 declared missing values by default, something that can be prevented
by either of the following two commands:</p>
<div class="sourceCode" id="cb20"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="fu"><a href="https://rdrr.io/r/base/character.html" class="external-link">as.character</a></span><span class="op">(</span><span class="fu"><a href="../reference/drop_undeclare.html">undeclare</a></span><span class="op">(</span><span class="va">x4</span><span class="op">)</span><span