Find a Fixed Point (Value equal to index) in a given array

Given an array of n distinct integers sorted in ascending order, write a function that returns a Fixed Point in the array, if there is any Fixed Point present in array, else returns -1. Fixed Point in an array is an index i such that arr[i] is equal to i. Note that integers in array can be negative.

Examples:

  Input: arr[] = {-10, -5, 0, 3, 7}
  Output: 3  // arr[3] == 3 

  Input: arr[] = {0, 2, 5, 8, 17}
  Output: 0  // arr[0] == 0 


  Input: arr[] = {-10, -5, 3, 4, 7, 9}
  Output: -1  // No Fixed Point

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

Method 1 (Linear Search)
Linearly search for an index i such that arr[i] == i. Return the first such index found. Thanks to pm for suggesting this solution.

C/C++

// C/C++ program to check fixed point  
// in an array using linear search 
#include<stdio.h> 
  
int linearSearch(int arr[], int n) 
{ 
    int i; 
    for(i = 0; i < n; i++) 
    { 
        if(arr[i] == i) 
            return i; 
    } 
  
    /* If no fixed point present then return -1 */
    return -1; 
} 
  
/* Driver program to check above functions */
int main() 
{ 
    int arr[] = {-10, -1, 0, 3, 10, 11, 30, 50, 100}; 
    int n = sizeof(arr)/sizeof(arr[0]); 
    printf("Fixed Point is %d", linearSearch(arr, n)); 
    getchar(); 
    return 0; 
} 

Java

// Java program to check fixed point  
// in an array using linear search 
   
class Main 
{ 
    static int linearSearch(int arr[], int n) 
    { 
        int i; 
        for(i = 0; i < n; i++) 
        { 
            if(arr[i] == i) 
                return i; 
        } 
        
        /* If no fixed point present  
           then return -1 */
        return -1; 
    } 
    //main function 
    public static void main(String args[]) 
    { 
        int arr[] = {-10, -1, 0, 3, 10, 11, 30, 50, 100}; 
        int n = arr.length; 
        System.out.println("Fixed Point is " 
                     + linearSearch(arr, n)); 
    } 
} 

Python

# Python program to check fixed point  
# in an array using linear search 
def linearSearch(arr, n): 
    for i in range(n): 
        if arr[i] is i: 
            return i 
    # If no fixed point present then return -1 
    return -1
  
# Driver program to check above functions 
arr = [-10, -1, 0, 3, 10, 11, 30, 50, 100] 
n = len(arr) 
print("Fixed Point is " + str(linearSearch(arr,n))) 
  
# This code is contributed by Pratik Chhajer 

C#

// C# program to check fixed point  
// in an array using linear search 
using System; 
  
class GFG 
{ 
    static int linearSearch(int []arr, int n) 
    { 
        int i; 
        for(i = 0; i < n; i++) 
        { 
            if(arr[i] == i) 
                return i; 
        } 
          
        /* If no fixed point present  
        then return -1 */
        return -1; 
    } 
    // Driver code 
    public static void Main() 
    { 
        int []arr = {-10, -1, 0, 3, 10, 11, 30, 50, 100}; 
        int n = arr.Length; 
        Console.Write("Fixed Point is "+ linearSearch(arr, n)); 
    } 
} 
  
// This code is contributed by Sam007 

PHP

<?php 
// PHP program to check fixed point  
// in an array using linear search 
  
function linearSearch($arr, $n) 
{ 
    for($i = 0; $i < $n; $i++) 
    { 
        if($arr[$i] == $i) 
            return $i; 
    } 
  
    // If no fixed point present then 
    // return -1 
    return -1; 
} 
  
    // Driver Code 
    $arr = array(-10, -1, 0, 3, 10,  
                  11, 30, 50, 100); 
    $n = count($arr); 
    echo "Fixed Point is ". 
            linearSearch($arr,$n); 
  
// This code is contributed by Sam007 
?> 

Output:

Fixed Point is 3

Time Complexity: O(n)

Method 2 (Binary Search)
First check whether middle element is Fixed Point or not. If it is, then return it; otherwise check whether index of middle element is greater than value at the index. If index is greater, then Fixed Point(s) lies on the right side of the middle point (obviously only if there is a Fixed Point). Else the Fixed Point(s) lies on left side.

C/C++

// C/C++ program to check fixed point  
// in an array using binary search 
#include<stdio.h> 
  
int binarySearch(int arr[], int low, int high) 
{ 
    if(high >= low) 
    { 
        int mid = (low + high)/2;  /*low + (high - low)/2;*/
        if(mid == arr[mid]) 
            return mid; 
        if(mid > arr[mid]) 
            return binarySearch(arr, (mid + 1), high); 
        else
            return binarySearch(arr, low, (mid -1)); 
    } 
  
    /* Return -1 if there is no Fixed Point */
    return -1; 
} 
  
/* Driver program to check above functions */
int main() 
{ 
    int arr[10] = {-10, -1, 0, 3, 10, 11, 30, 50, 100}; 
    int n = sizeof(arr)/sizeof(arr[0]); 
    printf("Fixed Point is %d", binarySearch(arr, 0, n-1)); 
    getchar(); 
    return 0; 
} 

Java

// Java program to check fixed point  
// in an array using binary search 
  
class Main 
{ 
    static int binarySearch(int arr[], int low, int high) 
    { 
        if(high >= low) 
        {    
            /* low + (high - low)/2; */
            int mid = (low + high)/2;   
            if(mid == arr[mid]) 
                return mid; 
            if(mid > arr[mid]) 
                return binarySearch(arr, (mid + 1), high); 
            else
                return binarySearch(arr, low, (mid -1)); 
        } 
        
        /* Return -1 if there is  
           no Fixed Point */
        return -1; 
    } 
        
    //main function 
    public static void main(String args[]) 
    { 
        int arr[] = {-10, -1, 0, 3 , 10, 11, 30, 50, 100}; 
        int n = arr.length; 
        System.out.println("Fixed Point is " 
                   + binarySearch(arr,0, n-1));         
    }  
} 

Python

# Python program to check fixed point  
# in an array using binary search 
def binarySearch(arr, low, high): 
    if high >= low: 
        mid = (low + high)//2
      
    if mid is arr[mid]: 
        return mid 
      
    if mid > arr[mid]: 
        return binarySearch(arr, (mid + 1), high) 
    else: 
        return binarySearch(arr, low, (mid -1)) 
      
    # Return -1 if there is no Fixed Point 
    return -1
  
  
# Driver program to check above functions */ 
arr = [-10, -1, 0, 3, 10, 11, 30, 50, 100] 
n = len(arr) 
print("Fixed Point is " + str(binarySearch(arr, 0, n-1))) 
  
  
# This code is contributed by Pratik Chhajer  

C#

// C# program to check fixed point  
// in an array using binary search 
using System; 
  
class GFG 
{ 
    static int binarySearch(int []arr, int low, int high) 
    { 
        if(high >= low) 
        {  
            // low + (high - low)/2;  
            int mid = (low + high)/2;  
              
            if(mid == arr[mid]) 
                return mid; 
            if(mid > arr[mid]) 
                return binarySearch(arr, (mid + 1), high); 
            else
                return binarySearch(arr, low, (mid -1)); 
        } 
          
        /* Return -1 if there is  
        no Fixed Point */
        return -1; 
    } 
          
    // Driver code 
    public static void Main() 
    { 
        int []arr = {-10, -1, 0, 3 , 10, 11, 30, 50, 100}; 
        int n = arr.Length; 
        Console.Write("Fixed Point is "+ binarySearch(arr,0, n-1));      
    }  
} 
// This code is contributed by Sam007 

PHP

<?php 
// PHP program to check fixed po 
// in an array using binary search 
  
function binarySearch($arr, $low, $high) 
{ 
    if($high >= $low) 
    { 
          
         /*low + (high - low)/2;*/
        $mid = (int)(($low + $high) / 2); 
        if($mid == $arr[$mid]) 
            return $mid; 
        if($mid > $arr[$mid]) 
            return binarySearch($arr, ($mid + 1), $high); 
        else
            return binarySearch($arr, $low, ($mid - 1)); 
    } 
  
    /* Return -1 if there is 
       no Fixed Po*/
    return -1; 
} 
  
    // Driver Code 
    $arr = array(-10, -1, 0, 3, 10, 
                  11, 30, 50, 100); 
    $n = count($arr); 
    echo "Fixed Point is: " 
        . binarySearch($arr, 0, $n - 1); 
          
// This code is contributed by Anuj_67 
?> 