title: Change elements at given positions in an arma::vec with corresponding elements in other vector
body: <p>I was wondering what is the most compact syntax in <code>Rcpp</code> to change given (noncontiguous) elements at positions <code>pos</code> in a vector <code>v1</code> with the corresponding elements in another vector (if I was using <code>arma::vec</code> class)? Say what in R I would do with</p>
  
  <pre><code>v1 = 1:10
  pos = c(2,4,10)
  v2 = c(3,8,2)
  v1[pos] = v2
  </code></pre>
  
  <p>Is this possible without doing an explicit <code>for</code> loop?</p>
  
  <p>Apologies if this is a trivial question...</p>