class BackpropagationAlgorithm(IterativeAlgorithm):
r"""Backpropagation algorithm for multilayer perceptrons.
INPUT:
- ``self`` -- object on which the function is invoked.
- ``sample`` -- list or tuple of ``LabeledExample`` containing the sample
to be learnt.
- ``dimensions`` -- list or tuple of numerical values describing the number
of layers and the number of units therein, including the input layer.
- ``threshold`` -- boolean (default: ``True``) flag setting the use of
thresholded perceptrons.
- ``activations`` -- ActivationFunction or list/tuple of ActivationFunction
(default: SigmoidActivationFunction()) activation functions to be used
for the perceptron units. When a single value is specified, it applies to
all units. When a list/tuple is specified, each element corresponds to
all units in a perceptron layer.
- ``weight_bound`` -- float (default: 0.1) upper bound of the interval in
which the initial weights and thresholds are chosen uniformly at random
(the lower bound of this interval is ``-1 * weight_bound``). A
``ValueError`` is thrown if this parameter is not positive.
OUTPUT:
``BackpropagationAlgorithm`` object.
EXAMPLES:
Consider the following data set summarizing the binary XOR function:
::
>>> from yaplf.data import LabeledExample
>>> xor_sample = [LabeledExample((0, 0), (0,)),
... LabeledExample((0, 1), (1,)), LabeledExample((1, 0), (1,)),
... LabeledExample((1, 1), (0,))]
This is a paradigmatical example of non-linearly separable data set which
needs a richer model than a perceptron in order to be learnt:
::
>>> from yaplf.utility.activation import SigmoidActivationFunction
>>> from yaplf.algorithms.neural.multilayer import BackpropagationAlgorithm
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... threshold = True, activations = SigmoidActivationFunction(10))
In order to actually run the algorithm it is necessary to specify a
stopping criterion (the default behaviour would only execute a learning
iteration, probably not going so far). In order to keep it simple, one can
chose ``FixedIterationsStoppingCriterion`` so as to run a fixed number of
iterations, say `5000`:
::
>>> from yaplf.utility.stopping import FixedIterationsStoppingCriterion
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(5000), learning_rate = .1)
The inferred model can be inspected through the ``model`` field in the
``LearningAlgorithm`` object:
::
>>> alg.model # random
MultilayerPerceptron((2, 2, 1), [array([[-0.49748418, -0.48592928],
[-0.72052151, -0.69609958]]), array([[ 0.78258238, -0.84286672]])],
thresholds = [array([ 0.71869556, 0.24337534]), array([-0.36203957])],
activations = SigmoidActivationFunction(10))
Note that the algorithm can be run in different flavours, described in the
documentation of ``run``.
One of the ways of assessing the performance of the algorithm is that of
invoking the ``test`` function inherited from the ``Model`` class in order
to see how each example has been classified:
::
>>> alg.model.test(xor_sample, verbose = True) # random
(0, 0) mapped to 0.0279358676426, label is (0,), error [ 0.00078041]
(0, 1) mapped to 0.968320708961, label is (1,), error [ 0.00100358]
(1, 0) mapped to 0.966511649371, label is (1,), error [ 0.00112147]
(1, 1) mapped to 0.0429967333857, label is (0,), error [ 0.00184872]
MSE 0.00118854472284
0.0011885447228408164
The named argument ``verbose`` activates the verbose output detailing how
error spreads on each example. Note that the output of ``test`` is likely
to be different on each run, as when the class constructor is called the
initial weights are chosen at random. It is also possible that the
performance is not satisfactory for some examples are not learnt at all.
This can be an effect of many factors. For instance:
- learning has not converged, and more iterations are needed; thus other
``run`` invocations should be performed, suitably chosing the function
parameters;
- despite the chosen multilayer perceptron architecture can learn the data
set, learning converged to an unsatisfactory model because of the initial
values randomly picked; thus learning should be restarted, either
invoking ``reset`` on the ``BackpropagationAlgorithm`` object or creating
a new instance of this class;
- the chosen architecture cannot learn the data set, thus the whole process
should be repeated modifying the initially chosen multilayer perceptron
architecture, for instance chosing a different number of layers or a
different number of units in layers.
Another way of evaluating the inferred model is in this case that of
graphically visualizing it. As the perceptron has two inputs it is indeed
possible to call its ``plot`` method specifying a region containing all
the pattern supplied to the learning algorithm:
::
>>> alg.model.plot((0, 1), (0, 1), shading = True)
Learning can be monitored using the class ``ErrorTrajectory`` in package
``yaplf.graph.trajectory`` as follows:
::
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... activations = SigmoidActivationFunction(10))
>>> errObs = ErrorTrajectory(alg)
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(1500))
>>> errObs.get_trajectory(color='red', joined = True)
In this way, at each learning iteration the inferred model is tested
against the training set, so that the ``get_trajectory`` function returns
a graph of the related error versus the iteration number.
As a final remark, it should be highlighted that these examples are shown
for illustrative purpose. The suitable way of assessing a learnt model
performance involves more complex techniques involving for instance the use
of:
- a test set in order to assess when learning should stop, using a
different stopping criterion such as ``TestSetStoppingCriterion``;
- a test set in order to graphically inspect how error changes as learning
proceeds, specifying different arguments to the constructor of
``ErrorTrajectory``;
- a cross validation procedure (see function ``cross_validation`` in
package ``yaplf.utility.validation``) in order to choose the best
perceptron architecture.
Concerning last point, the cross validation procedure can select among a
given set of choices the one minimizing the inferred error. More precisely,
consider the next code snippet:
::
>>> tc_sample = (LabeledExample((0.9, 0.9, 0.9, 0.9, 0.1, 0.1, 0.9,
... 0.9, 0.9), (0.1,)), LabeledExample((0.9, 0.9, 0.9, 0.1, 0.9, 0.1,
... 0.1, 0.9, 0.1), (0.9,)), LabeledExample((0.9, 0.9, 0.9, 0.9, 0.1,
... 0.9, 0.9, 0.1, 0.9), (0.1,)), LabeledExample((0.1, 0.1, 0.9, 0.9,
... 0.9, 0.9, 0.1, 0.1, 0.9), (0.9,)), LabeledExample((0.9, 0.9, 0.9,
... 0.1, 0.1, 0.9, 0.9, 0.9, 0.9), (0.1,)), LabeledExample((0.1, 0.9,
... 0.1, 0.1, 0.9, 0.1, 0.9, 0.9, 0.9), (0.9,)), LabeledExample((0.9,
... 0.1, 0.9, 0.9, 0.1, 0.9, 0.9, 0.9, 0.9), (0.1,)),
... LabeledExample((0.9, 0.1, 0.1, 0.9, 0.9, 0.9, 0.9, 0.1, 0.1),
... (0.9,)))
>>> from yaplf.utility.validation import cross_validation
>>> p = cross_validation(BackpropagationAlgorithm, tc_sample,
... {'activations': (SigmoidActivationFunction(2),
... SigmoidActivationFunction(3), SigmoidActivationFunction(5),
... SigmoidActivationFunction(10))},
... fixed_parameters = {'dimensions': (9, 2, 1)},
... run_parameters = {'stopping_criterion': \
... FixedIterationsStoppingCriterion(1000), 'learning_rate': 1},
... num_folds = 4, verbose = True)
Errors: [0.030319163781913711, 0.060571552036313515,
0.029535618302241346, 0.4099711142423762]
Minimum error in position 2
Selected parameters (SigmoidActivationFunction(5),)
Its effect is that of training a multilayer perceptron through the
backpropagation algorithm starting by the sample in ``tc_sample``, as
specified by the first two arguments and analyzing four different
activation functions (third argument, note that in this case it consists
of a singleton tuple and more generally can be a tuple involving different
named argument of ``BackpropagationAlgorithm`` or other learning
algorithms.) For each choice of the activation function, learning evolves
as follows:
- the sample is partitioned in 4 subsets, as specified by the ``num_folds``
named argument;
- the learning algorithm is instantiated excluding from the sample the
first subset in previous point, and using as named argument those
specified in ``fixed_parameters``, joined with the one specifying the
selected activation function; subsequently, the algorithm is run using
the named arguments in ``run_parameters``;
- the inferred multilayer perceptron is tested on the originally excluded
subset of examples;
- the whole process is repeated starting from the second subset, and so on
till the fourth; the test errors are then averaged and associated to the
initially chosen activation function.
Finally, the activation function yielding the minimum averaged test error
is selected and used in order to infer a new multilayer perceptron starting
from all examples.
AUTHORS:
- Dario Malchiodi (2010-03-31)
"""
def __init__(self, sample, dimensions=None, **kwargs):
r"""
See ``BackpropagationAlgorithm`` for full documentation.
"""
IterativeAlgorithm.__init__(self, sample)
# dimensions needs to be a named argument in order to be able to
# cross-validate on it. The default value will correspond to three
# layers, with the input and output one automatically sized in order
# to fit the provided data set, and the hidden one containing half of
# the biggest value between number of input and output units.
if dimensions is not None:
self.dimensions = dimensions
else:
n_in = len(sample[0].pattern)
n_out = len(sample[0].label)
self.dimensions = (n_in, int(max(n_in, n_out) / 2), n_out)
num_input = self.dimensions[0]
for example in sample:
if len(example.pattern) != num_input:
raise ValueError('Sample incompatible with number of units')
try:
self.activations = kwargs['activations']
if len(self.activations) != len(dimensions):
raise ValueError(\
'Activations incompatible with number of layers')
except KeyError:
self.activations = SigmoidActivationFunction()
except TypeError:
pass
# Raised by len if the argument is assigned a single activation
# this calms down pylint
self.threshold = None
self.reset(**kwargs)
def reset(self, **kwargs):
r"""
Reset weights and thresholds of the inferred MultilayerPerceptron
picking values at random.
INPUT:
- ``self`` -- object on which the function is invoked.
- ``threshold`` -- boolean (default: ``True``) flag setting the use of
thresholded perceptrons.
- ``weight_bound`` -- float (default: 0.1) upper bound of the interval
in which the initial weights and thresholds are chosen uniformly at
random (the lower bound of this interval is ``-1 * weight_bound``). A
``ValueError`` is thrown if this parameter is not positive.
OUTPUT:
No output. After the invocation the initialized model is available
through the ``model`` field, in form of a ``MultilayerPerceptron``
instance.
EXAMPLES:
Consider the following data set summarizing the binary XOR function:
::
>>> from yaplf.data import LabeledExample
>>> xor_sample = [LabeledExample((0, 0), (0,)),
... LabeledExample((0, 1), (1,)), LabeledExample((1, 0), (1,)),
... LabeledExample((1, 1), (0,))]
This is a paradigmatical example of non-linearly separable data set
which needs a richer model than a perceptron in order to be learnt:
::
>>> from yaplf.utility.validation import SigmoidActivationFunction
>>> from yaplf.algorithms.neural import BackpropagationAlgorithm
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... threshold = True, activations = SigmoidActivationFunction(10))
Suppose the algorithm is run for, say, 1000 iterations with learning
rate set to `0.1` with the following results:
::
>>> from yaplf.utility.stopping import \
... FixedIterationsStoppingCriterion
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(1000), learning_rate = .1)
>>> alg.model.test(xor_sample, verbose = True) # random
(0.100000000000000, 0.100000000000000) mapped to 0.107281883481,
label is (0.100000000000000,), error [ 5.30258270e-05]
(0.100000000000000, 0.900000000000000) mapped to 0.889216991161,
label is (0.900000000000000,), error [ 0.00011627]
(0.900000000000000, 0.100000000000000) mapped to 0.350484814876,
label is (0.900000000000000,), error [ 0.30196694]
(0.900000000000000, 0.900000000000000) mapped to 0.353978869018,
label is (0.100000000000000,), error [ 0.06450527]
MSE 0.0916603759241
0.0916603759241
It is clear that the algorithm has learnt only three examples out of four.
In order to check whether or not the algorithm converged, one can continue
its execution for, say, another thousand iterations:
::
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(1000))
>>> alg.model.test(xor_sample) # random
0.0916130351364
As the test error is essentially unchanged we can conclude that learning
converged to a local minima of the error function. In order to fresh start
another session, hoping that the random initialization can overcome this
problem, one can get a new instance of ``BackpropagationAlgorithm``, or
call ``reset`` on the alrerady available instance:
::
>>> alg.reset()
>>> alg.run(stopping_criterion =\
... FixedIterationsStoppingCriterion(1000))
>>> alg.model.test(xor_sample) # random
1.01130579975e-05
AUTHORS:
- Dario Malchiodi (2010-03-31)
"""
try:
self.threshold = kwargs['threshold']
except KeyError:
self.threshold = True
try:
# picks initial weights and threshold uniformly
# between -weight_bound and weight_bound
init_bound = kwargs['weight_bound']
if init_bound <= 0:
raise ValueError(
'The weight_bound parameter should be positive')
except KeyError:
init_bound = 0.1
dims = transpose((self.dimensions[1:], self.dimensions[:-1]))
connections = [random.uniform(-1 * init_bound, init_bound, shape)
for shape in dims]
perc_kwargs = {'activations': self.activations}
if self.threshold:
thr = [random.uniform(-1 * init_bound, init_bound, shape) \
for shape in self.dimensions[1:]]
perc_kwargs['thresholds'] = thr
self.model = MultilayerPerceptron(self.dimensions, connections,
**perc_kwargs)
def run(self, **kwargs):
r"""
Run the learning algorithm.
INPUT:
- ``self`` -- object on which the function is invoked.
- ``stopping_criterion`` -- ``StoppingCriterion`` instance (default:
``FixedIterationsStoppingCriterion()``, amounting to the execution of
one learning step) describing the criterion to be fulfilled in order
to stop the training phase.
- ``batch`` -- boolean (default: ``False``, amounting to online
learning mode) flag setting batch learning, i.e. model update after
the presentation of all examples, instead of online learning, i.e.
model update at each example presentation.
- ``selector`` -- iterator (default: ``sequential_selector``, amounting
to cycling through the available examples) selecting the next sample
to be fed to the learnin algorithm.
- ``learning_rate`` -- float (default: 0.1) value to be used as
learning rate.
- ``momentum_term`` -- float (default: 0) value tu be used as momentum
term.
- ``min_error`` -- float (default: 0, which means connections
and thresholds will always be updated) error value under which no
update will occur on connections and thresholds.
OUTPUT:
No output. After the invocation the inferred model is available through
the ``model`` field, in form of a ``Perceptron`` instance.
EXAMPLES:
Consider the following data set summarizing the binary XOR function,
and a ``BackpropagationAlgorithm`` instance for it:
::
>>> from yaplf.data import LabeledExample
>>> xor_sample = [LabeledExample((0, 0), (0,)),
... LabeledExample((0, 1), (1,)), LabeledExample((1, 0), (1,)),
... LabeledExample((1, 1), (0,))]
>>> from yaplf.utility import SigmoidActivationFunction
>>> from yaplf.algorithms.neural import BackpropagationAlgorithm
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... threshold = True, activations = SigmoidActivationFunction(10))
In order to actually run the algorithm it is necessary to specify a
stopping criterion (the default behaviour would only execute a learning
iteration, probably not going so far). In order to keep it simple, one
can chose ``FixedIterationsStoppingCriterion`` so as to run a fixed
number of iterations, say `5000`:
::
>>> from yaplf.utility.stopping import \
... FixedIterationsStoppingCriterion
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(5000), learning_rate = .1)
The inferred model can be inspected through the ``model`` field in the
``LearningAlgorithm`` object:
::
>>> alg.model # random
MultilayerPerceptron((2, 2, 1), [array([[-0.49748418, -0.48592928],
[-0.72052151, -0.69609958]]), array([[ 0.78258238, -0.84286672]])],
thresholds = [array([ 0.71869556, 0.24337534]),
array([-0.36203957])], activations = SigmoidActivationFunction(10))
One of the ways of assessing the performance of the algorithm is that
of invoking the ``test`` function inherited from the ``Model`` class in
order to see how each example has been classified:
::
>>> alg.model.test(xor_sample, verbose = True) # random
(0, 0) mapped to 0.0279358676426, label is (0,), error
[ 0.00078041]
(0, 1) mapped to 0.968320708961, label is (1,), error [ 0.00100358]
(1, 0) mapped to 0.966511649371, label is (1,), error [ 0.00112147]
(1, 1) mapped to 0.0429967333857, label is (0,), error
[ 0.00184872]
MSE 0.00118854472284
0.0011885447228408164
The named argument ``verbose`` activates the verbose output detailing
how error spreads on each example. Note that the output of ``test`` is
likely to be different on each run, as when the class constructor is
called the initial weights are chosen at random.
Another solution is that of running the algorithm until the error on
its training set is below a given threshold. This can be easily
attained using ``TrainErrorStoppingCriterion``:
::
>>> from yaplf.utility.stopping import TrainErrorStoppingCriterion
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... learning_rate = .1,
... activations = SigmoidActivationFunction(10))
>>> alg.run(stopping_criterion = TrainErrorStoppingCriterion(0.01))
The argument in the constructor of ``TrainErrorStoppingCriterion`` sets
the above mentioned threshold. It is also possible (and, besides, more
correct) to run the learning algorithm using a training set and
stopping the process when the test error on another data set goes
below a given threshold. This can be attained using
``TestErrorStoppingCriterion`` in package ``yaplf.utility.stopping``.
Another way of evaluating the inferred model is in this case that of
graphically visualizing it. As the perceptron has two inputs it is
indeed possible to call its ``plot`` method specifying a region
containing all the pattern supplied to the learning algorithm:
::
>>> alg.model.plot((0, 1), (0, 1), shading = True)
Learning can be also monitored using the class ``ErrorTrajectory`` in
package ``yaplf.graph.trajectory`` as follows:
::
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... activations = SigmoidActivationFunction(10))
>>> errObs = ErrorTrajectory(alg)
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(1500))
>>> errObs.get_trajectory(color='red', joined = True)
In this way, at each learning iteration the inferred model is tested
against the training set, so that the ``get_trajectory`` function
returns a graph of the related error versus the iteration number.
The ``run`` function has a number of named argument allowing to
tune how the backpropagation algorithm selects its output, and whose
meaning requires a bit more information about how the algorithm works:
basically, during the initialization of ``BackpropagationAlgorithm``
and at each invokation of ``reset`` a multilayer perceptron is created
picking its connection weights and its threshold values at random;
subsequently at each iteration an example is selected and fed to this
perceptron. The obtained output is compared to the expected one and
an error is computed. This error, together with other information,
the computation of a quantity `\Delta w` which will in turn be used
in order to modify connections and thresholds. More precisely at a
given time `t`, for each connection weight, say `w_{ij}(t)`, a
corresponding `\Delta w_{ij}(t)` is computed and the perceptron is
updated so that `w_{ij}(t+1) = w_{ij}(t) - \eta \Delta w_{ij}(t) +
\alpha \Delta w_{ij}(t-1)`. This rule implements a local descent in
the error space so that eventually the obtained minimizes locally the
training error. The values `\eta` and `\alpha` can be chosen through
the following named arguments:
- ``learning_rate`` corresponds to`\eta`, the so-called *learning
rate*, with a default value of 0.1. The higher this value, the more
extended will be the local steps of the algorithm. This will improve
convergence but will also raise the risk of outrunning an optimum
and starting to oscillate around it.
- ``momentum_term`` corresponds to `\alpha`, the so-called *momentum
term* as it features a momentum which increases the actual step
when the surface is smooth and tends to decrement it otherwise. Its
default value is 0, corresponding to the original version of the
backpropagation algorithm.
The following named argument also affect the learning behaviour:
- ``selector`` sets an iterator selecting the next example to be fed to
the learnin algorithm. Its default value cycles through the provided
sample.
- ``batch`` -- boolean flag setting batch learning mode, in which the
update values `\Delta w_{ij}` are cumulated for all example in the
sample and subsequently used in order to modify the perceptron,
rather than the standard online mode where the perceptron is updated
after each single example presentation. Its default value is
``False``, corresponding to the online mode previously illustrated.
It is worth noting that when the algorithm is run for a fixed number
of iterations (i.e. through ``FixedIterationsStoppingCriterion``),
an iteration corresponds to one example presentation for online mode
and to the presentation of the whole sample for batch mode.
- ``min_error`` -- error value under which no update will occur on
connections and thresholds. This argument can be used in order to
avoid that some examples are overlearnt at the expense of the
remaining ones. The default value is 0, leading to updating the
perceptron regardless of how small is the error on an example.
AUTHORS:
- Dario Malchiodi (2010-03-31)
"""
IterativeAlgorithm.run(self, **kwargs)
try:
# batch or online learning
batch = kwargs['batch']
except KeyError:
batch = False
try:
learning_rate = kwargs['learning_rate']
except KeyError:
learning_rate = 0.1
try:
momentum_term = kwargs['momentum_term']
except KeyError:
momentum_term = 0
try:
min_error = kwargs['min_error']
except KeyError:
min_error = 0
dims = transpose((self.dimensions[1:], self.dimensions[:-1]))
last_delta = [zeros(shape) for shape in dims]
if self.threshold:
last_delta_threshold = [zeros(shape) \
for shape in self.dimensions[1:]]
while self.stop_criterion.stop() == False:
if batch:
cumul_delta = last_delta = [zeros(shape) for shape in dims]
if self.threshold:
cumul_delta_thresholds = [zeros(shape) \
for shape in self.dimensions[1:]]
for elem in self.sample:
answer = self.model.compute(elem.pattern,
full_state=True, show_net=True, no_unbox=True)
delta = [[]] * (len(self.dimensions) - 1)
error = transpose(answer[-1])[0] - elem.label
if abs(error) < min_error:
continue
derivative = [\
self.model.get_activation(-1).compute_derivative(net,
func_value=val)
for (val, net) in answer[-1]]
delta[-1] = error * derivative
for lev in range(1, len(self.dimensions) - 1):
derivatives = [self.model.get_activation(-lev - \
1).compute_derivative(net, func_value=val) \
for (val, net) in answer[-lev - 1]]
prop_delta = \
dot(transpose(self.model.connections[-lev]),
delta[-lev])
delta[-lev - 1] = array(derivatives * prop_delta)
for lev in range(1, len(self.dimensions)):
new_delta = -learning_rate * outer(delta[-lev],
transpose(answer[-lev - 1])[0]) + \
momentum_term * last_delta[-lev]
cumul_delta[-lev] += new_delta
last_delta[-lev] = new_delta
if self.threshold:
new_delta = -learning_rate * delta[-lev] +\
momentum_term * last_delta_threshold[-lev]
cumul_delta_thresholds[-lev] += new_delta
last_delta_threshold[-lev] = new_delta
for lev in range(1, len(self.dimensions)):
self.model.connections[-lev] += cumul_delta[-lev]
if self.threshold:
self.model.thresholds[-lev] += \
cumul_delta_thresholds[lev]
else:
elem = self.sample_selector.next()
answer = self.model.compute(elem.pattern,
full_state=True, show_net=True, no_unbox=True)
delta = [[]] * (len(self.dimensions) - 1)
error = transpose(answer[-1])[0] - elem.label
if abs(error) < min_error:
continue
derivative = [\
self.model.get_activation(-1).compute_derivative(net,
func_value=val)
for (val, net) in answer[-1]]
delta[-1] = error * derivative
for lev in range(1, len(self.dimensions) - 1):
derivatives = [self.model.get_activation(-lev - \
1).compute_derivative(net, func_value=val) \
for (val, net) in answer[-lev - 1]]
prop_delta = dot(transpose(self.model.connections[-lev]),
delta[-lev])
delta[-lev - 1] = array(derivatives * prop_delta)
for lev in range(1, len(self.dimensions)):
new_delta = -learning_rate * outer(delta[-lev],
transpose(answer[-lev - 1])[0]) + \
momentum_term * last_delta[-lev]
self.model.connections[-lev] += new_delta
last_delta[-lev] = new_delta
if self.threshold:
new_delta = -learning_rate * delta[-lev] +\
momentum_term * last_delta_threshold[-lev]
self.model.thresholds[-lev] += new_delta
last_delta_threshold[-lev] = new_delta
self.notify_observers()
def reset(self, **kwargs):
r"""
Reset weights and thresholds of the inferred MultilayerPerceptron
picking values at random.
INPUT:
- ``self`` -- object on which the function is invoked.
- ``threshold`` -- boolean (default: ``True``) flag setting the use of
thresholded perceptrons.
- ``weight_bound`` -- float (default: 0.1) upper bound of the interval
in which the initial weights and thresholds are chosen uniformly at
random (the lower bound of this interval is ``-1 * weight_bound``). A
``ValueError`` is thrown if this parameter is not positive.
OUTPUT:
No output. After the invocation the initialized model is available
through the ``model`` field, in form of a ``MultilayerPerceptron``
instance.
EXAMPLES:
Consider the following data set summarizing the binary XOR function:
::
>>> from yaplf.data import LabeledExample
>>> xor_sample = [LabeledExample((0, 0), (0,)),
... LabeledExample((0, 1), (1,)), LabeledExample((1, 0), (1,)),
... LabeledExample((1, 1), (0,))]
This is a paradigmatical example of non-linearly separable data set
which needs a richer model than a perceptron in order to be learnt:
::
>>> from yaplf.utility.validation import SigmoidActivationFunction
>>> from yaplf.algorithms.neural import BackpropagationAlgorithm
>>> alg = BackpropagationAlgorithm(xor_sample, (2, 2, 1),
... threshold = True, activations = SigmoidActivationFunction(10))
Suppose the algorithm is run for, say, 1000 iterations with learning
rate set to `0.1` with the following results:
::
>>> from yaplf.utility.stopping import \
... FixedIterationsStoppingCriterion
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(1000), learning_rate = .1)
>>> alg.model.test(xor_sample, verbose = True) # random
(0.100000000000000, 0.100000000000000) mapped to 0.107281883481,
label is (0.100000000000000,), error [ 5.30258270e-05]
(0.100000000000000, 0.900000000000000) mapped to 0.889216991161,
label is (0.900000000000000,), error [ 0.00011627]
(0.900000000000000, 0.100000000000000) mapped to 0.350484814876,
label is (0.900000000000000,), error [ 0.30196694]
(0.900000000000000, 0.900000000000000) mapped to 0.353978869018,
label is (0.100000000000000,), error [ 0.06450527]
MSE 0.0916603759241
0.0916603759241
It is clear that the algorithm has learnt only three examples out of four.
In order to check whether or not the algorithm converged, one can continue
its execution for, say, another thousand iterations:
::
>>> alg.run(stopping_criterion = \
... FixedIterationsStoppingCriterion(1000))
>>> alg.model.test(xor_sample) # random
0.0916130351364
As the test error is essentially unchanged we can conclude that learning
converged to a local minima of the error function. In order to fresh start
another session, hoping that the random initialization can overcome this
problem, one can get a new instance of ``BackpropagationAlgorithm``, or
call ``reset`` on the alrerady available instance:
::
>>> alg.reset()
>>> alg.run(stopping_criterion =\
... FixedIterationsStoppingCriterion(1000))
>>> alg.model.test(xor_sample) # random
1.01130579975e-05
AUTHORS:
- Dario Malchiodi (2010-03-31)
"""
try:
self.threshold = kwargs['threshold']
except KeyError:
self.threshold = True
try:
# picks initial weights and threshold uniformly
# between -weight_bound and weight_bound
init_bound = kwargs['weight_bound']
if init_bound <= 0:
raise ValueError(
'The weight_bound parameter should be positive')
except KeyError:
init_bound = 0.1
dims = transpose((self.dimensions[1:], self.dimensions[:-1]))
connections = [random.uniform(-1 * init_bound, init_bound, shape)
for shape in dims]
perc_kwargs = {'activations': self.activations}
if self.threshold:
thr = [random.uniform(-1 * init_bound, init_bound, shape) \
for shape in self.dimensions[1:]]
perc_kwargs['thresholds'] = thr
self.model = MultilayerPerceptron(self.dimensions, connections,
**perc_kwargs)