/library.html'>library</a></span><span class='op'>(</span><span class='va'><a href='http://r-datatable.com'>data.table</a></span><span class='op'>)</span>
<span class='va'>ids</span> <span class='op'>=</span> <span class='va'>tab</span><span class='op'>[</span>, <span class='va'>.SD</span><span class='op'>[</span><span class='fu'><a href='https://rdrr.io/r/base/sample.html'>sample</a></span><span class='op'>(</span><span class='fu'><a href='https://rdrr.io/r/base/nrow.html'>nrow</a></span><span class='op'>(