Reorder vector using a vector of indices

Question

I\'d like to reorder the items in a vector, using another vector to specify the order:

char   A[]     = { \'a\', \'b\', \'c\' };
size_t ORDER[] = { 1, 0, 2 }         


        

Answer 1

It is not clear by the title and the question if the vector should be ordered with the same steps it takes to order vOrder or if vOrder already contains the indexes of the desired order. The first interpretation has already a satisfying answer (see chmike and Potatoswatter), I add some thoughts about the latter. If the creation and/or copy cost of object T is relevant

template 
void reorder( std::vector & data, std::vector & order )
{
 std::size_t i,j,k;
  for(i = 0; i < order.size() - 1; ++i) {
    j = order[i];
    if(j != i) {
      for(k = i + 1; order[k] != i; ++k);
      std::swap(order[i],order[k]);
      std::swap(data[i],data[j]);
    }
  }
}

If the creation cost of your object is small and memory is not a concern (see dribeas):

template 
void reorder( std::vector & data, std::vector const & order )
{
 std::vector tmp;         // create an empty vector
 tmp.reserve( data.size() ); // ensure memory and avoid moves in the vector
 for ( std::size_t i = 0; i < order.size(); ++i ) {
  tmp.push_back( data[order[i]] );
 }
 data.swap( tmp );          // swap vector contents
}

Note that the two pieces of code in dribeas answer do different things.