That's a rotate!

Sean Parent is a well known C++ expert who has given several talks. One of his most famous talks is called C++ Seasoning. I urge you to watch it yet, but not just yet! First I want to mention a couple of things that were important for me. This talk has succinct, invaluable lessons that almost sound too good to be true.

Primarily, he provides three goals you must have as a programmer to improve the quality of your code:

• Now Raw Loops
• Now Raw Synchronization Primitives
• Now Raw Pointers

Meme:

That's a rotate

• Sean Parent

Sean’s frequent reference to his favorite algorithm during the talk gave rise to this meme. Oddly, it makes think of trying to randomly apply rotate(). It’s like I’m looking to apply rotate() only to have the satisfaction of somehow attaching myself to this talk. I enjoyed this talk a lot!

Problem:

The gist of the problem is while trying to avoid using Raw Loops in your program, look for alternatives in the standard library since it typically has an optimized implementation without risk of introducing bugs.

So a problem that came to mind is with the following input:

Required Output is:

Bad Solution:

template <typename Container>
auto move_n_elements_to_back(Container& c, size_t n) {
    // Copy the first n elements to the end of the container
    for(auto it = c.begin(); it != std::next(c.begin(), n); ++it) {
      c.emplace_back(std::move(*it));
    }
    // Erase the copied elements from front of container
    c.erase(c.begin(), std::next(c.begin(), n));
}

Clever Solution (didn’t know rotate()):

Even if you did not know about the existence of rotate algorithm implementation you could solve the problem with

template <typename Container>
auto move_n_elements_to_back(Container& c, size_t n) {
    auto split_point = std::next(c.begin(), n);
    std::reverse(c.begin(), split_point);
	split_point++;
	std::reverse(split_point, c.end());	
	std::reverse(c.begin(), c.end());	
}

Good Solution:

template <typename Container>
auto move_n_elements_to_back(Container& c, size_t n) {
     auto new_begin = std::next(c.begin(), n);
     std::rotate(c.begin(), new_begin, c.end());
}