ode> (or <code>summarize_dt</code>) has a
parameter “by”, you can specify the group. We could find the number of
planes and the number of flights that go to each possible
destination:</p>
<div class="sourceCode" id="cb16"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="co"># the dplyr syntax:</span></span>
<span><span class="co"># destinations &lt;- group_by(flights, dest)</span></span>
<span><span class="co"># summarise(destinations,</span></span>
<span><span class="co"