1
0
Fork 0
QuaPy/quapy/functional.py

365 lines
15 KiB
Python

import itertools
from collections import defaultdict
from typing import Union, Callable
import scipy
import numpy as np
def prevalence_linspace(n_prevalences=21, repeats=1, smooth_limits_epsilon=0.01):
"""
Produces an array of uniformly separated values of prevalence.
By default, produces an array of 21 prevalence values, with
step 0.05 and with the limits smoothed, i.e.:
[0.01, 0.05, 0.10, 0.15, ..., 0.90, 0.95, 0.99]
:param n_prevalences: the number of prevalence values to sample from the [0,1] interval (default 21)
:param repeats: number of times each prevalence is to be repeated (defaults to 1)
:param smooth_limits_epsilon: the quantity to add and subtract to the limits 0 and 1
:return: an array of uniformly separated prevalence values
"""
p = np.linspace(0., 1., num=n_prevalences, endpoint=True)
p[0] += smooth_limits_epsilon
p[-1] -= smooth_limits_epsilon
if p[0] > p[1]:
raise ValueError(f'the smoothing in the limits is greater than the prevalence step')
if repeats > 1:
p = np.repeat(p, repeats)
return p
def prevalence_from_labels(labels, classes):
"""
Computed the prevalence values from a vector of labels.
:param labels: array-like of shape `(n_instances)` with the label for each instance
:param classes: the class labels. This is needed in order to correctly compute the prevalence vector even when
some classes have no examples.
:return: an ndarray of shape `(len(classes))` with the class prevalence values
"""
if labels.ndim != 1:
raise ValueError(f'param labels does not seem to be a ndarray of label predictions')
unique, counts = np.unique(labels, return_counts=True)
by_class = defaultdict(lambda:0, dict(zip(unique, counts)))
prevalences = np.asarray([by_class[class_] for class_ in classes], dtype=float)
prevalences /= prevalences.sum()
return prevalences
def prevalence_from_probabilities(posteriors, binarize: bool = False):
"""
Returns a vector of prevalence values from a matrix of posterior probabilities.
:param posteriors: array-like of shape `(n_instances, n_classes,)` with posterior probabilities for each class
:param binarize: set to True (default is False) for computing the prevalence values on crisp decisions (i.e.,
converting the vectors of posterior probabilities into class indices, by taking the argmax).
:return: array of shape `(n_classes,)` containing the prevalence values
"""
if posteriors.ndim != 2:
raise ValueError(f'param posteriors does not seem to be a ndarray of posteior probabilities')
if binarize:
predictions = np.argmax(posteriors, axis=-1)
return prevalence_from_labels(predictions, np.arange(posteriors.shape[1]))
else:
prevalences = posteriors.mean(axis=0)
prevalences /= prevalences.sum()
return prevalences
def as_binary_prevalence(positive_prevalence: Union[float, np.ndarray], clip_if_necessary=False):
"""
Helper that, given a float representing the prevalence for the positive class, returns a np.ndarray of two
values representing a binary distribution.
:param positive_prevalence: prevalence for the positive class
:param clip_if_necessary: if True, clips the value in [0,1] in order to guarantee the resulting distribution
is valid. If False, it then checks that the value is in the valid range, and raises an error if not.
:return: np.ndarray of shape `(2,)`
"""
if clip_if_necessary:
positive_prevalence = np.clip(positive_prevalence, 0, 1)
else:
assert 0 <= positive_prevalence <= 1, 'the value provided is not a valid prevalence for the positive class'
return np.asarray([1-positive_prevalence, positive_prevalence]).T
def HellingerDistance(P, Q) -> float:
"""
Computes the Hellingher Distance (HD) between (discretized) distributions `P` and `Q`.
The HD for two discrete distributions of `k` bins is defined as:
.. math::
HD(P,Q) = \\frac{ 1 }{ \\sqrt{ 2 } } \\sqrt{ \\sum_{i=1}^k ( \\sqrt{p_i} - \\sqrt{q_i} )^2 }
:param P: real-valued array-like of shape `(k,)` representing a discrete distribution
:param Q: real-valued array-like of shape `(k,)` representing a discrete distribution
:return: float
"""
return np.sqrt(np.sum((np.sqrt(P) - np.sqrt(Q))**2))
def TopsoeDistance(P, Q, epsilon=1e-20):
"""
Topsoe distance between two (discretized) distributions `P` and `Q`.
The Topsoe distance for two discrete distributions of `k` bins is defined as:
.. math::
Topsoe(P,Q) = \\sum_{i=1}^k \\left( p_i \\log\\left(\\frac{ 2 p_i + \\epsilon }{ p_i+q_i+\\epsilon }\\right) +
q_i \\log\\left(\\frac{ 2 q_i + \\epsilon }{ p_i+q_i+\\epsilon }\\right) \\right)
:param P: real-valued array-like of shape `(k,)` representing a discrete distribution
:param Q: real-valued array-like of shape `(k,)` representing a discrete distribution
:return: float
"""
return np.sum(P*np.log((2*P+epsilon)/(P+Q+epsilon)) + Q*np.log((2*Q+epsilon)/(P+Q+epsilon)))
def uniform_prevalence_sampling(n_classes, size=1):
"""
Implements the `Kraemer algorithm <http://www.cs.cmu.edu/~nasmith/papers/smith+tromble.tr04.pdf>`_
for sampling uniformly at random from the unit simplex. This implementation is adapted from this
`post <https://cs.stackexchange.com/questions/3227/uniform-sampling-from-a-simplex>_`.
:param n_classes: integer, number of classes (dimensionality of the simplex)
:param size: number of samples to return
:return: `np.ndarray` of shape `(size, n_classes,)` if `size>1`, or of shape `(n_classes,)` otherwise
"""
if n_classes == 2:
u = np.random.rand(size)
u = np.vstack([1-u, u]).T
else:
u = np.random.rand(size, n_classes-1)
u.sort(axis=-1)
_0s = np.zeros(shape=(size, 1))
_1s = np.ones(shape=(size, 1))
a = np.hstack([_0s, u])
b = np.hstack([u, _1s])
u = b-a
if size == 1:
u = u.flatten()
return u
uniform_simplex_sampling = uniform_prevalence_sampling
def strprev(prevalences, prec=3):
"""
Returns a string representation for a prevalence vector. E.g.,
>>> strprev([1/3, 2/3], prec=2)
>>> '[0.33, 0.67]'
:param prevalences: a vector of prevalence values
:param prec: float precision
:return: string
"""
return '['+ ', '.join([f'{p:.{prec}f}' for p in prevalences]) + ']'
def adjusted_quantification(prevalence_estim, tpr, fpr, clip=True):
"""
Implements the adjustment of ACC and PACC for the binary case. The adjustment for a prevalence estimate of the
positive class `p` comes down to computing:
.. math::
ACC(p) = \\frac{ p - fpr }{ tpr - fpr }
:param prevalence_estim: float, the estimated value for the positive class
:param tpr: float, the true positive rate of the classifier
:param fpr: float, the false positive rate of the classifier
:param clip: set to True (default) to clip values that might exceed the range [0,1]
:return: float, the adjusted count
"""
den = tpr - fpr
if den == 0:
den += 1e-8
adjusted = (prevalence_estim - fpr) / den
if clip:
adjusted = np.clip(adjusted, 0., 1.)
return adjusted
def normalize_prevalence(prevalences):
"""
Normalize a vector or matrix of prevalence values. The normalization consists of applying a L1 normalization in
cases in which the prevalence values are not all-zeros, and to convert the prevalence values into `1/n_classes` in
cases in which all values are zero.
:param prevalences: array-like of shape `(n_classes,)` or of shape `(n_samples, n_classes,)` with prevalence values
:return: a normalized vector or matrix of prevalence values
"""
prevalences = np.asarray(prevalences)
n_classes = prevalences.shape[-1]
accum = prevalences.sum(axis=-1, keepdims=True)
prevalences = np.true_divide(prevalences, accum, where=accum>0)
allzeros = accum.flatten()==0
if any(allzeros):
if prevalences.ndim == 1:
prevalences = np.full(shape=n_classes, fill_value=1./n_classes)
else:
prevalences[accum.flatten()==0] = np.full(shape=n_classes, fill_value=1./n_classes)
return prevalences
def __num_prevalence_combinations_depr(n_prevpoints:int, n_classes:int, n_repeats:int=1):
"""
Computes the number of prevalence combinations in the n_classes-dimensional simplex if `nprevpoints` equally distant
prevalence values are generated and `n_repeats` repetitions are requested.
:param n_classes: integer, number of classes
:param n_prevpoints: integer, number of prevalence points.
:param n_repeats: integer, number of repetitions for each prevalence combination
:return: The number of possible combinations. For example, if n_classes=2, n_prevpoints=5, n_repeats=1, then the
number of possible combinations are 5, i.e.: [0,1], [0.25,0.75], [0.50,0.50], [0.75,0.25], and [1.0,0.0]
"""
__cache={}
def __f(nc,np):
if (nc,np) in __cache: # cached result
return __cache[(nc,np)]
if nc==1: # stop condition
return 1
else: # recursive call
x = sum([__f(nc-1, np-i) for i in range(np)])
__cache[(nc,np)] = x
return x
return __f(n_classes, n_prevpoints) * n_repeats
def num_prevalence_combinations(n_prevpoints:int, n_classes:int, n_repeats:int=1):
"""
Computes the number of valid prevalence combinations in the n_classes-dimensional simplex if `n_prevpoints` equally
distant prevalence values are generated and `n_repeats` repetitions are requested.
The computation comes down to calculating:
.. math::
\\binom{N+C-1}{C-1} \\times r
where `N` is `n_prevpoints-1`, i.e., the number of probability mass blocks to allocate, `C` is the number of
classes, and `r` is `n_repeats`. This solution comes from the
`Stars and Bars <https://brilliant.org/wiki/integer-equations-star-and-bars/>`_ problem.
:param n_classes: integer, number of classes
:param n_prevpoints: integer, number of prevalence points.
:param n_repeats: integer, number of repetitions for each prevalence combination
:return: The number of possible combinations. For example, if n_classes=2, n_prevpoints=5, n_repeats=1, then the
number of possible combinations are 5, i.e.: [0,1], [0.25,0.75], [0.50,0.50], [0.75,0.25], and [1.0,0.0]
"""
N = n_prevpoints-1
C = n_classes
r = n_repeats
return int(scipy.special.binom(N + C - 1, C - 1) * r)
def get_nprevpoints_approximation(combinations_budget:int, n_classes:int, n_repeats:int=1):
"""
Searches for the largest number of (equidistant) prevalence points to define for each of the `n_classes` classes so
that the number of valid prevalence values generated as combinations of prevalence points (points in a
`n_classes`-dimensional simplex) do not exceed combinations_budget.
:param combinations_budget: integer, maximum number of combinations allowed
:param n_classes: integer, number of classes
:param n_repeats: integer, number of repetitions for each prevalence combination
:return: the largest number of prevalence points that generate less than combinations_budget valid prevalences
"""
assert n_classes > 0 and n_repeats > 0 and combinations_budget > 0, 'parameters must be positive integers'
n_prevpoints = 1
while True:
combinations = num_prevalence_combinations(n_prevpoints, n_classes, n_repeats)
if combinations > combinations_budget:
return n_prevpoints-1
else:
n_prevpoints += 1
def check_prevalence_vector(p, raise_exception=False, toleranze=1e-08):
"""
Checks that p is a valid prevalence vector, i.e., that it contains values in [0,1] and that the values sum up to 1.
:param p: the prevalence vector to check
:return: True if `p` is valid, False otherwise
"""
p = np.asarray(p)
if not all(p>=0):
if raise_exception:
raise ValueError('the prevalence vector contains negative numbers')
return False
if not all(p<=1):
if raise_exception:
raise ValueError('the prevalence vector contains values >1')
return False
if not np.isclose(p.sum(), 1, atol=toleranze):
if raise_exception:
raise ValueError('the prevalence vector does not sum up to 1')
return False
return True
def get_divergence(divergence: Union[str, Callable]):
if isinstance(divergence, str):
if divergence=='HD':
return HellingerDistance
elif divergence=='topsoe':
return TopsoeDistance
else:
raise ValueError(f'unknown divergence {divergence}')
elif callable(divergence):
return divergence
else:
raise ValueError(f'argument "divergence" not understood; use a str or a callable function')
def argmin_prevalence(loss, n_classes, method='optim_minimize'):
if method == 'optim_minimize':
return optim_minimize(loss, n_classes)
elif method == 'linear_search':
return linear_search(loss, n_classes)
elif method == 'ternary_search':
raise NotImplementedError()
else:
raise NotImplementedError()
def optim_minimize(loss, n_classes):
"""
Searches for the optimal prevalence values, i.e., an `n_classes`-dimensional vector of the (`n_classes`-1)-simplex
that yields the smallest lost. This optimization is carried out by means of a constrained search using scipy's
SLSQP routine.
:param loss: (callable) the function to minimize
:param n_classes: (int) the number of classes, i.e., the dimensionality of the prevalence vector
:return: (ndarray) the best prevalence vector found
"""
from scipy import optimize
# the initial point is set as the uniform distribution
uniform_distribution = np.full(fill_value=1 / n_classes, shape=(n_classes,))
# solutions are bounded to those contained in the unit-simplex
bounds = tuple((0, 1) for _ in range(n_classes)) # values in [0,1]
constraints = ({'type': 'eq', 'fun': lambda x: 1 - sum(x)}) # values summing up to 1
r = optimize.minimize(loss, x0=uniform_distribution, method='SLSQP', bounds=bounds, constraints=constraints)
return r.x
def linear_search(loss, n_classes):
"""
Performs a linear search for the best prevalence value in binary problems. The search is carried out by exploring
the range [0,1] stepping by 0.01. This search is inefficient, and is added only for completeness (some of the
early methods in quantification literature used it, e.g., HDy). A most powerful alternative is `optim_minimize`.
:param loss: (callable) the function to minimize
:param n_classes: (int) the number of classes, i.e., the dimensionality of the prevalence vector
:return: (ndarray) the best prevalence vector found
"""
assert n_classes==2, 'linear search is only available for binary problems'
prev_selected, min_score = None, None
for prev in prevalence_linspace(n_prevalences=100, repeats=1, smooth_limits_epsilon=0.0):
score = loss(np.asarray([1 - prev, prev]))
if min_score is None or score < min_score:
prev_selected, min_score = prev, score
return np.asarray([1 - prev_selected, prev_selected])