<p>In <a href="https://github.com/mlpack/mlpack/pull/746#discussion_r76269981">src/mlpack/core/tree/binary_space_tree/ub_tree_split_impl.hpp</a>:</p>
<pre style='color:#555'>&gt; +      size_t oldI = newToOld[i];
&gt; +      size_t newIndex = newFromOld[index];
&gt; +
&gt; +      data.swap_cols(i, newFromOld[index]);
&gt; +
&gt; +      size_t tmp = newFromOld[index];
&gt; +      newFromOld[index] = i;
&gt; +      newFromOld[oldI] = tmp;
&gt; +
&gt; +      tmp = newToOld[i];
&gt; +      newToOld[i] = newToOld[newIndex];
&gt; +      newToOld[newIndex] = tmp;
&gt; +
&gt; +      tmp = oldFromNew[i];
&gt; +      oldFromNew[i] = oldFromNew[newIndex];
&gt; +      oldFromNew[newIndex] = tmp;
</pre>
<p>I believe this rearranging is only done in the first level of building the tree (at which point <code>oldFromNew</code> is initialized in the same way <code>newToOld</code> is initialized here), so does it make sense to simply remove the <code>newToOld</code> member used here and instead use <code>oldFromNew</code>?  If so, maybe it makes sense to have the other overload of <code>PerformSplit</code> call this one after initializing the <code>oldFromNew</code> vector.  I suspect I may have overlooked something here, so let me know if I have. :)</p>

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