Page 69 - Introduction to AI Robotics

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CONNECTS(D1, R1, R2) 2 The Hierarchical Paradigm
CONNECTS(D1, R2, R1)
STATUS(D1,OPEN)
NEXTTO(IT, D1)

Strips exits and the plan for the robot to physically execute (in reverse
order on the stack) is: GOTODOOR(IT, D1), GOTHRUDOOR(IT, D1).

2.2.3 Strips summary

Strips works recursively; if it can’t reach a goal directly, it identifies the prob-
lem (a failed precondition), then makes the failed precondition a subgoal.
Once the subgoal is reached, Strips puts the operator for reaching the sub-
goal on a list, then backs up (pops the stack) and resumes trying to reach the
previous goal. Strips plans rather than execute: it creates a list of operators
to apply; it does not apply the operator as it goes. Strips implementations
requires the designer to set up a:

world model representation
difference table with operators, preconditions, add, and delete lists
difference evaluator

The steps in executing Strips are:

1. Compute the difference between the goal state and the initial state using
the difference evaluation function. If there is no difference, terminate.
2. If there is a difference, reduce the difference by selecting the first operator
from the difference table whose add-list has a predicate which negates the
difference.

3. Next, examine the preconditions to see if a set of bindings for the variables
can be obtained which are all true. If not, take the first false precondition,
make it the new goal and store the original goal by pushing it on a stack.
Recursively reduce that difference by repeating step 2 and 3.
4. When all preconditions for an operator match, push the operator onto
the plan stack and update a copy of the world model. Then return to
the operator with the failed precondition so it can apply its operator or
recurse on another failed precondition.