 can use <code>i</code> along with <code>:=</code> in <code>j</code> the very same way as we have already seen in the <em>“Introduction to data.table”</em> vignette.</p></li>
<li><p>Column <code>hour</code> is replaced with <code>0</code> only on those <em>row indices</em> where the condition <code>hour == 24L</code> specified in <code>i</code> evaluates to <code>TRUE</code>.</p></li>
<li><p><code>:=</code> returns the result invisibly. Sometimes it might be necessary to see the result after the assignm