title: what does lapply(Output_data,"[[",2) mean in R
body: <p>In RHadoop, when we make the results readable, it will use the code:</p>
  
  <pre><code>results &lt;- data.frame(words=unlist(lapply(Output_data,"[[",1)), count
  =unlist(lapply(Output_data,"[[",2)))
  </code></pre>
  
  <p>but what does <code>lapply(Output_data,"[[",1)</code>mean? especially the "[[" and '1' in lapply.</p>