title: Syntax in R regarding ddply
body: <p>can anyone explain a reason why there would be a period before the <code>playerID</code> in the following R statement?</p>
  
  <pre><code>dataframe.AB&lt;-ddply(Batting, .(playerID), summarize, Career.AB=sum(AB, na.rm=TRUE))
  </code></pre>
  
  <p>I've seen that argument passed something with the c(...) function...not sure what's going on here.  I should mention that <code>playerID</code> is a variable in the <code>Batting</code> dataframe.</p>
  
  <p>Thanks.</p>