What is the most useful R trick?
In order to share some more tips and tricks for [**R**][1], what is you single-most useful feature or trick?  Clever vectorization?  Data input/output?  Visualization and graphics?  Statistical analysis?  Special functions?  The interactive environment itself? 

One item per post, and we will see if we get a winner by means of votes.


  [1]: http://www.r-project.org
What is the most useful R trick?
===

In order to share some more tips and tricks for [**R**][1], what is you single-most useful feature or trick?  Clever vectorization?  Data input/output?  Visualization and graphics?  Statistical analysis?  Special functions?  The interactive environment itself? 

One item per post, and we will see if we get a winner by means of votes.


  [1]: http://www.r-project.org