title: Set column name ddply
body: <p>How to set the column name of the summarized data in </p>
  
  <pre><code>library(plyr)
  ddply(data,.(col1,col2),nrow)
  </code></pre>
  
  <p>like in</p>
  
  <pre><code>ddply(data,.(col1,col2),function(x) data.frame(number=nrow(x)))
  </code></pre>