Source code for ray.data.preprocessors.normalizer

from typing import List

import numpy as np
import pandas as pd

from ray.data.preprocessor import Preprocessor
from ray.util.annotations import PublicAPI


@PublicAPI(stability="alpha")
class Normalizer(Preprocessor):
    r"""Scales each sample to have unit norm.

    This preprocessor works by dividing each sample (i.e., row) by the sample's norm.
    The general formula is given by

    .. math::

        s' = \frac{s}{\lVert s \rVert_p}

    where :math:`s` is the sample, :math:`s'` is the transformed sample,
    :math:\lVert s \rVert`, and :math:`p` is the norm type.

    The following norms are supported:

    * `"l1"` (:math:`L^1`): Sum of the absolute values.
    * `"l2"` (:math:`L^2`): Square root of the sum of the squared values.
    * `"max"` (:math:`L^\infty`): Maximum value.

    Examples:
        >>> import pandas as pd
        >>> import ray
        >>> from ray.data.preprocessors import Normalizer
        >>>
        >>> df = pd.DataFrame({"X1": [1, 1], "X2": [1, 0], "X3": [0, 1]})
        >>> ds = ray.data.from_pandas(df)  # doctest: +SKIP
        >>> ds.to_pandas()  # doctest: +SKIP
           X1  X2  X3
        0   1   1   0
        1   1   0   1

        The :math:`L^2`-norm of the first sample is :math:`\sqrt{2}`, and the
        :math:`L^2`-norm of the second sample is :math:`1`.

        >>> preprocessor = Normalizer(columns=["X1", "X2"])
        >>> preprocessor.fit_transform(ds).to_pandas()  # doctest: +SKIP
                 X1        X2  X3
        0  0.707107  0.707107   0
        1  1.000000  0.000000   1

        The :math:`L^1`-norm of the first sample is :math:`2`, and the
        :math:`L^1`-norm of the second sample is :math:`1`.

        >>> preprocessor = Normalizer(columns=["X1", "X2"], norm="l1")
        >>> preprocessor.fit_transform(ds).to_pandas()  # doctest: +SKIP
            X1   X2  X3
        0  0.5  0.5   0
        1  1.0  0.0   1

        The :math:`L^\infty`-norm of the both samples is :math:`1`.

        >>> preprocessor = Normalizer(columns=["X1", "X2"], norm="max")
        >>> preprocessor.fit_transform(ds).to_pandas()  # doctest: +SKIP
            X1   X2  X3
        0  1.0  1.0   0
        1  1.0  0.0   1

    Args:
        columns: The columns to scale. For each row, these colmumns are scaled to
            unit-norm.
        norm: The norm to use. The supported values are ``"l1"``, ``"l2"``, or
            ``"max"``. Defaults to ``"l2"``.

    Raises:
        ValueError: if ``norm`` is not ``"l1"``, ``"l2"``, or ``"max"``.
    """

    _norm_fns = {
        "l1": lambda cols: np.abs(cols).sum(axis=1),
        "l2": lambda cols: np.sqrt(np.power(cols, 2).sum(axis=1)),
        "max": lambda cols: np.max(abs(cols), axis=1),
    }

    _is_fittable = False

    def __init__(self, columns: List[str], norm="l2"):
        self.columns = columns
        self.norm = norm

        if norm not in self._norm_fns:
            raise ValueError(
                f"Norm {norm} is not supported."
                f"Supported values are: {self._norm_fns.keys()}"
            )

    def _transform_pandas(self, df: pd.DataFrame):
        columns = df.loc[:, self.columns]
        column_norms = self._norm_fns[self.norm](columns)

        df.loc[:, self.columns] = columns.div(column_norms, axis=0)
        return df

    def __repr__(self):
        return (
            f"{self.__class__.__name__}(columns={self.columns!r}, norm={self.norm!r})"
        )