<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69918719">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>&gt; +  {
&gt; +    // Nothing to initialize here.
&gt; +  }
&gt; +
&gt; +
&gt; +  template &lt;typename MapType&gt;
&gt; +  mapped_type MapString(const std::string&amp; string,
&gt; +                        const size_t dimension,
&gt; +                        MapType maps,
&gt; +                        std::vector&lt;Datatype&gt;&amp; types)
&gt; +  {
&gt; +    // If this condition is true, either we have no mapping for the given string
&gt; +    // or we have no mappings for the given dimension at all.  In either case,
&gt; +    // we create a mapping.
&gt; +    const double NaN = std::numeric_limits&lt;double&gt;::quiet_NaN();
&gt; +    if (missingSet.count(string) != 0 &amp;&amp;
</pre>
<p>The role of missingSet and maps are different. <br>
missingSet specify which value/string we should map to<br>
maps record the value/string we already mapped to</p>

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