<p>In <a href="https://github.com/mlpack/mlpack/pull/746#discussion_r75311483">src/mlpack/core/tree/binary_space_tree/ub_tree_split_impl.hpp</a>:</p>
<pre style='color:#555'>&gt; +                                                const size_t begin,
&gt; +                                                const size_t count,
&gt; +                                                size_t&amp; splitCol)
&gt; +{
&gt; +  constexpr size_t order = sizeof(AddressElemType) * CHAR_BIT;
&gt; +  if (begin == 0 &amp;&amp; count == data.n_cols)
&gt; +  {
&gt; +    // Calculate all addresses.
&gt; +    InitializeAddresses(data);
&gt; +
&gt; +    // Probably this is not a good idea. Maybe it is better to get
&gt; +    // a number of distinct samples and find the median.
&gt; +    std::sort(addresses.begin(), addresses.end(), ComparePair);
&gt; +
&gt; +    // Rearrange dataset.
&gt; +    PerformSplit(data, count);
</pre>
<p>I've just verified, <code>sort_index(X)</code> provides the sorted order of the elements of <code>X</code> instead of the columns. It seems, the function is not applicable in this case,</p>

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