<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69818637">src/mlpack/methods/preprocess/preprocess_imputer_main.cpp</a>:</p>
<pre style='color:#555'>&gt; +#include &lt;mlpack/core.hpp&gt;
&gt; +#include &lt;mlpack/core/data/imputer.hpp&gt;
&gt; +#include &lt;mlpack/core/data/dataset_info.hpp&gt;
&gt; +#include &lt;mlpack/core/data/map_policies/increment_policy.hpp&gt;
&gt; +#include &lt;mlpack/core/data/map_policies/missing_policy.hpp&gt;
&gt; +#include &lt;mlpack/core/data/imputation_methods/mean_imputation.hpp&gt;
&gt; +#include &lt;mlpack/core/data/imputation_methods/median_imputation.hpp&gt;
&gt; +#include &lt;mlpack/core/data/imputation_methods/custom_imputation.hpp&gt;
&gt; +#include &lt;mlpack/core/data/imputation_methods/listwise_deletion.hpp&gt;
&gt; +
&gt; +PROGRAM_INFO(&quot;Impute Data&quot;, &quot;This utility takes a dataset and converts user &quot;
&gt; +    &quot;defined missing variable to another to provide more meaningful analysis &quot;
&gt; +    &quot;\n\n&quot;
&gt; +    &quot;The program does not modify the original file, but instead makes a &quot;
&gt; +    &quot;separate file to save the output data; The program requires you to &quot;
&gt; +    &quot;specify the file name with --output_file (-o).&quot;
</pre>
<p>It says here that --output_file is required, but in the code it isn't required and only a warning is issued if --output_file is not specified. I think issuing a warning is a better approach, so maybe we should update the documentation here?</p>

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