title: Data table error could not find function "."
body: <p>I am trying to run the following piece of code. Whenever I try to run the code I get the following error: <code>Error in eval(expr, envir, enclos) : could not find function "."</code>
  How can I fix it? Could someone help? </p>
  
  <pre><code>data(mtcars)
  library(data.table)
  mtcarsDT &lt;- data.table(mtcars)
  mtcarsDT[ mpg &gt; 20,
          .(AvgHP = mean(hp),
          "MinWT(kg)" = min(wt * 453.6)), # wt lbs
          by = .(cyl, under5gears = gear &lt; 5)
          ]
  </code></pre>
  
  <p>Here is the session info</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.1.2 (2014-10-31)
  Platform: x86_64-apple-darwin10.8.0 (64-bit)
  
  locale:
  [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] data.table_1.9.2
  
  loaded via a namespace (and not attached):
  [1] digest_0.6.8    htmltools_0.2.6 plyr_1.8.1      Rcpp_0.11.3     reshape2_1.4.1  rmarkdown_0.3.3 stringr_0.6.2  
  [8] tools_3.1.2 
  </code></pre>