Exponential Search

The name of this searching algorithm may be misleading as it works in O(Log n) time. The name comes from the way it searches an element.

```Given a sorted array an element x to be
searched, find position of x in the array.

Input:  arr[] = {10, 20, 40, 45, 55}
x = 45
Output: Element found at index 3

Input:  arr[] = {10, 15, 25, 45, 55}
x = 15
Output: Element found at index 1```

We have discussed, linear search, binary search for this problem.

Exponential search involves two steps:

1. Find range where element is present
2. Do Binary Search in above found range.

How to find the range where element may be present?
The idea is to start with subarray size 1 compare its last element with x, then try size 2, then 4 and so on until last element of a subarray is not greater.
Once we find an index i (after repeated doubling of i), we know that the element must be present between i/2 and i (Why i/2? because we could not find a greater value in previous iteration)

Below is implementation of above steps.

`// C++ program to find an element x in a`
`// sorted array using <a href="#">Exponential search</a>.`
`#include <bits/stdc++.h>`
`using` `namespace` `std;`
`int` `binarySearch(``int` `arr[], ``int``, ``int``, ``int``);`
`// Returns position of first ocurrence of`
`// x in array`
`int` `exponentialSearch(``int` `arr[], ``int` `n, ``int` `x)`
`{`
`    ``// If x is present at firt location itself`
`    ``if` `(arr[0] == x)`
`        ``return` `0;`
`    ``// Find range for binary search by`
`    ``// repeated doubling`
`    ``int` `i = 1;`
`    ``while` `(i < n && arr[i] <= x)`
`        ``i = i*2;`
`    ``//  Call binary search for the found range.`
`    ``return` `binarySearch(arr, i/2, min(i, n), x);`
`}`
`// 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 n 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`
`int` `main(``void``)`
`{`
`   ``int` `arr[] = {2, 3, 4, 10, 40};`
`   ``int` `n = ``sizeof``(arr)/ ``sizeof``(arr[0]);`
`   ``int` `x = 10;`
`   ``int` `result = exponentialSearch(arr, n, x);`
`   ``(result == -1)? ``printf``(``"Element is not present in array"``)`
`                 ``: ``printf``(``"Element is present at index %d"``,`
`                                                    ``result);`
`   ``return` `0;`
`}`

Output :

`Element is present at index 3`

Time Complexity : O(Log n)
Auxiliary Space : The above implementation of Binary Search is recursive and requires O()Log n) space. With iterative Binary Search, we need only O(1) space.

Applications of Exponential Search:

1. Exponential Binary Search is particularly useful for unbounded searches, where size of array is infinite. Please refer Unbounded Binary Search for an example.
2. It works better than Binary Search for bounded arrays also when the element to be searched is closer to the first element.

Disclaimer: This does not belong to TechCodeBit, its an article taken from the below
source and credits.
source and credits: http://www.geeksforgeeks.org
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