de>i</code> and <code>by</code> the same way as we have seen in the <em>Introduction to data.table</em> vignette. We can in the same way use <code>keyby</code>, chain operations together, and pass expressions to <code>by</code> as well all in the same way. The syntax is <em>consistent</em>.</p></li>
<li><p>We can use <code>:=</code> for its side effect or use <code>copy()</code> to not modify the original object while updating by reference.</p></li>
</ul>
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<div id="section-16" class="sec