Binary Search - GeeksforGeeks

Given a sorted array arr[] of n elements, write a function to search a given element x in arr[].

A simple approach is to do linear search.The time complexity of above algorithm is O(n). Another approach to perform the same task is using Binary Search.

Binary Search: Search 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.

Example :
Image Source : http://www.nyckidd.com/bob/Linear%20Search%20and%20Binary%20Search_WorkingCopy.pdf

The idea of binary search is to use the information that the array is sorted and reduce the time complexity to O(Log n).

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

We basically ignore half of the elements just after one comparison.

Compare x with the middle element.
If x matches with middle element, we return the mid index.
Else If x is greater than the mid element, then x can only lie in right half subarray after the mid element. So we recur for right half.
Else (x is smaller) recur for the left half.

Recursive implementation of Binary Search

C/C++

// C program to implement recursive Binary Search 
#include <stdio.h> 
  
// A recursive binary search function. It returns  
// location of x in given array arr[l..r] is present,  
// otherwise -1 
int binarySearch(int arr[], int l, int r, int x) 
{ 
   if (r >= l) 
   { 
        int mid = l + (r - l)/2; 
  
        // If the element is present at the middle  
        // itself 
        if (arr[mid] == x)   
            return mid; 
  
        // If element is smaller than mid, then  
        // it can only be present in left subarray 
        if (arr[mid] > x)  
            return binarySearch(arr, l, mid-1, x); 
  
        // Else the element can only be present 
        // in right subarray 
        return binarySearch(arr, mid+1, r, x); 
   } 
  
   // We reach here when element is not  
   // present in array 
   return -1; 
} 
  
int main(void) 
{ 
   int arr[] = {2, 3, 4, 10, 40}; 
   int n = sizeof(arr)/ sizeof(arr[0]); 
   int x = 10; 
   int result = binarySearch(arr, 0, n-1, x); 
   (result == -1)? printf("Element is not present in array") 
                 : printf("Element is present at index %d", 
                                                   result); 
   return 0; 
} 

Java

// Java implementation of recursive Binary Search 
class BinarySearch 
{ 
    // Returns index of x if it is present in arr[l.. 
    // r], else return -1 
    int binarySearch(int arr[], int l, int r, int x) 
    { 
        if (r>=l) 
        { 
            int mid = l + (r - l)/2; 
  
            // If the element is present at the  
            // middle itself 
            if (arr[mid] == x) 
               return mid; 
  
            // If element is smaller than mid, then  
            // it can only be present in left subarray 
            if (arr[mid] > x) 
               return binarySearch(arr, l, mid-1, x); 
  
            // Else the element can only be present 
            // in right subarray 
            return binarySearch(arr, mid+1, r, x); 
        } 
  
        // We reach here when element is not present 
        //  in array 
        return -1; 
    } 
  
    // Driver method to test above 
    public static void main(String args[]) 
    { 
        BinarySearch ob = new BinarySearch(); 
        int arr[] = {2,3,4,10,40}; 
        int n = arr.length; 
        int x = 10; 
        int result = ob.binarySearch(arr,0,n-1,x); 
        if (result == -1) 
            System.out.println("Element not present"); 
        else
            System.out.println("Element found at index " +  
                                                 result); 
    } 
} 
/* This code is contributed by Rajat Mishra */

Python

# Python Program for recursive binary search. 
  
# Returns index of x in arr if present, else -1 
def binarySearch (arr, l, r, x): 
  
    # Check base case 
    if r >= l: 
  
        mid = l + (r - l)/2
  
        # If element is present at the middle itself 
        if arr[mid] == x: 
            return mid 
          
        # If element is smaller than mid, then it  
        # can only be present in left subarray 
        elif arr[mid] > x: 
            return binarySearch(arr, l, mid-1, x) 
  
        # Else the element can only be present  
        # in right subarray 
        else: 
            return binarySearch(arr, mid+1, r, x) 
  
    else: 
        # Element is not present in the array 
        return -1
  
# Test array 
arr = [ 2, 3, 4, 10, 40 ] 
x = 10
  
# Function call 
result = binarySearch(arr, 0, len(arr)-1, x) 
  
if result != -1: 
    print "Element is present at index %d" % result 
else: 
    print "Element is not present in array"

C#

// C# implementation of recursive Binary Search 
using System; 
  
class GFG 
{ 
    // Returns index of x if it is present in  
    // arr[l..r], else return -1 
    static int binarySearch(int []arr, int l,  
                                  int r, int x) 
    { 
        if (r >= l) 
        { 
            int mid = l + (r - l)/2; 
  
            // If the element is present at the  
            // middle itself 
            if (arr[mid] == x) 
                return mid; 
  
            // If element is smaller than mid, then  
            // it can only be present in left subarray 
            if (arr[mid] > x) 
                return binarySearch(arr, l, mid-1, x); 
  
            // Else the element can only be present 
            // in right subarray 
            return binarySearch(arr, mid+1, r, x); 
        } 
  
        // We reach here when element is not present 
        // in array 
        return -1; 
    } 
  
    // Driver method to test above 
    public static void Main() 
    { 
          
        int []arr = {2, 3, 4, 10, 40}; 
        int n = arr.Length; 
        int x = 10; 
          
        int result = binarySearch(arr, 0, n-1, x); 
          
        if (result == -1) 
            Console.WriteLine("Element not present"); 
        else
            Console.WriteLine("Element found at index " 
                                             + result); 
    } 
} 
  
// This code is contributed by Sam007. 

PHP

<?php 
// PHP program to implement 
// recursive Binary Search 
  
// A recursive binary search 
// function. It returns location 
// of x in given array arr[l..r]  
// is present, otherwise -1 
function binarySearch($arr, $l, $r, $x) 
{ 
if ($r >= $l) 
{ 
        $mid = $l + ($r - $l) / 2; 
  
        // If the element is present  
        // at the middle itself 
        if ($arr[$mid] == $x)  
            return floor($mid); 
  
        // If element is smaller than  
        // mid, then it can only be  
        // present in left subarray 
        if ($arr[$mid] > $x)  
            return binarySearch($arr, $l,  
                                $mid - 1, $x); 
  
        // Else the element can only  
        // be present in right subarray 
        return binarySearch($arr, $mid + 1,  
                            $r, $x); 
} 
  
// We reach here when element  
// is not present in array 
return -1; 
} 
  
// Driver Code 
$arr = array(2, 3, 4, 10, 40); 
$n = count($arr); 
$x = 10; 
$result = binarySearch($arr, 0, $n - 1, $x); 
if(($result == -1)) 
echo "Element is not present in array"; 
else
echo "Element is present at index ", 
                            $result; 
                              
// This code is contributed by anuj_67. 
?> 

Output :

Element is present at index 3

Iterative implementation of Binary Search

C/C++

// C program to implement iterative Binary Search 
#include <stdio.h> 
  
// A iterative binary search function. It returns 
// location of x in given array arr[l..r] if present, 
// otherwise -1 
int binarySearch(int arr[], int l, int r, int x) 
{ 
    while (l <= r) 
    { 
        int m = l + (r-l)/2; 
  
        // Check if x is present at mid 
        if (arr[m] == x) 
            return m; 
  
        // If x greater, ignore left half 
        if (arr[m] < x) 
            l = m + 1; 
  
        // If x is smaller, ignore right half 
        else
            r = m - 1; 
    } 
  
    // if we reach here, then element was 
    // not present 
    return -1; 
} 
  
int main(void) 
{ 
    int arr[] = {2, 3, 4, 10, 40}; 
    int n = sizeof(arr)/ sizeof(arr[0]); 
    int x = 10; 
    int result = binarySearch(arr, 0, n-1, x); 
    (result == -1)? printf("Element is not present"
                                       " in array") 
               : printf("Element is present at "
                                "index %d", result); 
    return 0; 
} 

Java

// Java implementation of iterative Binary Search 
class BinarySearch 
{ 
    // Returns index of x if it is present in arr[],  
    // else return -1 
    int binarySearch(int arr[], int x) 
    { 
        int l = 0, r = arr.length - 1; 
        while (l <= r) 
        { 
            int m = l + (r-l)/2; 
  
            // Check if x is present at mid 
            if (arr[m] == x) 
                return m; 
  
            // If x greater, ignore left half 
            if (arr[m] < x) 
                l = m + 1; 
  
            // If x is smaller, ignore right half 
            else
                r = m - 1; 
        } 
  
        // if we reach here, then element was  
        // not present 
        return -1; 
    } 
  
    // Driver method to test above 
    public static void main(String args[]) 
    { 
        BinarySearch ob = new BinarySearch(); 
        int arr[] = {2, 3, 4, 10, 40}; 
        int n = arr.length; 
        int x = 10; 
        int result = ob.binarySearch(arr, x); 
        if (result == -1) 
            System.out.println("Element not present"); 
        else
            System.out.println("Element found at " +  
                                   "index " + result); 
    } 
} 

Python

# Python code to implement iterative Binary  
# Search. 
  
# It returns location of x in given array arr  
# if present, else returns -1 
def binarySearch(arr, l, r, x): 
  
    while l <= r: 
  
        mid = l + (r - l)/2; 
          
        # Check if x is present at mid 
        if arr[mid] == x: 
            return mid 
  
        # If x is greater, ignore left half 
        elif arr[mid] < x: 
            l = mid + 1
  
        # If x is smaller, ignore right half 
        else: 
            r = mid - 1
      
    # If we reach here, then the element 
    # was not present 
    return -1
  
  
# Test array 
arr = [ 2, 3, 4, 10, 40 ] 
x = 10
  
# Function call 
result = binarySearch(arr, 0, len(arr)-1, x) 
  
if result != -1: 
    print "Element is present at index %d" % result 
else: 
    print "Element is not present in array"

C#

// C# implementation of iterative Binary Search 
using System; 
  
class GFG 
{ 
    // Returns index of x if it is present in arr[],  
    // else return -1 
    static int binarySearch(int []arr, int x) 
    { 
        int l = 0, r = arr.Length - 1; 
        while (l <= r) 
        { 
            int m = l + (r-l)/2; 
  
            // Check if x is present at mid 
            if (arr[m] == x) 
                return m; 
  
            // If x greater, ignore left half 
            if (arr[m] < x) 
                l = m + 1; 
  
            // If x is smaller, ignore right half 
            else
                r = m - 1; 
        } 
  
        // if we reach here, then element was  
        // not present 
        return -1; 
    } 
  
    // Driver method to test above 
    public static void Main() 
    { 
        int []arr = {2, 3, 4, 10, 40}; 
        int n = arr.Length; 
        int x = 10; 
        int result = binarySearch(arr, x); 
        if (result == -1) 
            Console.WriteLine("Element not present"); 
        else
            Console.WriteLine("Element found at " +  
                                "index " + result); 
    } 
} 
// This code is contributed by Sam007 

PHP

<?php 
// PHP program to implement 
// iterative Binary Search 
  
// A iterative binary search  
// function. It returns location  
// of x in given array arr[l..r]  
// if present, otherwise -1 
function binarySearch($arr, $l,  
                      $r, $x) 
{ 
    while ($l <= $r) 
    { 
        $m = $l + ($r - $l) / 2; 
  
        // Check if x is present at mid 
        if ($arr[$m] == $x) 
            return floor($m); 
  
        // If x greater, ignore 
        // left half 
        if ($arr[$m] < $x) 
            $l = $m + 1; 
  
        // If x is smaller,  
        // ignore right half 
        else
            $r = $m - 1; 
    } 
  
    // if we reach here, then  
    // element was not present 
    return -1; 
} 
  
// Driver Code 
$arr = array(2, 3, 4, 10, 40); 
$n = count($arr); 
$x = 10; 
$result = binarySearch($arr, 0,  
                       $n - 1, $x); 
if(($result == -1)) 
echo "Element is not present in array"; 
else
echo "Element is present at index ",  
                            $result; 
  
// This code is contributed by anuj_67. 
?> 

Output :

Element is present at index 3

Time Complexity:
The time complexity of Binary Search can be written as

T(n) = T(n/2) + c

The above recurrence can be solved either using Recurrence T ree method or Master method. It falls in case II of Master Method and solution of the recurrence is $Theta(Logn)$ .