<p>In <a href="https://github.com/mlpack/mlpack/pull/694#discussion_r69866700">src/mlpack/core/data/map_policies/missing_policy.hpp</a>:</p>
<pre style='color:#555'>> +#include <limits>
> +
> +
> +using namespace std;
> +
> +namespace mlpack {
> +namespace data {
> +
> +/**
> + * Same as increment map policy, but does not change type of features.
> + */
> +class MissingPolicy
> +{
> + public:
> + // typedef of mapped_type
> + using mapped_type = double;
</pre>
<p><a href="https://github.com/zoq" class="user-mention">@zoq</a> it is because mappings can be any type of pairs. DatasetMapper takes care of mapping string values to any type of numeric values. <br>
For example, IncrementPolicy defines mapped_type as size_t. Therefore the DatasetMapper knows how to map and <a href="https://github.com/mlpack/mlpack/pull/694/files#diff-4654fc03017f8f55e68d7e012a8c74f9R78">return</a> the data. (string to size_t mapping)<br>
However, MissingPolicy uses NaN so the mapped_type is defined as double. </p>
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