lace</em>.</p></li>
<li><p>We could have also provided <code>by</code> with a <em>character vector</em> as we saw in the <em>Introduction to data.table</em> vignette, e.g., <code>by = c(&quot;origin&quot;, &quot;dest&quot;)</code>.</p></li>
</ul>
</div>
</div>
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<div id="section-8" class="section level1">
<h1></h1>
<div id="e-multiple-columns-and" class="section level3">
<h3>e) Multiple columns and <code>:=</code></h3>
<div id="how-can-we-add-two-more-columns-computing-max-of-dep_delay-and-arr_delay-fo