Creating a vector of indices of a sorted vector [duplicate]

Answer 1

1) Create y as a vector of index (an integer range)

2) Sort this range with a comparator that returns the indexes elements from x Using the Standard Library, that gives :

#include 
#include 
#include 

int main() {

    std::vector x = {15, 3, 0, 20};

    std::vector y;

    std::vector y(x.size());
    std::size_t n(0);
    std::generate(std::begin(y), std::end(y), [&]{ return n++; });

    std::sort(  std::begin(y), 
                std::end(y),
                [&](int i1, int i2) { return x[i1] < x[i2]; } );

    for (auto v : y)
        std::cout << v << ' ';

    return 0;
}

Live demo.