Question

I have a vector of doubles. I want to sort it from highest to lowest, and get the indices of the top K elements. std::sort just sorts in place, and does not return the indices I believe. What would be a quick way to get the top K indices of largest elements?

Answer 1

The first thing that comes to mind is somewhat hackish, but you could define a struct that stored both the double and its original index, then overload the < operator to sort based on the double:

struct s {
    double d;
    int index;
    bool operator < (const struct &s) const {
        return d < s.d;
    }
};

Then you could retrieve the original indices from the struct.

Fuller example:

vector<double> orig;
vector<s> v;
...
for (int i=0; i < orig.size(); ++i) {
    s s_temp;
    s_temp.d = orig[i];
    s_temp.index = i;
    v.push_back(s);
}
sort(v.begin(), v.end());
//now just retrieve v[i].index

This will leave them sorted from smallest to largest, but you could overload the > operator instead and then pass in greater to the sort function if wanted.