(Array, Int32, Int32, Object, IComparer)
• BinarySearch<T>(T[], T)
• BinarySearch<T>(T[], T, IComparer<T>)
• BinarySearch<T>(T[], Int32, Int32, T)
• BinarySearch<T>(T[], Int32, Int32, T, IComparer<T>)

BinarySearch(Array, Object) Method

This method is used to search a specific element in the entire 1-D sorted array. It used the IComparable interface that is implemented by each element of the 1-D array and the specified object. This method is an O(log n) operation, where n is the Length of the specified array. 

Syntax: public static int BinarySearch (Array arr, object val);
Parameters:
arr: It is the sorted 1-D array to search. 
val: It is the object to search for. 

Return Value: It returns the index of the specified valin the specified arr if the val is found otherwise it returns a negative number. There are different cases of return values as follows:

• If the val is not found and valis less than one or more elements in the arr, the negative number returned is the bitwise complement of the index of the first element that is larger than val.
• If the val is not found and val is greater than all elements in the arr, 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 the val is present in the arr.

Exceptions:

• ArgumentNullException: If the arr is null.
• RankException: If the arr is multidimensional.
• ArgumentException: If the val is of a type which is not compatible with the elements of the arr.
• InvalidOperationException: If the val does not implement the IComparable interface, and the search encounters an element that does not implement the IComparable interface.

Below programs illustrate the above-discussed method:

Example 1:

C#

// C# program to illustrate the
// Array.BinarySearch(Array, Object)
// Method
using System;

class GFG {

    // Main Method
    public static void Main(String[] args)
    {

        // taking an 1-D Array
        int[] arr = new int[7] { 1, 5, 7, 4, 6, 2, 3 };

        // for this method array
        // must be sorted
        Array.Sort(arr);

        Console.Write("The elements of Sorted Array: ");

        // calling the method to
        // print the values
        display(arr);

        // taking the element which is
        // to search for in a variable
        // It is not present in the array
        object s = 8;

        // calling the method containing
        // BinarySearch method
        result(arr, s);

        // taking the element which is
        // to search for in a variable
        // It is present in the array
        object s1 = 4;

        // calling the method containing
        // BinarySearch method
        result(arr, s1);
    }

    // containing BinarySearch Method
    static void result(int[] arr2, object k)
    {

        // using the method
        int res = Array.BinarySearch(arr2, k);

        if (res < 0) {
            Console.WriteLine("\nThe element to search for "
                                  + "({0}) is not found.",
                              k);
        }

        else {
            Console.WriteLine("The element to search for "
                                  + "({0}) is at index {1}.",
                              k, res);
        }
    }

    // display method
    static void display(int[] arr1)
    {

        // Displaying Elements of array
        foreach(int i in arr1)
            Console.Write(i + " ");
    }
}

Output:

The elements of Sorted Array: 1 2 3 4 5 6 7 
The element to search for (8) is not found.
The element to search for (4) is at index 3.

Example 2:

C#

// C# program to illustrate the
// Array.BinarySearch(Array, Object)
// Method
using System;

class GFG {

    // Main Method
    public static void Main(String[] args)
    {

        // taking an 1-D Array
        int[] arr = new int[7] { 1, 5, 7, 4, 6, 2, 3 };

        // for this method array
        // must be sorted
        Array.Sort(arr);

        Console.Write("The elements of Sorted Array: ");

        // calling the method to
        // print the values
        display(arr);

        // it will return a negative value as
        // 9 is not present in the array
        Console.WriteLine("\nIndex of 9 is: " + Array.BinarySearch(arr, 9));
    }

    // display method
    static void display(int[] arr1)
    {

        // Displaying Elements of array
        foreach(int i in arr1)
            Console.Write(i + " ");
    }
}

Output:

The elements of Sorted Array: 1 2 3 4 5 6 7 
Index of 9 is: -8

BinarySearch(Array, Object, IComparer) Method

This method is used to search a specific element in the entire 1-D sorted array using the specified IComparer interface.

Syntax: public static int BinarySearch(Array arr, Object val, IComparer comparer)
Parameters:
arr : The one-dimensional sorted array in which the search will happen. 
val : The object value which is to search for. 
comparer : When comparing elements then the IComparer implementation is used. 

Return Value: It returns the index of the specified val in the specified arr if the val is found otherwise it returns a negative number. There are different cases of return values as follows: 

• If the val is not found and val is less than one or more elements in the arr, the negative number returned is the bitwise complement of the index of the first element that is larger than val.
• If the val is not found and val is greater than all elements in the arr, 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 the val is present in the arr.

Exceptions: 

• ArgumentNullException: If the arr is null.
• RankException: If arr is multidimensional.
• ArgumentException: If the range is less than lower bound OR length is less than 0.
• ArgumentException: If the comparer is null, and value is of a type that is not compatible with the elements of arr.
• InvalidOperationException: If the comparer is null, value does not implement the IComparable interface, and the search encounters an element that does not implement the IComparable interface.

Example:

C#

// C# program to demonstrate the
// Array.BinarySearch(Array,
// Object, IComparer) Method
using System;

class GFG {

    // Main Method
    public static void Main()
    {

        // initializes a new Array.
        Array arr = Array.CreateInstance(typeof(Int32), 5);

        // Array elements
        arr.SetValue(20, 0);
        arr.SetValue(10, 1);
        arr.SetValue(30, 2);
        arr.SetValue(40, 3);
        arr.SetValue(50, 4);

        Console.WriteLine("The original Array");

        // calling "display" function
        display(arr);

        Console.WriteLine("\nsorted array");

        // sorting the Array
        Array.Sort(arr);

        display(arr);

        Console.WriteLine("\n1st call");

        // search for object 10
        object obj1 = 10;

        // call the "FindObj" function
        FindObj(arr, obj1);

        Console.WriteLine("\n2nd call");
        object obj2 = 60;
        FindObj(arr, obj2);
    }

    // find object method
    public static void FindObj(Array Arr,
                               object Obj)
    {
        int index = Array.BinarySearch(Arr, Obj,
                                       StringComparer.CurrentCulture);

        if (index < 0) {
            Console.WriteLine("The object {0} is not found\nNext"
                                  + " larger object is at index {1}",
                              Obj, ~index);
        }
        else {
            Console.WriteLine("The object {0} is at index {1}",
                              Obj, index);
        }
    }

    // display method
    public static void display(Array arr)
    {
        foreach(int g in arr)
        {
            Console.WriteLine(g);
        }
    }
}

Output:

The original Array
20
10
30
40
50

sorted array
10
20
30
40
50

1st call
The object 10 is at index 0

2nd call
The object 60 is not found
Next larger object is at index 5

BinarySearch(Array, Int32, Int32, Object) Method

This method is used to search a value in the range of elements in a 1-D sorted array. It uses the IComparable interface implemented by each element of the array and the specified value. It searches only in a specified boundary which is defined by the user.

Syntax: public static int BinarySearch(Array arr, int i, int len, object val);
Parameters:
arr: It is 1-D array in which the user have to search for an element. 
i: It is the starting index of the range from where the user want to start the search. 
len: It is the length of the range in which the user want to search. 
val: It is the value which the user to search for. 

Return Value: It returns the index of the specified val in the specified arr if the val is found otherwise it returns a negative number. There are different cases of return values as follows: 

• If the val is not found and val is less than one or more elements in the arr, the negative number returned is the bitwise complement of the index of the first element that is larger than val.
• If the val is not found and val is greater than all elements in the arr, 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 the val is present in the arr.

Exceptions: 

• ArgumentNullException: If the arr is null.
• RankException: If arr is multidimensional.
• ArgumentOutOfRangeException: If the index is less than lower bound of array OR length is less than 0.
• ArgumentException: If the index and length do not specify the valid range in array OR the value is of the type which is not compatible with the elements of the array.
• InvalidOperationException: If value does not implement the IComparable interface, and the search encounters an element that does not implement the IComparable interface.

Example:

C#

// C# Program to illustrate the use of
// Array.BinarySearch(Array, Int32,
// Int32, Object) Method
using System;
using System.IO;

class GFG {

    // Main Method
    static void Main()
    {
        // initializing the integer array
        int[] intArr = { 42, 5, 7, 12, 56, 1, 32 };

        // sorts the intArray as it must be
        // sorted before using method
        Array.Sort(intArr);

        // printing the sorted array
        foreach(int i in intArr) Console.Write(i + " "
                                               + "\n");

        // intArr is the array we want to find
        // and 1 is the starting index
        // of the range to search. 5 is the
        // length of the range to search.
        // 32 is the object to search
        int index = Array.BinarySearch(intArr, 1, 5, 32);

        if (index >= 0) {

            // if the element is found it
            // returns the index of the element
            Console.WriteLine("Index of 32 is : " + index);
        }

        else {

            // if the element is not
            // present in the array or
            // if it is not in the
            // specified range it prints this
            Console.Write("Element is not found");
        }

        // intArr is the array we want to
        // find. and 1 is the starting
        // index of the range to search. 5 is
        // the length of the range to search
        // 44 is the object to search
        int index1 = Array.BinarySearch(intArr, 1, 5, 44);

        // as the element is not present
        // it prints a negative value.
        Console.WriteLine("Index of 44 is :" + index1);
    }
}

Output:

1 
5 
7 
12 
32 
42 
56 
Index of 32 is : 4
Index of 44 is :-7

BinarySearch(Array, Int32, Int32, Object, IComparer) Method

This method is used to search a value in the range of elements in a 1-D sorted array using a specified IComparer interface. 

Syntax: public static int BinarySearch(Array arr, int index, int length, Object value, IComparer comparer)
Parameters:
arr : The sorted one-dimensional Array which is to be searched. 
index : The starting index of the range from which searching will start. 
length : The length of the range in which the search will happen. 
value : The object to search for. 
comparer : When comparing elements then use the IComparer implementation. 

Return Value: It returns the index of the specified value in the specified arr, if the value is found otherwise it returns a negative number. There are different cases of return values as follows: 

• If the value is not found and value is less than one or more elements in the array, the negative number returned is the bitwise complement of the index of the first element that is larger than value.
• If the value is not found and value is greater than all elements in the 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 the value is present in the array.

Example: In this example, here we use “CreateInstance()” method to create a typed array and stores some integer value and search some values after sort the array. 

C#

// C# program to demonstrate the
// Array.BinarySearch(Array,
// Int32, Int32, Object,
// IComparer) Method
using System;

class GFG {

    // Main Method
    public static void Main()
    {
        // initializes a new Array.
        Array arr = Array.CreateInstance(typeof(Int32), 8);

        // Array elements
        arr.SetValue(20, 0);
        arr.SetValue(10, 1);
        arr.SetValue(30, 2);
        arr.SetValue(40, 3);
        arr.SetValue(50, 4);
        arr.SetValue(80, 5);
        arr.SetValue(70, 6);
        arr.SetValue(60, 7);

        Console.WriteLine("The original Array");

        // calling "display" function
        display(arr);

        Console.WriteLine("\nsorted array");

        // sorting the Array
        Array.Sort(arr);

        display(arr);

        Console.WriteLine("\n1st call");

        // search for object 10
        object obj1 = 10;

        // call the "FindObj" function
        FindObj(arr, obj1);

        Console.WriteLine("\n2nd call");
        object obj2 = 60;
        FindObj(arr, obj2);
    }

    // find object method
    public static void FindObj(Array Arr,
                               object Obj)
    {
        int index = Array.BinarySearch(Arr, 1, 4,
                                       Obj, StringComparer.CurrentCulture);

        if (index < 0) {
            Console.WriteLine("The object {0} is not found\n"
                                  + "Next larger object is at index {1}",
                              Obj, ~index);
        }
        else {
            Console.WriteLine("The object {0} is at "
                                  + "index {1}",
                              Obj, index);
        }
    }

    // display method
    public static void display(Array arr)
    {
        foreach(int g in arr)
        {
            Console.WriteLine(g);
        }
    }
}

Output:

The original Array
20
10
30
40
50
80
70
60

sorted array
10
20
30
40
50
60
70
80

1st call
The object 10 is not found
Next larger object is at index 1

2nd call
The object 60 is not found
Next larger object is at index 5