<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69918063">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>&gt; +#define MLPACK_CORE_DATA_MAP_POLICIES_MISSING_POLICY_HPP
&gt; +
&gt; +#include &lt;mlpack/core.hpp&gt;
&gt; +#include &lt;unordered_map&gt;
&gt; +#include &lt;boost/bimap.hpp&gt;
&gt; +#include &lt;mlpack/core/data/map_policies/datatype.hpp&gt;
&gt; +#include &lt;limits&gt;
&gt; +
&gt; +
&gt; +using namespace std;
&gt; +
&gt; +namespace mlpack {
&gt; +namespace data {
&gt; +
&gt; +/**
&gt; + * Same as increment map policy, but does not change type of features.
</pre>
<p>MissingPolicy only map desired value/string to nan.</p>

<p>Example : <br>
MissingPolicy miss({"a"});<br>
0 a 2 3 4 map to 0 nan 2 3 4</p>

<p>IncrementPocliy inc;<br>
0 a 2 3 4 map to 0 1 2 3 4</p>

<p>The role of missingSet and maps are different. </p>

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