Generating vectors of consecutive map element subsets in C++

Question

Question:

How can I create vectors that represent all possible subsets of a specific length containing consecutive elements from a map, according to the natural ordering of the map's keys?

For instance, consider the following map where Sample is a user-defined class:

std::map<int, Sample> myMap = {{15, 'a'}, {21, 'b'}, {33, 'c'}, {37, 'd'}, {49, 'e'}};

I would like to generate vectors of length 3 that contain the values of consecutive keys in the map. The result should look like this:

std::vector<Sample> subset1 = {'a', 'b', 'c'};
std::vector<Sample> subset2 = {'b', 'c', 'd'};
std::vector<Sample> subset3 = {'c', 'd', 'e'};

How can I achieve this in C++?

Answer 1

You may iterate iver the map as key are ordered:

std::vector<std::array<Sample, 3u>> get_triplets(const std::map<int, Sample>& samples)
{
    if (samples.size() < 3) {
        return {};
    }
    std::vector<std::array<Sample, 3u>> res;

    auto it = samples.begin();
    auto it1 = std::next(it);
    auto it2 = std::next(it1);

    for (; it2 != samples.end(); ++it, ++it1, ++it2) {
        res.push_back({{it->second, it1->second, it2->second}});
    }
    return res;
}

Live Demo

Edit: to have n-uplets, small changes from previous triplet version:

std::vector<std::vector<Sample>> get_n_uplets(std::size_t n, const std::map<int, Sample>& samples)
{
    if (samples.size() < n) {
        return {};
    }
    std::vector<std::vector<Sample>> res;

    auto first = samples.begin();
    auto last = std::next(first, n - 1);

    for (; last != samples.end(); ++first, ++last) {
        std::vector<Sample> inner;

        for (auto it = first; it != std::next(last); ++it) {
            inner.push_back(it->second);
        }
        res.push_back(inner);
    }
    return res;
}

Live Demo