### An attempt to Implement Binary Search STL (associative) Containers agnostic way.

Binary Search is a very trivial algorithm to search a target value from a sorted array. It's popular among students of computer science and also during interviews it gets asked by the interviewer. It's easy. We take an array of integers already sorted and apply binary search on that array to figure out if the target value does present in that sorted array or not.

The condition can be either present or not present. If it is present we return the array index of the element else we return -1.

The algorithm is very simple, it's broken down into three parts.

1. Find and compare the middle element of the search space with the key.

2. If the key is found in the middle, just return the array index.

3. If the key is not found then choose half of the array space, based on whether the key value is smaller or greater than the mid element.

a. If the key element is smaller than the mid element, then the left side of the search space will be used otherwise, the right side of the search space will be used.

This process continues until the element is found in the search space or the total search space is exhausted.

The very standard implementation is more or less like the below in C++:

int BinarySearch(const std::vector<int>& vec, const int&& target)
{
if (vec.size() == 0) return -1;
int left = 0;
int right = static_cast<int>(vec.size() - 1);

while (left <= right)
{
int mid = left + ((right - left) >> 2);
if (vec[mid] == target) return mid;
if (vec[mid] < target) left = mid + 1;
else right = mid - 1;
}
return -1;
}

int main()
{
std::vector<int> v =
{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,
17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44,
45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58,
59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72,
73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86,
87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 };

std::cout << "Non templated Imp(std::vector used): " << BinarySearch(v, 2) << "\n";
std::cout << "Non templated Imp(std::vector used): " << BinarySearch(v, 99) << "\n";
std::cout << "Non templated Imp(std::vector used): " << BinarySearch(v, 300) << "\n";

return 0;
}

And it works. But now comes the twist, what if we want a binary search implementation that should work for most of the STLs associative containers like vector, list, array, and deque. In summary, want an implementation for STLs associative containers like vector, array, list, and deque. The idea is to develop the binary search in such a way that it can work container agnostic way. Hence followed templated approach and instead of passing the container itself used Iterator. It's a templatized Iterator-based approach. The below implementation works for std::vector, std::list, std::array, std::deque, and std::set. I've used visual studio 2022 for my implementation and the code looks like the below:

template<typename T, typename It>
_int64 BinarySearchT(It begin, It end, const T&& target)
{
if (begin == end)
return -1;

It left = begin;
It right = std::prev(end);

// Check for a container with one element
if (left == right)
{
if (*left == target) return 0;
else return -1;
}

while (left != end)
{
auto mid = left;
if (*mid == target) return std::distance(begin, mid);
else if (*mid < target) {
left = mid;
}
else
{
right = mid;
end = right;
}
}

return -1;
}

As we can see I have used Iterators to search the key element in search space(containers) which could be a std::array or std::vector or std::list or std::deque or std::set.

Here is the complete demo code:

*For GCC compiler function return shall be int64_t instead of _int64.
** The implementation doesn't work for std::forward_list.