ak.to_categorical
-----------------

Defined in `awkward.behaviors.categorical <https://github.com/scikit-hep/awkward-1.0/blob/80bbef0738a6b7928333d7c705ee1b359991de5b/src/awkward/behaviors/categorical.py>`__ on `line 172 <https://github.com/scikit-hep/awkward-1.0/blob/80bbef0738a6b7928333d7c705ee1b359991de5b/src/awkward/behaviors/categorical.py#L172>`__.

.. py:function:: ak.to_categorical(array, highlevel=True)


    :param array: Data convertible to an Awkward Array
    :param highlevel: If True, return an :py:obj:`ak.Array`; otherwise, return
                  a low-level :py:obj:`ak.layout.Content` subclass.
    :type highlevel: bool

Creates a categorical dataset, which has the following properties:

   * only distinct values (categories) are stored in their entirety,
   * pointers to those distinct values are represented by integers
     (an :py:obj:`ak.layout.IndexedArray` or :py:obj:`ak.layout.IndexedOptionArray`
     labeled with parameter ``"__array__" = "categorical"``.

This is equivalent to R's "factor", Pandas's "categorical", and
Arrow/Parquet's "dictionary encoding." It differs from generic uses of
:py:obj:`ak.layout.IndexedArray` and :py:obj:`ak.layout.IndexedOptionArray` in Awkward
Arrays by the guarantee of no duplicate categories and the ``"categorical"``
parameter.

.. code-block:: python


    >>> array = ak.Array([["one", "two", "three"], [], ["three", "two"]])
    >>> categorical = ak.to_categorical(array)
    >>> categorical
    <Array [['one', 'two', ... 'three', 'two']] type='3 * var * categorical[type=str...'>
    >>> ak.type(categorical)
    3 * var * categorical[type=string]
    >>> ak.to_list(categorical) == ak.to_list(array)
    True
    >>> ak.categories(categorical)
    <Array ['one', 'two', 'three'] type='3 * string'>
    >>> ak.is_categorical(categorical)
    True
    >>> ak.from_categorical(categorical)
    <Array [['one', 'two', ... 'three', 'two']] type='3 * var * string'>

This function descends through nested lists, but not into the fields of
records, so records can be categories. To make categorical record
fields, split up the record, apply this function to each desired field,
and :py:obj:`ak.zip` the results together.

.. code-block:: python


    >>> records = ak.Array([
    ...     {"x": 1.1, "y": "one"},
    ...     {"x": 2.2, "y": "two"},
    ...     {"x": 3.3, "y": "three"},
    ...     {"x": 2.2, "y": "two"},
    ...     {"x": 1.1, "y": "one"}
    ... ])
    >>> records
    <Array [{x: 1.1, y: 'one'}, ... y: 'one'}] type='5 * {"x": float64, "y": string}'>
    >>> categorical_records = ak.zip({
    ...     "x": ak.to_categorical(records["x"]),
    ...     "y": ak.to_categorical(records["y"]),
    ... })
    >>> categorical_records
    <Array [{x: 1.1, y: 'one'}, ... y: 'one'}] type='5 * {"x": categorical[type=floa...'>
    >>> ak.type(categorical_records)
    5 * {"x": categorical[type=float64], "y": categorical[type=string]}
    >>> ak.to_list(categorical_records) == ak.to_list(records)
    True

The check for uniqueness is currently implemented in a Python loop, so
conversion to categorical should be regarded as expensive. (This can
change, but it would always be an _n log(n)_ operation.)

See also :py:obj:`ak.is_categorical`, :py:obj:`ak.categories`, :py:obj:`ak.from_categorical`.

