## C++, search, ternary_search.cpp

``````/**
* \file
* \brief [Ternary search](https://en.wikipedia.org/wiki/Ternary_search)
* algorithm
*
* This is a divide and conquer algorithm.
* It does this by dividing the search space by 3 parts and
* using its property (usually monotonic property) to find
* the desired index.
*
* * Time Complexity : O(log3 n)
* * Space Complexity : O(1) (without the array)
*/

#include <iostream>

/**
* The absolutePrecision can be modified to fit preference but
* it is recommended to not go lower than 10 due to errors that
* may occur.
*/
#define absolutePrecision 10
/**
* The value of _target should be decided or can be decided later
* by using the variable of the function.
*/
#define _target 10

#define MAX 10000000  ///< Maximum length of array

/**
* get_input function is to receive input from standard IO
* @todo @christianbender Get input from STDIO or write input to memory as done
* above.
*/
void get_input() {}

/**
* This is the iterative method of the ternary search which returns the index of
* the element.
* \param[in] left lower interval limit
* \param[in] right upper interval limit
* \param[in] A array to search in
* \param[in] target value to search for
* \returns index where the target value was found
*/
int it_ternary_search(int left, int right, int A[], int target) {
while (1) {
if (left < right) {
if (right - left < absolutePrecision) {
for (int i = left; i <= right; i++)
if (A[i] == target)
return i;

return -1;
}

int oneThird = (left + right) / 3 + 1;
int twoThird = (left + right) * 2 / 3 + 1;

if (A[oneThird] == target)
return oneThird;
else if (A[twoThird] == target)
return twoThird;

else if (target > A[twoThird])
left = twoThird + 1;
else if (target < A[oneThird])
right = oneThird - 1;

else
left = oneThird + 1, right = twoThird - 1;
} else {
return -1;
}
}
}

/**
* This is the recursive method of the ternary search which returns the index of
* the element.
* \param[in] left lower interval limit
* \param[in] right upper interval limit
* \param[in] A array to search in
* \param[in] target value to search for
* \returns index where the target value was found
*/
int rec_ternary_search(int left, int right, int A[], int target) {
if (left < right) {
if (right - left < absolutePrecision) {
for (int i = left; i <= right; i++)
if (A[i] == target)
return i;

return -1;
}

int oneThird = (left + right) / 3 + 1;
int twoThird = (left + right) * 2 / 3 + 1;

if (A[oneThird] == target)
return oneThird;
if (A[twoThird] == target)
return twoThird;

if (target < A[oneThird])
return rec_ternary_search(left, oneThird - 1, A, target);
if (target > A[twoThird])
return rec_ternary_search(twoThird + 1, right, A, target);

return rec_ternary_search(oneThird + 1, twoThird - 1, A, target);
} else {
return -1;
}
}

/**
* ternary_search is a template function
* You could either use it_ternary_search or rec_ternary_search according to
* preference.
* \param [in] N length of array
* \param[in] A array to search in
* \param[in] target value to search for
*/
void ternary_search(int N, int A[], int target) {
std::cout << it_ternary_search(0, N - 1, A, target) << '\t';
std::cout << rec_ternary_search(0, N - 1, A, target) << '\t';
std::cout << std::endl;
}

/** Main function */
int main() {
int N = 21;
int A[] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 3, 4, 10};
get_input();
ternary_search(N, A, _target);
return 0;
}
``````