/code> performs operations by reference. Since <code>DT</code> (the function argument) and <code>flights</code> refer to the same object in memory, modifying <code>DT</code> also reflects on <code>flights</code>.</p></li>
<li><p>And <code>ans</code> contains the maximum speed for each month.</p></li>
</ul>
</div>
</div>
<div id="b-the-copy-function" class="section level3">
<h3>b) The <code>copy()</code> function</h3>
<p>In the previous section, we used <code>:=</code> for its side effect. But of course this