sorting two vectors using c++ with condition

Question

I have two vectors vector a and vector b and I want to sort them both with the condition that a[i]/b[i]>a[i+1]/b[b+1]. How do I implement the code in C++ for that?

Answer 1

Say you start with

#include <vector>
#include <algorithm>

int main() {
    std::vector<int> a{2, 7, 3};
    std::vector<int> b{4, 2, 1};

Create a vector of indices, inds:

    std::vector<std::size_t> inds;
    for(std::size_t i = 0; i < a.size(); ++i)
        inds.push_back(i);

also create a comparison function describing the criteria in your question:

    auto cmp = [&](std::size_t lhs, std::size_t rhs) {
        return static_cast<double>(a[lhs]) / b[lhs] > static_cast<double>(a[rhs]) / b[rhs];
    };

and sort according to it:

    std::sort(std::begin(inds), std::end(inds), cmp);
}

At this point, inds will be organized according to your criteria.

Finally, use the answers in reorder a vector using a vector of indices to reorder each of a and b according to inds.