# Source code for mars.tensor.statistics.cov

```#!/usr/bin/env python
# -*- coding: utf-8 -*-
# Copyright 1999-2021 Alibaba Group Holding Ltd.
#
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#
# Unless required by applicable law or agreed to in writing, software
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and

import warnings

import numpy as np

from ..core import Tensor
from ..datasource import tensor as astensor, array
from ..base.squeeze import squeeze
from ..base.where import where
from .average import average

[docs]def cov(m, y=None, rowvar=True, bias=False, ddof=None, fweights=None, aweights=None):
"""
Estimate a covariance matrix, given data and weights.

Covariance indicates the level to which two variables vary together.
If we examine N-dimensional samples, :math:`X = [x_1, x_2, ... x_N]^T`,
then the covariance matrix element :math:`C_{ij}` is the covariance of
:math:`x_i` and :math:`x_j`. The element :math:`C_{ii}` is the variance
of :math:`x_i`.

See the notes for an outline of the algorithm.

Parameters
----------
m : array_like
A 1-D or 2-D array containing multiple variables and observations.
Each row of `m` represents a variable, and each column a single
observation of all those variables. Also see `rowvar` below.
y : array_like, optional
An additional set of variables and observations. `y` has the same form
as that of `m`.
rowvar : bool, optional
If `rowvar` is True (default), then each row represents a
variable, with observations in the columns. Otherwise, the relationship
is transposed: each column represents a variable, while the rows
contain observations.
bias : bool, optional
Default normalization (False) is by ``(N - 1)``, where ``N`` is the
number of observations given (unbiased estimate). If `bias` is True,
then normalization is by ``N``. These values can be overridden by using
the keyword ``ddof`` in numpy versions >= 1.5.
ddof : int, optional
If not ``None`` the default value implied by `bias` is overridden.
Note that ``ddof=1`` will return the unbiased estimate, even if both
`fweights` and `aweights` are specified, and ``ddof=0`` will return
the simple average. See the notes for the details. The default value
is ``None``.
fweights : array_like, int, optional
1-D tensor of integer freguency weights; the number of times each
observation vector should be repeated.
aweights : array_like, optional
1-D tensor of observation vector weights. These relative weights are
typically large for observations considered "important" and smaller for
observations considered less "important". If ``ddof=0`` the array of
weights can be used to assign probabilities to observation vectors.

Returns
-------
out : Tensor
The covariance matrix of the variables.

--------
corrcoef : Normalized covariance matrix

Notes
-----
Assume that the observations are in the columns of the observation
array `m` and let ``f = fweights`` and ``a = aweights`` for brevity. The
steps to compute the weighted covariance are as follows::

>>> w = f * a
>>> v1 = mt.sum(w)
>>> v2 = mt.sum(w * a)
>>> m -= mt.sum(m * w, axis=1, keepdims=True) / v1
>>> cov = mt.dot(m * w, m.T) * v1 / (v1**2 - ddof * v2)

Note that when ``a == 1``, the normalization factor
``v1 / (v1**2 - ddof * v2)`` goes over to ``1 / (np.sum(f) - ddof)``
as it should.

Examples
--------
Consider two variables, :math:`x_0` and :math:`x_1`, which
correlate perfectly, but in opposite directions:

>>> import mars.tensor as mt

>>> x = mt.array([[0, 2], [1, 1], [2, 0]]).T
>>> x.execute()
array([[0, 1, 2],
[2, 1, 0]])

Note how :math:`x_0` increases while :math:`x_1` decreases. The covariance
matrix shows this clearly:

>>> mt.cov(x).execute()
array([[ 1., -1.],
[-1.,  1.]])

Note that element :math:`C_{0,1}`, which shows the correlation between
:math:`x_0` and :math:`x_1`, is negative.

Further, note how `x` and `y` are combined:

>>> x = [-2.1, -1,  4.3]
>>> y = [3,  1.1,  0.12]
>>> X = mt.stack((x, y), axis=0)
>>> print(mt.cov(X).execute())
[[ 11.71        -4.286     ]
[ -4.286        2.14413333]]
>>> print(mt.cov(x, y).execute())
[[ 11.71        -4.286     ]
[ -4.286        2.14413333]]
>>> print(mt.cov(x).execute())
11.71

"""
from ..merge import vstack
from ..linalg import dot

if ddof is not None and ddof != int(ddof):
raise ValueError("ddof must be integer")

m = astensor(m)
if m.ndim > 2:
raise ValueError("m has more than 2 dimensions")

if y is None:
dtype = np.result_type(m.dtype, np.float64)
else:
y = astensor(y)
if y.ndim > 2:
raise ValueError("y has more than 2 dimensions")
dtype = np.result_type(m.dtype, y.dtype, np.float64)

X = array(m, ndmin=2, dtype=dtype)
if not rowvar and X.shape[0] != 1:
X = X.T
if y is not None:
y = array(y, copy=False, ndmin=2, dtype=dtype)
if not rowvar and y.shape[0] != 1:
y = y.T
X = vstack((X, y))

if ddof is None:
if bias == 0:
ddof = 1
else:
ddof = 0

# Get the product of frequencies and weights
w = None
if fweights is not None:
fweights = astensor(fweights, dtype=float)
if fweights.ndim > 1:
raise RuntimeError("cannot handle multidimensional fweights")
if fweights.shape[0] != X.shape[1]:
raise RuntimeError("incompatible numbers of samples and fweights")
if any(fweights < 0):
raise ValueError("fweights cannot be negative")
w = fweights
if aweights is not None:
aweights = astensor(aweights, dtype=float)
if aweights.ndim > 1:
raise RuntimeError("cannot handle multidimensional aweights")
if aweights.shape[0] != X.shape[1]:
raise RuntimeError("incompatible numbers of samples and aweights")
if any(aweights < 0):
raise ValueError("aweights cannot be negative")
if w is None:
w = aweights
else:
w *= aweights

avg, w_sum = average(X, axis=1, weights=w, returned=True)
w_sum = w_sum[0]

# Determine the normalization
if w is None:
fact = X.shape[1] - ddof
elif ddof == 0:
fact = w_sum
elif aweights is None:
fact = w_sum - ddof
else:
fact = w_sum - ddof * sum(w * aweights) / w_sum

X -= avg[:, None]
if w is None:
X_T = X.T
else:
X_T = (X * w).T
c = dot(X, X_T.conj())
if isinstance(fact, Tensor):
fact = where(fact <= 0, 0.0, fact)
fact = fact.astype(float)
else:
if fact <= 0:
warnings.warn(
"Degrees of freedom <= 0 for slice", RuntimeWarning, stacklevel=2
)
fact = 0.0
fact = np.float64(fact)
c = c * (1.0 / fact)
return squeeze(c)
```