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Introduction to Artificial Intelligence & Expert Systems

Notes y are variables matching any character of the input string alphabet. Matching resumes with P1
once the replacement has been made.
The string “ABC”, for instance, undergoes the following sequence of transformations under
these production rules:
$ABC (P6)
B$AC (P5)
BC$A (P5)
$BC$A (P6)
C$B$A (P5)
$C$B$A (P6)
$$C$B$A (P6)
*C$B$A (P1)
C*$B$A (P3)
C*B$A (P2)
CB*$A (P3)
CB*A (P2)
CBA* (P3)
CBA (P4)

In such a simple system, the ordering of the production rules is crucial. Often, the lack of control
structure makes production systems difficult to design. It is, of course, possible to add control
structure to the production systems model, namely in the inference engine, or in the working
memory.

13.3.2 Frame Architecture

In a toy simulation world where a monkey in a room can grab different objects and climb on
others, an example production rule to grab an object suspended from the ceiling would look
like:
(p Holds::Object-Ceiling
{(goal ^status active ^type holds ^objid <O1>) <goal>}
{(physical-object
^id <O1>
^weight light
^at <p>
^on ceiling) <object-1>}
{(physical-object ^id ladder ^at <p> ^on floor) <object-2>}
{(monkey ^on ladder ^holds NIL) <monkey>}
-(physical-object ^on <O1>)
-- >
(write (crlf) Grab <O1> (crlf))
(modify <object1> ^on NIL)
(modify <monkey> ^holds <O1>)
(modify <goal> ^status satisfied)
)
In this example, data in working memory is structured and variables appear between angle
brackets. The name of the data structure, such as “goal” and “physical-object”, is the first literal
in conditions; the fields of a structure are prefixed with “^”. The “-” indicates a negative condition.
Self Assessment

State whether the following statements are true or false:
7. A production system also contains a database called working memory.
8. The “-” indicates a positive condition.
9. The lack of control structure makes production systems difficult to design.

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