<span class="r-pr">#&gt;</span> 4:     NA     4     0     0     1</span>
<span class="r-in"><span></span></span>
<span class="r-in"><span><span class="co"># when NA  and "NA" both exist, they would be merged</span></span></span>
<span class="r-in"><span><span class="va">df</span> <span class="op">&lt;-</span> <span class="fu"><a href="https://rdatatable.gitlab.io/data.table/reference/data.table.html" class="external-link">data.table</a></span><span class="op">(</span>x <span class="op">=</span> <span cla