title: Should I use a data.frame or a matrix?
body: <p>When should one use a <code>data.frame</code>, and when is it better to use a <code>matrix</code>?</p>
  
  <p>Both keep data in a rectangular format, so sometimes it's unclear.</p>
  
  <p>Are there any general rules of thumb for when to use which data type?</p>