ode></pre></div>
<div id="update-by-reference-question" class="section level4 bs-callout bs-callout-warning">
<h4 class="bs-callout bs-callout-warning">Exercise:</h4>
<p>What is the difference between <code>flights[hour == 24L, hour := 0L]</code> and <code>flights[hour == 24L][, hour := 0L]</code>? Hint: The latter needs an assignment (<code>&lt;-</code>) if you would want to use the result later.</p>
<p>If you can’t figure it out, have a look at the <code>Note</code> section of <code>?&quot;:=&quot;</cod