"""
.. module:: CClassifierGradientSGDMixin
:synopsis: Mixin for SGD classifier gradients.
.. moduleauthor:: Ambra Demontis <ambra.demontis@unica.it>
.. moduleauthor:: Marco Melis <marco.melis@unica.it>
"""
from secml.array import CArray
from secml.ml.classifiers.gradients import CClassifierGradientLinearMixin
from secml.ml.classifiers.clf_utils import convert_binary_labels
[docs]class CClassifierGradientSGDMixin(CClassifierGradientLinearMixin):
"""Mixin class for CClassifierSGD gradients."""
# train derivatives:
[docs] def grad_tr_params(self, x, y):
"""
Derivative of the classifier training objective function w.r.t. the
classifier parameters
Parameters
----------
x : CArray
Features of the dataset on which the training objective is computed.
y : CArray
dataset labels
"""
raise NotImplementedError
# test derivatives:
def _grad_f_x(self, x=None, y=1):
"""Computes the gradient of the linear classifier's decision function
wrt decision function input.
For linear classifiers, the gradient wrt input is equal
to the weights vector w. The point x can be in fact ignored.
Parameters
----------
x : CArray or None, optional
The gradient is computed in the neighborhood of x.
y : int, optional
Binary index of the class wrt the gradient must be computed.
Default is 1, corresponding to the positive class.
Returns
-------
gradient : CArray
The gradient of the linear classifier's decision function
wrt decision function input. Vector-like array.
"""
x = x.atleast_2d()
if self.is_kernel_linear(): # Simply return w for a linear Ridge
return CClassifierGradientLinearMixin._grad_f_x(self, y=y)
# Point is required in the case of non-linear
if x is None:
raise ValueError("point 'x' is required to compute the gradient")
gradient = self.kernel.gradient(self._tr, x).atleast_2d()
# Few shape check to ensure broadcasting works correctly
if gradient.shape != (self._tr.shape[0], self.n_features):
raise ValueError("Gradient shape must be ({:}, {:})".format(
x.shape[0], self.n_features))
w_2d = self.w.atleast_2d()
if gradient.issparse is True: # To ensure the sparse dot is used
w_2d = w_2d.tosparse()
if w_2d.shape != (1, self._tr.shape[0]):
raise ValueError(
"Weight vector shape must be "
"({:}, {:}) or ravel equivalent".format(1, self._tr.shape[0]))
gradient = w_2d.dot(gradient)
# Gradient sign depends on input label (0/1)
return convert_binary_labels(y) * gradient.ravel()