</span><span class='va'>.SD</span><span class='op'>)</span>, <span class='fl'>5</span><span class='op'>)</span><span class='op'>]</span>, by <span class='op'>=</span> <span class='fu'><a href='https://rdrr.io/r/base/c.html'>c</a></span><span class='op'>(</span><span class='st'>"problem"</span>, <span class='st'>"algorithm"</span>, <span class='st'>"y"</span><span class='op'>)</span><span class='op'>]</span>
<span class='fu'><a href='https://Rdatatable.gitlab.io/data.table/reference/setkey.html'>setkeyv</a><