title: What do two commas mean?
body: <p>First I have a slightly embarrassing question. What do the commas in R represent? 
  For example, whenever there's code that's like <code>unique[x3,]</code> or something similar, What is the comma doing before the bracket?</p>
  
  <p>Second,</p>
  
  <pre><code>mosaicplot(UCBAdmissions[,,i],)
  </code></pre>
  
  <p>what do the two commas inside the square bracket mean?</p>