Search an element in an unsorted array using minimum number of comparisons

Given an array of n distinct integers and an element x. Search the element x in the array using minimum number of comparisons. Any sort of comparison will contribute 1 to the count of comparisons. For example, the condition used to terminate a loop, will also contribute 1 to the count of comparisons each time it gets executed. Expressions like while (n) {n–;} also contribute to the count of comparisons as value of n is being compared internally so as to decide whether or not to terminate the loop. 
Examples: 

Input : arr[] = {4, 6, 1, 5, 8}, 
        x = 1
Output : Found

Input : arr[] = {10, 3, 12, 7, 2, 11, 9}, 
        x = 15
Output : Not Found

Asked in Adobe Interview

Below simple method to search requires 2n + 1 comparisons in worst case. 

for (i = 0; i < n; i++)  // Worst case n+1
   if (arr[i] == x)  // Worst case n
       return i;

How to reduce number of comparisons?
The idea is to copy x (element to be searched) to last location so that one last comparison when x is not present in arr[] is saved.
Algorithm:

search(arr, n, x)
    if arr[n-1] == x  // 1 comparison
       return "true"
    backup = arr[n-1]
    arr[n-1] = x

    for i=0, i++ // no termination condition
        if arr[i] == x // execute at most n times
                       // that is at-most n comparisons
            arr[n-1] = backup
            return (i < n-1) // 1 comparison

C++

// C++ implementation to search an element in
// the unsorted array using minimum number of
// comparisons
#include <bits/stdc++.h>
using namespace std;

// function to search an element in
// minimum number of comparisons
string search(int arr[], int n, int x)
{
 // 1st comparison
 if (arr[n - 1] == x)
  return "Found";

 int backup = arr[n - 1];
 arr[n - 1] = x;

 // no termination condition and thus
 // no comparison
 for (int i = 0;; i++) {
  // this would be executed at-most n times
  // and therefore at-most n comparisons
  if (arr[i] == x) {
   // replace arr[n-1] with its actual element
   // as in original 'arr[]'
   arr[n - 1] = backup;

   // if 'x' is found before the '(n-1)th'
   // index, then it is present in the array
   // final comparison
   if (i < n - 1)
    return "Found";

   // else not present in the array
   return "Not Found";
  }
 }
}

// Driver program to test above
int main()
{
 int arr[] = { 4, 6, 1, 5, 8 };
 int n = sizeof(arr) / sizeof(arr[0]);
 int x = 1;
 cout << search(arr, n, x);
 return 0;
}

Java

// Java implementation to search an element in
// the unsorted array using minimum number of
// comparisons
import java.io.*;

class GFG {

    // Function to search an element in
    // minimum number of comparisons
    static String search(int arr[], int n, int x)
    {
        // 1st comparison
        if (arr[n - 1] == x)
            return "Found";

        int backup = arr[n - 1];
        arr[n - 1] = x;

        // no termination condition and thus
        // no comparison
        for (int i = 0;; i++) {
            // this would be executed at-most n times
            // and therefore at-most n comparisons
            if (arr[i] == x) {
                // replace arr[n-1] with its actual element
                // as in original 'arr[]'
                arr[n - 1] = backup;

                // if 'x' is found before the '(n-1)th'
                // index, then it is present in the array
                // final comparison
                if (i < n - 1)
                    return "Found";

                // else not present in the array
                return "Not Found";
            }
        }
    }

    // driver program
    public static void main(String[] args)
    {
        int arr[] = { 4, 6, 1, 5, 8 };
        int n = arr.length;
        int x = 1;
        System.out.println(search(arr, n, x));
    }
}

// Contributed by Pramod Kumar

Python3

# Python3 implementation to search an
# element in the unsorted array using
# minimum number of comparisons

# function to search an element in
# minimum number of comparisons
def search(arr, n, x):

    # 1st comparison
    if (arr[n-1] == x) :
        return "Found"

    backup = arr[n-1]
    arr[n-1] = x

    # no termination condition and
    # thus no comparison
    i = 0
    while(i < n) :

        # this would be executed at-most n times
        # and therefore at-most n comparisons
        if (arr[i] == x) :

            # replace arr[n-1] with its actual
            # element as in original 'arr[]'
            arr[n-1] = backup

            # if 'x' is found before the '(n-1)th'
            # index, then it is present in the
            # array final comparison
            if (i < n-1):
                return "Found"

            # else not present in the array
            return "Not Found"
        i = i + 1

# Driver Code
arr = [4, 6, 1, 5, 8]
n = len(arr)
x = 1
print (search(arr, n, x))

# This code is contributed by rishabh_jain

C#

// C# implementation to search an
// element in the unsorted array
// using minimum number of comparisons
using System;

class GFG {

    // Function to search an element in
    // minimum number of comparisons
    static String search(int[] arr, int n, int x)
    {
        // 1st comparison
        if (arr[n - 1] == x)
            return "Found";

        int backup = arr[n - 1];
        arr[n - 1] = x;

        // no termination condition and thus
        // no comparison
        for (int i = 0;; i++) {

            // this would be executed at-most n times
            // and therefore at-most n comparisons
            if (arr[i] == x) {

                // replace arr[n-1] with its actual element
                // as in original 'arr[]'
                arr[n - 1] = backup;

                // if 'x' is found before the '(n-1)th'
                // index, then it is present in the array
                // final comparison
                if (i < n - 1)
                    return "Found";

                // else not present in the array
                return "Not Found";
            }
        }
    }

    // driver program
    public static void Main()
    {
        int[] arr = { 4, 6, 1, 5, 8 };
        int n = arr.Length;
        int x = 1;
        Console.WriteLine(search(arr, n, x));
    }
}

// This code is contributed by Sam007

PHP

<?php
// PHP implementation to
// search an element in
// the unsorted array
// using minimum number of
// comparisons

// function to search an
// element in minimum
// number of comparisons
function search($arr, $n, $x)
{

    // 1st comparison
    if ($arr[$n - 1] == $x)
        return "Found";

    $backup = $arr[$n - 1];
    $arr[$n - 1] = $x;

    // no termination
    // condition and thus
    // no comparison
    for ($i = 0; ; $i++)
    {

        // this would be executed
        // at-most n times and
        // therefore at-most
        // n comparisons
        if ($arr[$i] == $x)
        {

            // replace arr[n-1]
            // with its actual element
            // as in original 'arr[]'
            $arr[$n - 1] = $backup;

            // if 'x' is found before
            // the '(n-1)th' index,
            // then it is present
            // in the array
            // final comparison
            if ($i < $n - 1)
                return "Found";

            // else not present
            // in the array
            return "Not Found";
        }
    }
}

// Driver Code
$arr = array( 4, 6, 1, 5, 8 );
$n = sizeof($arr);
$x = 1;
echo(search($arr, $n, $x));

// This code is contributed by Ajit.
?>

Javascript

<script>
    // Javascript implementation to search an
    // element in the unsorted array
    // using minimum number of comparisons

    // Function to search an element in
    // minimum number of comparisons
    function search(arr, n, x)
    {
        // 1st comparison
        if (arr[n - 1] == x)
            return "Found";

        let backup = arr[n - 1];
        arr[n - 1] = x;

        // no termination condition and thus
        // no comparison
        for (let i = 0;; i++) {

            // this would be executed at-most n times
            // and therefore at-most n comparisons
            if (arr[i] == x) {

                // replace arr[n-1] with its actual element
                // as in original 'arr[]'
                arr[n - 1] = backup;

                // if 'x' is found before the '(n-1)th'
                // index, then it is present in the array
                // final comparison
                if (i < n - 1)
                    return "Found";

                // else not present in the array
                return "Not Found";
            }
        }
    }

    let arr = [ 4, 6, 1, 5, 8 ];
    let n = arr.length;
    let x = 1;
    document.write(search(arr, n, x));

</script>

Output: 

Found

Time Complexity: O(n) 
Auxiliary Space: O(1)
Number of Comparisons: Atmost (n+2) comparisons
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