title: extract elements from a list based on a vector of indices
body: <p>I want to extract elements from a list based on indices stored in a separate vector. </p>
  
  <p>This is my attempt at it:</p>
  
  <pre><code>list_positions&lt;-c(2,3,4)
  my_list&lt;-list(c(1,3,4),c(2,3,4,5,6),c(1,2,3,4,6))
  
  my_fun&lt;-function(x,y){
    x[y]
  }
  
  mapply(my_fun,x=my_list,y=list_positions)
  </code></pre>
  
  <p>Maybe somebody can suggest a faster solution. My list is has around 14 million elements. I tried parallel solutions, where instead of mapply I used clusterMap but still I would like to have a better performance.</p>