# Authors: Ashim Bhattarai <ashimb9@gmail.com>
# Thomas J Fan <thomasjpfan@gmail.com>
# License: BSD 3 clause
import numpy as np
from ._base import _BaseImputer
from ..utils.validation import FLOAT_DTYPES
from ..metrics import pairwise_distances_chunked
from ..metrics.pairwise import _NAN_METRICS
from ..neighbors._base import _get_weights
from ..neighbors._base import _check_weights
from ..utils import is_scalar_nan
from ..utils._mask import _get_mask
from ..utils.validation import check_is_fitted
[docs]class KNNImputer(_BaseImputer):
"""Imputation for completing missing values using k-Nearest Neighbors.
Each sample's missing values are imputed using the mean value from
`n_neighbors` nearest neighbors found in the training set. Two samples are
close if the features that neither is missing are close.
Read more in the :ref:`User Guide <knnimpute>`.
.. versionadded:: 0.22
Parameters
----------
missing_values : int, float, str, np.nan or None, default=np.nan
The placeholder for the missing values. All occurrences of
`missing_values` will be imputed. For pandas' dataframes with
nullable integer dtypes with missing values, `missing_values`
should be set to np.nan, since `pd.NA` will be converted to np.nan.
n_neighbors : int, default=5
Number of neighboring samples to use for imputation.
weights : {'uniform', 'distance'} or callable, default='uniform'
Weight function used in prediction. Possible values:
- 'uniform' : uniform weights. All points in each neighborhood are
weighted equally.
- 'distance' : weight points by the inverse of their distance.
in this case, closer neighbors of a query point will have a
greater influence than neighbors which are further away.
- callable : a user-defined function which accepts an
array of distances, and returns an array of the same shape
containing the weights.
metric : {'nan_euclidean'} or callable, default='nan_euclidean'
Distance metric for searching neighbors. Possible values:
- 'nan_euclidean'
- callable : a user-defined function which conforms to the definition
of ``_pairwise_callable(X, Y, metric, **kwds)``. The function
accepts two arrays, X and Y, and a `missing_values` keyword in
`kwds` and returns a scalar distance value.
copy : bool, default=True
If True, a copy of X will be created. If False, imputation will
be done in-place whenever possible.
add_indicator : bool, default=False
If True, a :class:`MissingIndicator` transform will stack onto the
output of the imputer's transform. This allows a predictive estimator
to account for missingness despite imputation. If a feature has no
missing values at fit/train time, the feature won't appear on the
missing indicator even if there are missing values at transform/test
time.
Attributes
----------
indicator_ : :class:`~sklearn.impute.MissingIndicator`
Indicator used to add binary indicators for missing values.
``None`` if add_indicator is False.
n_features_in_ : int
Number of features seen during :term:`fit`.
.. versionadded:: 0.24
feature_names_in_ : ndarray of shape (`n_features_in_`,)
Names of features seen during :term:`fit`. Defined only when `X`
has feature names that are all strings.
.. versionadded:: 1.0
See Also
--------
SimpleImputer : Imputation transformer for completing missing values
with simple strategies.
IterativeImputer : Multivariate imputer that estimates each feature
from all the others.
References
----------
* Olga Troyanskaya, Michael Cantor, Gavin Sherlock, Pat Brown, Trevor
Hastie, Robert Tibshirani, David Botstein and Russ B. Altman, Missing
value estimation methods for DNA microarrays, BIOINFORMATICS Vol. 17
no. 6, 2001 Pages 520-525.
Examples
--------
>>> import numpy as np
>>> from sklearn.impute import KNNImputer
>>> X = [[1, 2, np.nan], [3, 4, 3], [np.nan, 6, 5], [8, 8, 7]]
>>> imputer = KNNImputer(n_neighbors=2)
>>> imputer.fit_transform(X)
array([[1. , 2. , 4. ],
[3. , 4. , 3. ],
[5.5, 6. , 5. ],
[8. , 8. , 7. ]])
"""
def __init__(
self,
*,
missing_values=np.nan,
n_neighbors=5,
weights="uniform",
metric="nan_euclidean",
copy=True,
add_indicator=False,
):
super().__init__(missing_values=missing_values, add_indicator=add_indicator)
self.n_neighbors = n_neighbors
self.weights = weights
self.metric = metric
self.copy = copy
def _calc_impute(self, dist_pot_donors, n_neighbors, fit_X_col, mask_fit_X_col):
"""Helper function to impute a single column.
Parameters
----------
dist_pot_donors : ndarray of shape (n_receivers, n_potential_donors)
Distance matrix between the receivers and potential donors from
training set. There must be at least one non-nan distance between
a receiver and a potential donor.
n_neighbors : int
Number of neighbors to consider.
fit_X_col : ndarray of shape (n_potential_donors,)
Column of potential donors from training set.
mask_fit_X_col : ndarray of shape (n_potential_donors,)
Missing mask for fit_X_col.
Returns
-------
imputed_values: ndarray of shape (n_receivers,)
Imputed values for receiver.
"""
# Get donors
donors_idx = np.argpartition(dist_pot_donors, n_neighbors - 1, axis=1)[
:, :n_neighbors
]
# Get weight matrix from from distance matrix
donors_dist = dist_pot_donors[
np.arange(donors_idx.shape[0])[:, None], donors_idx
]
weight_matrix = _get_weights(donors_dist, self.weights)
# fill nans with zeros
if weight_matrix is not None:
weight_matrix[np.isnan(weight_matrix)] = 0.0
# Retrieve donor values and calculate kNN average
donors = fit_X_col.take(donors_idx)
donors_mask = mask_fit_X_col.take(donors_idx)
donors = np.ma.array(donors, mask=donors_mask)
return np.ma.average(donors, axis=1, weights=weight_matrix).data
[docs] def fit(self, X, y=None):
"""Fit the imputer on X.
Parameters
----------
X : array-like shape of (n_samples, n_features)
Input data, where `n_samples` is the number of samples and
`n_features` is the number of features.
y : Ignored
Not used, present here for API consistency by convention.
Returns
-------
self : object
The fitted `KNNImputer` class instance.
"""
# Check data integrity and calling arguments
if not is_scalar_nan(self.missing_values):
force_all_finite = True
else:
force_all_finite = "allow-nan"
if self.metric not in _NAN_METRICS and not callable(self.metric):
raise ValueError("The selected metric does not support NaN values")
if self.n_neighbors <= 0:
raise ValueError(
"Expected n_neighbors > 0. Got {}".format(self.n_neighbors)
)
X = self._validate_data(
X,
accept_sparse=False,
dtype=FLOAT_DTYPES,
force_all_finite=force_all_finite,
copy=self.copy,
)
_check_weights(self.weights)
self._fit_X = X
self._mask_fit_X = _get_mask(self._fit_X, self.missing_values)
super()._fit_indicator(self._mask_fit_X)
return self