Question

I have several std::vector, all of the same length. I want to sort one of these vectors, and apply the same transformation to all of the other vectors. Is there a neat way of doing this? (preferably using the STL or Boost)? Some of the vectors hold ints and some of them std::strings.

Pseudo code:

std::vector<int> Index = { 3, 1, 2 }; std::vector<std::string> Values = { 'Third', 'First', 'Second' };  Transformation = sort(Index); Index is now { 1, 2, 3};  ... magic happens as Transformation is applied to Values ... Values are now { 'First', 'Second', 'Third' }; 

Answer 1

friol’s approach is good when coupled with yours. First, build a vector consisting of the numbers 1…n, along with the elements from the vector dictating the sorting order:

typedef vector<int>::const_iterator myiter;  vector<pair<size_t, myiter> > order(Index.size());  size_t n = 0; for (myiter it = Index.begin(); it != Index.end(); ++it, ++n)     order[n] = make_pair(n, it); 

Now you can sort this array using a custom sorter:

struct ordering {     bool operator ()(pair<size_t, myiter> const& a, pair<size_t, myiter> const& b) {         return *(a.second) < *(b.second);     } };  sort(order.begin(), order.end(), ordering()); 

Now you’ve captured the order of rearrangement inside order (more precisely, in the first component of the items). You can now use this ordering to sort your other vectors. There’s probably a very clever in-place variant running in the same time, but until someone else comes up with it, here’s one variant that isn’t in-place. It uses order as a look-up table for the new index of each element.

template <typename T> vector<T> sort_from_ref(     vector<T> const& in,     vector<pair<size_t, myiter> > const& reference ) {     vector<T> ret(in.size());      size_t const size = in.size();     for (size_t i = 0; i < size; ++i)         ret[i] = in[reference[i].first];      return ret; }