C++ sorting and keeping track of indexes

Question

Using C++, and hopefully the standard library, I want to sort a sequence of samples in ascending order, but I also want to remember the original indexes of the new samples.<

Answer 1

Consider using std::multimap as suggested by @Ulrich Eckhardt. Just that the code could be made even simpler.

Given

std::vector a = {5, 2, 1, 4, 3};  // a: 5 2 1 4 3

To sort in the mean time of insertion

std::multimap mm;
for (std::size_t i = 0; i != a.size(); ++i)
    mm.insert({a[i], i});

To retrieve values and original indices

std::vector b;
std::vector c;
for (const auto & kv : mm) {
    b.push_back(kv.first);             // b: 1 2 3 4 5
    c.push_back(kv.second);            // c: 2 1 4 3 0
}

The reason to prefer a std::multimap to a std::map is to allow equal values in original vectors. Also please note that, unlike for std::map, operator[] is not defined for std::multimap.