the <code>flights</code> <em>data.table</em>:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true"></a><span class="co"># get all &#39;hours&#39; in flights</span></span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true"></a>flights[, <span class="kw">sort</span>(<span class="kw">unique</span>(hour))]</span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true"></a><span class="co">#  [1]  0  1  2  3  4  5  6  7  8  