on’t know the columns to assign values to in advance.</p></li>
<li><p>On the other hand, (b) is handy if you would like to jot some comments down for later.</p></li>
<li><p>The result is returned <em>invisibly</em>.</p></li>
<li><p>Since <code>:=</code> is available in <code>j</code>, we can combine it with <code>i</code> and <code>by</code> operations just like the aggregation operations we saw in the previous vignette.</p></li>
</ul>
</div>
</div>
<div id="section-4" class="section level1">
<h1></h1>
<p