Question

I’d like to find the sort order of a vector, for example, without reordering the vector.

I can think of several ways to do this, I’m wondering if I’m missing some built-in STL or BOOST way to do this.

I imagine if the functionality were available the code would end up looking something like this:

std::vector<float> unsortedSeq;
unsortedSeq.push_back( 1.1 );
unsortedSeq.push_back( 1.0 );
unsortedSeq.push_back( 0.5 );
unsortedSeq.push_back( 1.2 );
unsortedSeq.push_back( 1.15 );

std::list<std::size_t> sortOrder;

std::sort_indices( unsortedSeq.begin(), unsortedSeq.end(), sortOrder.begin() );

BOOST_FOREACH( std::size_t index, sortOrder )
{
    std::cout << index << "\n"
}



2
1
0
4
3

Anyone know any STL or BOOST-sims that would do what I’m asking about as simply as shown?

Answer 1

You would do something like this:

template<typename T>
class SortOrder
{
public:
    SortOrder(const std::vector<T> *_sortArray) : sortArray(_sortArray) {;}

    bool operator()(int lhs, int rhs) const
    {
        return sortArray[lhs] < sortArray[rhs];
    }

private:
    const std::vector<T> *sortArray;
};

//To do the sorting:

#include <boost/range/counting_range.hpp>
auto countRange = boost::range::counting_range(0, myListOfStuff.size());

//Build a vector<int> that has one index for every value in your actual vector.
vector<int> indexList(countRange.begin(), countRange.end());

std::sort(indexList.begin(), indexList.end(), SortOrder(&myListOfStuff));

This generates a vector of indices, and then sorts them based on the vector of actual stuff.