Ch' 3 Search

1
Ch. 3 Search

• Supplemental slides for CSE 327
• Prof. Jeff Heflin

2
Problem Solving Agent Algorithm

• function SIMPLE-PROBLEM-SOLVING-AGENT(percept) r
  eturns an action
• static seq, an action sequence, initially
  empty state, some description of the current
  world state goal, a goal, initially
  null problem, a problem formulation
• state ? UPDATE-STATE(state,percept) if seq is
  empty then do goal ? FORMULATE-GOAL(state) pro
  blem ?FORMULATE-PROBLEM(state,goal) seq ?
  SEARCH(problem) action ? FIRST(seq) seq ?
  REST(seq) return action
• From Figure 3.1, p. 61

3
8-puzzle Successor Function
blank-right
blank-down
blank-left
blank-up
4
8-puzzle Search Tree
initial state
5
Tree Search Algorithm
function TREE-SEARCH(problem,fringe) returns a
solution, or failure fringe ? INSERT(MAKE-NODE(IN
ITIAL-STATEproblem,fringe) loop do if
EMPTY?(fringe) then return failure node ?
REMOVE-FIRST(fringe) if GOAL-TESTproblem
applied to STATEnode succeeds then return
SOLUTION(node) fringe ? INSERT-ALL(EXPAND(node,p
roblem),fringe)
Notes 1. fringe argument should be an empty
queue. The type of the queue (e.g., LIFO, FIFO,
etc.) will affect the order of the search 2.
INITIAL-STATEproblem is just syntax for
accessing an objects data (think
problem.initialState in C/Java) From
Figure 3.9, p. 72
6
Depth-first Search
1
A
not generated
on fringe
2
7
C
B
in memory
deleted
3
8
6
9
D
E
F
G
4
5
H
I
7
Breadth-first Search
1
A
not generated
on fringe
2
3
C
B
in memory
deleted
4
7
6
5
D
E
F
G
8
9
H
I
8
Uniform Cost Search
State space
Search tree
1
10
I
G

g(n)0
I
4
B
5
2
C
3
g(n)4
G
B
g(n)10
not generated
4
3
on fringe
G
g(n)9
C
g(n)7
in memory
deleted
9
Uninformed Search Summary
10
Water Jug Problem

• state ltJ12, J8, J3gt
• initial state lt0, 0, 0gt
• goal test lt1, x, ygt
• x and y can be any value
• path cost
• 1 per solution step?
• 1 per gallon of water moved?

• actions/successor function
• let C1212, C88, C33
• fill-jug-i
• if Ji lt Ci then JiCi
• empty-jug-i-into-jug-j
• if Ji lt Cj Jj thenJj Jj Ji, Ji0
• fill-jug-i-from-jug-j
• if Jj gt Ci Ji thenJj Jj (Ci Ji),
  JiCi