Semantic
Nets:
These are an alternative
to predicate logic as a form of knowledge representation.
The idea is that we can
store our knowledge in form of a graph with nodes representing object in the
world and arcs representing relationships between those objects.
For example:
It is intended to represent
the data:
Tom is a cat.
Tom caught a bird.
Tom is owned by John.
Tom is ginger in color.
Cat likes cream.
The cat sat on the mat.
A cat is a mammal.
A bird is an animal.
All mammals are animals.
Mammals have fur.
The is_a link has two
different meanings. It can mean that one object is an individual item from a
class and other is one class is a subset of another.
We can clean up the representation
by distinguishing between nodes representing individual or instances and nodes
and representing classes.
The name instance and
subclass are often used in place of is_a and ako(a kind of).
Prolog representation can
be used, with classes represented by predicates.
Cat(tom)
Cat(cat1)
Mat(mat1)
Sat_on(cat1,mat1).
Bird(bird1).
Caught(tom,bird1).
Like(x,cream):-cat(x).
Mammal(x):-cat(x).
Has(x,fur):-mammal(x).
Animal(x):-mammal(x).
Animal(x):-bird(x).
Owns(John,tom).
Is_colored(tom,ginger).
In general, an is_a link
between a class c and an individual m is represented by fact c(m).
An a_kind_of link between
a subclass c and a superclass s is represented by s(x):-c(x).
INHERITANCE:
The prolog equivalent
captures an important property of semantic nets that they may be used for a
form of inference known as inheritance.
The idea of this is that
if an object belongs to class, it inherits all the properties of that class.
For example: As we have a
“likes” link between cats and cream, meaning in “all the cats like cream”, we
can infer that any object which has an
is_a link to cats will like cream. So both TOM and CAT1 will like cream.
Inheritance also applies
across the a_kind_of links. For example, any property of mammals or animals
will automatic be a property of cats. So, we can infer, for example, that TOM
has fur, since TOM is a cat, a cat is a kind of animal, all animals are mammals
and all mammals have fur.
FRAMES:
Frames are a variant of semantic networks. A
frame is a data structure for representing a stereo typical situation.
All the information
relevant to a concept is stored in a single complex entity called a frame.
Superficially, frames
look like record, data structures or class, frames also support inheritance.
In frame based systems we
refer to:
·
Objects -
mammal
·
Slots -
properties such as color and size
·
Slot values- values stored in slots such as
grey and large.
Slots and the
corresponding slot values are inherited through the class hierarchy.
FEATURES:
These are more natural
support of values then semantic nets.
Frames can be easily implemented
using object oriented programming technique.
In Frames based system,
Inheritance is easily controlled.
BENEFITS:
·
Easily understood by non-developers.
·
Expressive power.
·
Easy to include default information and
detect missing values.
DRAWBACKS:
·
No standards.
·
No inference mechanism.
