Units are the basic computational elements of networks. They typically
integrate information from a number of sources (inputs), and perform
some relatively simple type of processing on these inputs, and then
output a single value which somehow summarizes its response to the
inputs.
The basic Unit class in PDP++ contains real valued variables for
representing a unit's activation and its net input from other units as
well as its target pattern and/or external training input. In addition
the unit class contains subgroups of sending and receiving connections
between other units, and a 'bias' connection, which may or may not be
present depending on the algorithm. A unit also has a position which
represents its relative offset from its layer's position in the netview
(see section 10.6 Network Viewer).
As with many objects in PDP++, the Unit relies on a corresponding
UnitSpec to provide most of the functions and parameters that
control the unit's behavior. The unit itself contains the state
variables. Thus, different units can have different parameters and
functions simply by changing which UnitSpec they point to.
The following variables are found on the Unit:
UnitSpec_SPtr spec
-
A pointer to the unit specifications for this unit (see below).
Geometry pos
-
Specifies the unit's 3-D position relative to the layer. Used for
display purposes and optionally for certain projection patterns
ExtType ext_flag
-
This flag indicates which kind of external input unit last
received. This may have one of four values:
NO_EXTERNAL
-
Indicates that the unit received no input.
TARG
-
Indicates that the unit received a target value, which is in the
targ field.
EXT
-
Indicates that the unit received external input, which is in the
ext field.
TARG_EXT
-
Indicates that the unit received both a target and external input.
COMP
-
Indicates that the unit has a comparison value in its
targ field.
This is for computing an error statistic or other comparisons, but not
for training the network.
COMP_TARG
-
Both a comparison and a target (this is redundant, since all target
values are included in comparisons anyway..)
COMP_EXT
-
Both a comparsion and an external input.
COMP_TARG_EXT
-
All three.
float targ
-
The target value that the unit is being taught to achieve (i.e.,
for output units in a backpropagation network).
float ext
-
The external input that the unit received. Depending on the
algorithm, this can be added into the net input for the unit (soft
clamping), or the unit' activation can be set to this value (hard
clamping).
float act
-
The unit's activation value, which is typically a function of its net
input.
float net
-
The unit's net input value, which is typically computed as a function of
the sending unit's activations times their weights.
Con_Group recv
-
This group contains the unit's receiving connections. Each projection
creates its own sub-group within this group (see section 8.2 Groups), so
recv just contains sub-groups which themselves contain the actual
connections.
Con_Group send
-
This group contains sub-groups containing the unit's sending
connections, one sub-group per projection (just like
recv).
Connection* bias
-
A pointer to a Connection object which contains the bias weight for
this Unit. Bias weights are treated as special connections which do not
have a corresponding sending unit. This pointer may be NULL if the unit
does not have a bias weight. The type of connection created here is
specified by the
bias_con_type member of the UnitSpec, and the
ConSpec for this connection is in the bias_spec member of the
UnitSpec.
The basic UnitSpec class defines the set of computational functions
on the unit, and has parameters which control the unit's behavior.
Specific algorithms add more parameters to this object.
MinMaxRange act_range
-
The legal range of activation values for the unit.
TypeDef* bias_con_type
-
The type of bias connection to create in the unit. The default value
for this is set by different algorithms, and it can be NULL if no bias
connections are to be created. The 'Build' operation should be
performed if this connection type is changed manually.
ConSpec_SPtr bias_spec
-
This ConSpec controls the behavior of the bias on the unit in an
algorithm-dependent fashion.
Note: the following information should be useful to those who wish to
program in PDP++, but is not necessary for the average user to
understand.