5.4. State Machines 161
(b)Use the Invariant Method to prove that the algorithm is partially correct—that
is, ifsD 0 , thenaDxy.
(c)Prove that the algorithm terminates after at most 1 Clog 3 yexecutions of the
body of thedostatement.
Problem 5.33.
A robot named Wall-E wanders around a two-dimensional grid. He starts out at
.0;0/and is allowed to take four different types of steps:
1..C2; 1/2..C1; 2/3..C1;C1/4.. 3;0/Thus, for example, Wall-E might walk as follows. The types of his steps are
listed above the arrows.
.0;0/
1
!.2; 1/
3
!.3;0/
2
!.4; 2/
4
!.1; 2/!:::
Wall-E’s true love, the fashionable and high-powered robot, Eve, awaits at.0;2/.
(a)Describe a state machine model of this problem.(b)Will Wall-E ever find his true love? Either find a path from Wall-E to Eve, or
use the Invariant Principle to prove that no such path exists.
Problem 5.34.
A hungry ant is placed on an unbounded grid. Each square of the grid either con-
tains a crumb or is empty. The squares containing crumbs form a path in which,
except at the ends, every crumb is adjacent to exactly two other crumbs. The ant is
placed at one end of the path and on a square containing a crumb. For example, the
figure below shows a situation in which the ant faces North, and there is a trail of
food leading approximately Southeast. The ant has already eaten the crumb upon
which it was initially placed.
The ant can only smell food directly in front of it. The ant can only remember
a small number of things, and what it remembers after any move only depends on
what it remembered and smelled immediately before the move. Based on smell and