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Data Dictionaries
=================

A data dictionary is the metadata that describes features of a data
set. The NumericTableFeature and DataSourceFeature structures
describe a particular feature within a dictionary of the associated
numeric table and data source respectively. These structures specify:

-  Whether the feature is continuous, categorical, or ordinal
-  Underlying data types (double, integer, and so on) used to
   represent feature values

The DataSourceFeature structure also specifies:

-  Possible values for a categorical feature
-  The feature name

The DataSourceDictionary class is a data dictionary that describes
raw data associated with the corresponding data source. The
NumericTableDictionary class is a data dictionary that describes
in-memory numeric data associated with the corresponding numeric
table. Both classes provide generic methods for dictionary
manipulation, such as accessing a particular data feature, setting
and retrieving the number of features, and adding a new feature.
Respective DataSource and NumericTable classes have generic
dictionary manipulation methods, such as getDictionary() and
setDictionary().

To create a dictionary from the data source context, you can do one
of the following:

-  Set the doDictionaryFromContext flag in the DataSource
   constructor.
-  Call to the createDictionaryFromContext() method.

Examples
********

C++:

-  :cpp_example:`datasource/datastructures_aos.cpp`
-  :cpp_example:`datasource/datastructures_soa.cpp`
-  :cpp_example:`datasource/datastructures_homogen.cpp`