Page 62 - Introduction to AI Robotics

This could lead to a data structure called a difference table of how to
DIFFERENCE TABLE 2.2 Attributes of the Hierarchical Paradigm 45
make decisions:
difference operator
d 200 fly
100<d < 200 ride_train
d 100 drive
d< 1 walk

Different modes of transportation are appropriate for different distances.
A mode of transportation, fly, ride_train, drive, walk,inthe table
is really a function in the robot’s program. It is also called an operator,be-
cause it reduces the value stored in difference as to the distance from be-
ing in the initial state of Tampa and wanting to be at the goal state.
A robot following this difference table would begin by flying as close as it
could to SAIL.
But suppose the robot flew into the San Francisco airport. It’d be within
100 miles of SAIL, so the robot appears to have made an intelligent deci-
sion. But now the robot has a new difference to reduce. It examines the
difference table with a new value of difference. The table says the ro-
bot should drive. Drive what? A car? Ooops: if the robot did have a
personal car, it would be back in Tampa. The robot needs to be able to distin-
guish between driving its car and driving a rental car. This is done by listing
PRECONDITIONS the preconditions that have to be true in order to execute that particu-
lar operator. The preconditions are a column in the difference table, where
a single operator can have multiple preconditions. In practice, the list of
preconditions is quite lengthy, but for the purposes of this example, only
drive_rental, drive_personal will be shown with preconditions.

difference operator preconditions
d 200 fly
100<d<200 ride_train
d 100 drive_rental at airport
drive_personal at home
d<1 walk
The difference table is now able to handle the issue of deciding to drive a
rental car. But this introduces a new issue: how does the robot know where
it is at? This is done by monitoring the state of the robot and its world. If
it took an airplane from Tampa to the San Francisco airport, its state has
changed. Its initial state is now at the San Francisco airport, and no