<p>In <a href="https://github.com/mlpack/mlpack/pull/740#discussion_r72339180">src/mlpack/core/data/imputer.hpp</a>:</p>
<pre style='color:#555'>&gt; +              const std::string&amp; missingValue)
&gt; +  {
&gt; +    if (columnMajor)
&gt; +    {
&gt; +      for (size_t i = 0; i &lt; input.n_rows; ++i)
&gt; +      {
&gt; +        if (mapper.NumMappings(i) &gt; 0)
&gt; +        {
&gt; +          T mappedValue = static_cast&lt;T&gt;(mapper.UnmapValue(missingValue, i));
&gt; +          strategy.Impute(input, mappedValue, i, columnMajor);
&gt; +        }
&gt; +      }
&gt; +    }
&gt; +    else
&gt; +    {
&gt; +      for (size_t i = 0; i &lt; input.n_rows; ++i)
</pre>
<p>I fixed it.<br>
I got a little creative and added a user friendly output messages in the commit.</p>

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