title: What does ".N" mean in data.table?
body: <p>I have a data.table <code>dt</code>:</p>
  <pre><code>library(data.table)
  dt = data.table(a=LETTERS[c(1,1:3)],b=4:7)
  
     a b
  1: A 4
  2: A 5
  3: B 6
  4: C 7
  </code></pre>
  <p>The result of <code>dt[, .N, by=a]</code> is</p>
  <pre><code>   a N
  1: A 2
  2: B 1
  3: C 1
  </code></pre>
  <p>I know the <code>by=a</code> or <code>by=&quot;a&quot;</code> means grouped by <code>a</code> column and the <code>N</code> column is the sum of duplicated times of <code>a</code>. However, I don't use <code>nrow()</code> but I get the result. The <code>.N</code> is not just the column name? I can't find the document by <code>??&quot;.N&quot;</code> in R. I tried to use <code>.K</code>, but it doesn't work. What does <code>.N</code> means?</p>