p>then see how we can <em>add/update/delete</em> columns <em>by reference</em> in <code>j</code> using the <code>:=</code> operator and how to combine with <code>i</code> and <code>by</code>.</p></li>
<li><p>and finally we will look at using <code>:=</code> for its <em>side-effect</em> and how we can avoid the side effects using <code>copy()</code>.</p></li>
</ol>
</div>
<div id="reference-semantics" class="section level2">
<h2>1. Reference semantics</h2>
<p>All the operations we have seen so far in the pre