Question

I have a vector of vectors, say

std::vector< std::vector<int> > my2dArray;

Now I would like to have a vector that contains the sizes of the vectors inside my2dArray. Manually this would look something like

std::vector<int> mySizes;
for (int i = 0; i < my2dArray.size(); i++ )
{
  mySizes.push_back( my2dArray[i].size() );
}

Is there a more “elegant” way of doing this – without manually writing a loop – by making use of some STL algorithm or the like?

Answer 1

C++03:

typedef std::vector<int> IntVec;
std::vector<size_t> sizes(my2dArray.size());
std::transform(my2dArray.begin(), my2dArray.end(), sizes.begin(), 
               std::mem_fun_ref(&IntVec::size));