A ComparatorMechanism is a subclass of ObjectiveMechanism that receives two inputs (a sample and a target), compares
them using its
function, and places the calculated discrepancy between the two in its
Creating a ComparatorMechanism¶
ComparatorMechanisms are generally created automatically when other PsyNeuLink components are created (such as
LearningMechanisms). A ComparatorMechanism can also be created directly by calling
its constructor. Its sample and target arguments are used to specify the OutputPorts that provide the
sample and target inputs, respectively (see
ObjectiveMechanism_Monitored_ports for details concerning their
specification, which are special versions of an ObjectiveMechanism’s monitor argument). When the
ComparatorMechanism is created, two InputPorts are created, one each for its sample and target inputs (and named,
by default, SAMPLE and TARGET). Each is assigned a MappingProjection from the corresponding OutputPort specified
in the sample and target arguments.
It is important to recognize that the value of the SAMPLE and TARGET InputPorts must have the same length and type,
so that they can be compared using the ComparatorMechanism’s
function. By default,
they use the format of the OutputPorts specified in the sample and target arguments, respectively,
and the MappingProjection to each uses an
IDENTITY_MATRIX. Therefore, for the default configuration, the
OutputPorts specified in the sample and target arguments must have values of the same length and type.
If these differ, the input_ports argument can be used to explicitly specify the format of the ComparatorMechanism’s
SAMPLE and TARGET InputPorts, to insure they are compatible with one another (as well as to customize their
names, if desired). If the input_ports argument is used, both the sample and target InputPorts must be
specified. Any of the formats for specifying InputPorts can be used in the argument.
If values are assigned for the InputPorts, they must be of equal length and type. Their types must
also be compatible with the value of the OutputPorts specified in the sample and target arguments. However,
the length specified for an InputPort can differ from its corresponding OutputPort; in that case, by default, the
MappingProjection created uses a
FULL_CONNECTIVITY matrix. Thus, OutputPorts of differing lengths can be mapped
to the sample and target InputPorts of a ComparatorMechanism (see the example below),
so long as the latter are of the same length. If a projection other than a
FULL_CONNECTIVITY matrix is needed, this
can be specified using the PROJECTION entry of a Port specification dictionary for the
InputPort in the input_ports argument.
A ComparatorMechanism has two
input_ports, each of which receives a
MappingProjection from a corresponding OutputPort specified in the sample and target arguments of its
constructor. The InputPorts are listed in the Mechanism’s
and named, respectively, SAMPLE and TARGET. The OutputPorts from which they receive their projections (specified
in the sample and target arguments) are listed in the Mechanism’s
target attributes as well as in its
function compares the value of the sample and target
InputPorts. By default, it uses a
LinearCombination function, assigning the sample InputPort a
weight of -1 and the target a
weight of 1, so that the sample
is subtracted from the target. However, the
function can be customized, so long as
it is replaced with one that takes two arrays with the same format as its inputs and generates a similar array as its
result. The result is assigned as the value of the Comparator Mechanism’s OUTCOME (primary)
When a ComparatorMechanism is executed, it updates its input_ports with the values of the OutputPorts specified
in its sample and target arguments, and then uses its
compare these. By default, the result is assigned to the
value of its OUTCOME
output_port, and as the first item of the Mechanism’s
Formatting InputPort values
The default_variable argument can be used to specify a particular format for the SAMPLE and/or TARGET InputPorts
of a ComparatorMechanism. This can be useful when one or both of these differ from the format of the
OutputPort(s) specified in the sample and target arguments. For example, for
Reinforcement Learning, a ComparatorMechanism is used to monitor an action selection Mechanism (the sample), and compare
this with a reinforcement signal (the target). In the example below, the action selection Mechanism is a
TransferMechanism that uses the
SoftMax function (and the
PROB as its output format) to select
an action. This generates a vector with a single non-zero value (the selected action). Because the output is a vector,
specifying it as the ComparatorMechanism’s sample argument will generate a corresponding InputPort with a vector
as its value. This will not match the reward signal specified in the ComparatorMechanism’s target argument, the
value of which is a single scalar. This can be dealt with by explicitly specifying the format for the SAMPLE and
TARGET InputPorts in the default_variable argument of the ComparatorMechanism’s constructor, as follows:
>>> import psyneulink as pnl >>> my_action_selection_mech = pnl.TransferMechanism(size=5, ... function=pnl.SoftMax(output=pnl.PROB)) >>> my_reward_mech = pnl.TransferMechanism() >>> my_comparator_mech = pnl.ComparatorMechanism(default_variable = [,], ... sample=my_action_selection_mech, ... target=my_reward_mech)
my_action_selection_mechanism is specified to take an array of length 5 as its input, and therefore
generate one of the same length as its primary output. Since it is assigned as the sample
of the ComparatorMechanism, by default this will create a SAMPLE InputPort of length 5, that will not match the
length of the TARGET InputPort (the default for which is length 1). This is taken care of, by specifying the
default_variable argument as an array with two single-value arrays (corresponding to the SAMPLE and TARGET
InputPorts). (In this example, the sample and target arguments are specified as Mechanisms since,
by default, each has only a single (primary) OutputPort, that will be used; if either had
more than one OutputPort, and one of those was desired, it would have had to be specified explicitly in the
sample or target argument).
- class psyneulink.library.components.mechanisms.processing.objective.comparatormechanism.ComparatorMechanism(sample, target, input_ports=[SAMPLE,TARGET] function=LinearCombination(weights=[[-1],], output_ports=OUTCOME)¶
Subclass of ObjectiveMechanism that compares the values of two OutputPorts. See ObjectiveMechanism for additional arguments and attributes.
sample (OutputPort, Mechanism, value, or string) – specifies the value to compare with the
target (OutputPort, Mechanism, value, or string) – specifies the value with which the
sampleis compared by the
input_ports (List[InputPort, value, str or dict] or Dict : default [SAMPLE, TARGET]) – specifies the names and/or formats to use for the values of the sample and target InputPorts; by default they are named SAMPLE and TARGET, and their formats are match the value of the OutputPorts specified in the sample and target arguments, respectively (see Structure for additional details).
function (Function, function or method : default Distance(metric=DIFFERENCE)) – specifies the
functionused to compare the
determines the value to compare with the
determines the value with which
sampleis compared by the
contains the two InputPorts named, by default, SAMPLE and TARGET, each of which receives a MappingProjection from the OutputPorts referenced by the
targetattributes (see Structure for additional details).
used to compare the
target. It can be any CombinationFunction, or a python function that takes a 2d array with two items and returns a 1d array of the same length as the two input items.
CombinationFunction, function or method
contains the primary OutputPort of the ComparatorMechanism; the default is its OUTCOME OutputPort, the value of which is equal to the
valueattribute of the ComparatorMechanism.
contains, by default, only the OUTCOME (primary) OutputPort of the ComparatorMechanism.
contains one item that is the value of the OUTCOME OutputPort.
list of Standard OutputPorts that includes the following in addition to the
standard_output_portsof an ObjectiveMechanism:
the sum of the terms in in the array returned by the Mechanism’s function.
the sum of squares of the terms in the array returned by the Mechanism’s function.
the mean of the squares of the terms returned by the Mechanism’s function.
- _validate_params(request_set, target_set=None, context=None)¶
If sample and target values are specified, validate that they are compatible
- exception psyneulink.library.components.mechanisms.processing.objective.comparatormechanism.ComparatorMechanismError(error_value)¶