title: How to reverse order a vector?
body: <p>Suppose I have a vector <code>v</code>, how do I get its reverse, i.e. last element first?</p>
  
  <p>The first thing that comes to me is <code>v[length(v):1]</code>, but it returns NA when <code>v</code> is <code>numeric(0)</code>, while user normally expect sorting nothing returns nothing, not sorting nothing returns the unavailable thing - it does make a big difference in my case.</p>