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Unit 9: Search and Control Strategies
IF there is less than 4l in the 4l container THEN fill the 4l container. Notes
Adding preconditions to operators => generation of production rules.
Forwarded form of rule # 1:
IF (x,y| x?4) THEN (4,y)
The forwarded set of production rules:
R1 IF (x,y| x?4) THEN (4,y)
R2 IF (x,y| y?3) THEN (x,3)
R3 IF (x,y| x>0) THEN (0,y)
R4 IF (x,y| y>0) THEN (x,0)
R5 IF (x,y| x+y>=4 ? y>0 ? x?4) THEN (4,y-(4-x))
R6 IF (x,y| x+y>=3 ? x>0 ? y?3) THEN (x-(3-y),3)
R7 IF (x,y| x+y?=4 ? y>0) THEN (x+y,0)
R8 IF (x,y| x+y?=3 ? x>0) THEN (0,x+y)
In certain states, more than one rule can be applied.
e.g.: (4,0) satisfies the preconditions of R2, R3 ? R6
Control or Search Strategy: Selecting rules; keeping track of those sequences of rules that have
already been tried and the states produced by them. Goal state provides a basis for the termination
of the problem solving task.
(i) Pattern Matching Stage: Execution of a rule requires a match.
preconditions match content of
of a rule <=====> the working memory.
when match is found => rule is applicable
several rules may be applicable
(ii) Conflict Resolution (Selection Strategy) Stage: Selecting one rule to execute;
(iii) Action Stage: Applying the action part of the rule => changing the content of the workspace
=>new patterns, new matches => new set of rules eligible for execution
Recognize-act control cycle
Production System
1. Content of workspace => candidate rules for execution (search strategy)
2. Each rule surveys the entire workspace
3. A rule activates itself when its condition matches the content of the workspace
Adv: Adding, deleting, revising rules without disturbing the rest of the system.
Water Container Problem
Production Rules Facts in Working Memory
IF (x<4) THEN FILL x (4,y)
IF (y<3) THEN FILL y (x,3)
IF (x>0) THEN EMPTY x (0,y)
IF (y>0) THEN EMPTY y (x,0)
IF (x+y>=4 AND y>0) THEN FILL x FROM y (4,y-(4-x))
IF (x+y>=3 AND x>0) THEN FILL y FROM x (x-(3-y),3)
LOVELY PROFESSIONAL UNIVERSITY 169
