>)</span></span></code></pre></div>
<p>The <code><a href="https://rdrr.io/pkg/data.table/man/copy.html" class="external-link">copy()</a></code> function ensures that “by-reference” changes
to <code>student</code>, for example, have no effect on
<code>source_student</code> <span class="citation">(<a href="#ref-data.table-reference-semantics">Dowle and Srinivasan
2022</a>)</span>. Thus the <code>source_*</code> objects retain all the
original columns, if needed later.</p>
</div>
<div class="section level2