an class="r-pr">#&gt;</span>         x     y   x_a   x_b  x_NA</span>
<span class="r-out co"><span class="r-pr">#&gt;</span>    &lt;char&gt; &lt;int&gt; &lt;num&gt; &lt;num&gt; &lt;num&gt;</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> 1:      a     1     1     0     0</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> 2:      b     2     0     1     0</span>
<span class="r-out co"><span class="r-pr">#&gt;</span> 3:     NA     3     0     0     1</span>
<span class="r-out co">