Find min element of a "masked vector"

I have a vector of some values and a mask vector of 0’s and 1’s. For example:

std::vector<int>   mask{0,   0,   1,   0,   1,   1,   0};
std::vector<double> vec{7.1, 1.0, 3.2, 2.0, 1.8, 5.0, 0.0};

and I need to find the min element (its index) in vec but only where mask is 1. In this example it would be 1.8 at index 4. Is there a way to do it by using the standard library (e.g. std::min_element and lambdas)?

>Solution :

You can make a helper class that’ll combine those vectors into a single vector of structs to make it easy to use min_element:

template <typename T>
class Masker {
private:
    template <typename V>
    struct Item {
        bool mask;
        V val;
    };
    std::vector<Item<T>> vec;
public:
    Masker(std::vector<bool> mask, std::vector<T> vals) {
        for(size_t i = 0; i < vals.size(); i++) {
            vec.push_back({mask[i], vals[i]});
        }
    }
    const T& find_min() {
        return (*min_element(vec.begin(), vec.end(),
            [](const auto& lhs, const auto& rhs){ 
                if(!lhs.mask) { return false; }
                if(!rhs.mask) { return true; }
                return lhs.val < rhs.val;
            })).val;
    }
};

And then call it like:

std::cout << Masker<double>(mask, vec).find_min();

Live example: https://ideone.com/G6ymGH