ctors in <code>R</code> can only have a single class.</p>
<p>The strategy goes as follows:</p>
<ol style="list-style-type: decimal">
<li>Use <code>check()</code> like in the previous exercise to identify where there are problems. As we just saw, when called on a full dataset, this function returns a list (of variables) of lists (of checks) of lists (problem status, message and problem values).</li>
<li>We use <code>sapply()</code> to select the problem values from each <code>identifyMissing</code> result. N