Fastest way to sort a list of number and their index

Question

I have a question that could seem very basic, but it is in a context where \"every CPU tick counts\" (this is a part of a larger algorithm that will be used on supercomputers).<

Answer 1

std::pair and std::sort fit your requirements ideally: if you put the value into the pair.first and the index in pair.second, you can simply call a sort on a vector of pairs, like this:

// This is your original data. It does not need to be in a vector
vector orig;
orig.push_back(10);
orig.push_back(3);
orig.push_back(6);
orig.push_back(11);
orig.push_back(2);
orig.push_back(19);
orig.push_back(7);
// This is a vector of {value,index} pairs
vector > vp;
vp.reserve(orig.size());
for (size_t i = 0 ; i != orig.size() ; i++) {
    vp.push_back(make_pair(orig[i], i));
}
// Sorting will put lower values ahead of larger ones,
// resolving ties using the original index
sort(vp.begin(), vp.end());
for (size_t i = 0 ; i != vp.size() ; i++) {
    cout << vp[i].first << " " << vp[i].second << endl;
}