title: Armadillo - how to extract rows?
body: <p>I'm using Armadillo C++ library for matrices.</p>
  
  <p>I would like to copy some of the rows to an external array (I need to copy them to the gpu).
   Is there a fast way to do this?</p>
  
  <p>If I use <code>.rows</code> , it gives me a subview with no access to data pointers, so I must iterate on the values and copy them one by one. This is very slow.</p>
  
  <p>Is there another option?</p>
  
  <p>Thanks.</p>