Quiz 1 : Queue based search
 q1a1: Any complete search algorithm must have
exponential (or worse) time complexity.
 q1a2: DFS requires more space than BFS.
 q1a3: BFS always returns the optimal cost path.
 q1a4: BFS uses a FIFO queue as fringe.
 q1a5: UCS uses a LIFO queue as fringe.
 q1a6: BFS has the best time complexity out of UCS,
BFS, DFS, ID.
 Yes





No
No
Yes
No
No
CSE511a: Artificial
Intelligence
Spring 2013
Lecture 3: A* Search
1/24/2012
Robert Pless – Wash U.
Multiple slides over the course adapted from Kilian
Weinberger, originally by Dan Klein (or Stuart Russell or
Andrew Moore)
Announcements
 Projects:
 Project 0 due tomorrow night.
 Project 1 (Search) is out soon, will be due Thursday 2/7.
 Find project partner
 Try pair programming, not divide-and-conquer
 Exercise more and try to eat more fruit.
Today
 A* Search
 Heuristic Design
Recap: Search
 Search problem:
 States (configurations of the world)
 Successor function: a function from states to
lists of (action, state, cost) triples; drawn as a graph
 Start state and goal test
 Search tree:
 Nodes: represent plans for reaching states
 Plans have costs (sum of action costs)
 Search Algorithm:
 Systematically builds a search tree
 Chooses an ordering of the fringe (unexplored nodes)
Machinarium
Machinarium: Search Problem
Youtube
A*-Search
General Tree Search
Uniform Cost Search
 Strategy: expand lowest
path cost
…
c1
c2
c3
 The good: UCS is
complete and optimal!
 The bad:
 Explores options in every
“direction”
 No information about goal
location
Start
Goal
[demo: countours UCS]
Best First (Greedy)
 Strategy: expand a node
that you think is closest
to a goal state
…
b
 Heuristic: estimate of
distance to nearest goal
for each state
 A common case:
 Best-first takes you
straight to the (wrong) goal
…
b
 Worst-case: like a badlyguided DFS
[demo: countours greedy]
Example: Heuristic Function
h(x)
Combining UCS and Greedy
 Uniform-cost orders by path cost, or backward cost g(n)
 Best-first orders by goal proximity, or forward cost h(n)
5
e
h=1
1
S
h=6
c
h=7
1
a
h=5
1
1
3
2
d
h=2
G
h=0
b
h=6
 A* Search orders by the sum: f(n) = g(n) + h(n)
Example: Teg Grenager
Three Questions
 When should A* terminate?
 Is A* optimal?
 What heuristics are valid?
When should A* terminate?
 Should we stop when we enqueue a goal?
2
A
2
h=2
S
G
h=3
2
B
h=0
3
h=1
 No: only stop when we dequeue a goal
Is A* Optimal?
1
A
h=6
3
h=0
S
h=7
G
5
 What went wrong?
 Actual bad goal cost < estimated good goal cost
 We need estimates to be less than actual costs!
Admissible Heuristics
 A heuristic h is admissible (optimistic) if:
where
is the true cost to a nearest goal
 Example:
15
 Coming up with admissible heuristics is most of
what’s involved in using A* in practice.
Example: Pancake Problem
Cost: Number of pancakes flipped
Pancake: State Space Object
Example: Pancake Problem
State space graph with costs as weights
4
2
3
2
3
4
3
4
3
2
2
2
4
3
Example: Heuristic Function
Heuristic: the largest pancake that is still out of place
3
h(x)
4
3
4
3
0
4
4
3
4
4
2
3
Example: Pancake Problem
Optimality of A*: Blocking
Notation:
 g(n) = cost to node n
 h(n) = estimated cost from n
to the nearest goal (heuristic)
 f(n) = g(n) + h(n) =
estimated total cost via n
 G*: a lowest cost goal node
 G: another goal node
…
Optimality of A*: Blocking
Proof:
 What could go wrong?
 We’d have to have to pop a
suboptimal goal G off the
fringe before G*
 This can’t happen:
 Imagine a suboptimal
goal G is on the queue
 Some node n which is a
subpath of G* must also
be on the fringe (why?)
 n will be popped before G
…
Properties of A*
Uniform-Cost
…
b
A*
…
b
UCS vs A* Contours
 Uniform-cost expanded
in all directions
 A* expands mainly
toward the goal, but
does hedge its bets to
ensure optimality
Start
Goal
Start
Goal
[demo: countours UCS / A*]
Creating Admissible Heuristics
 Most of the work in solving hard search problems
optimally is in coming up with admissible heuristics
 Often, admissible heuristics are solutions to relaxed
problems, where new actions are available
4
15
 Inadmissible heuristics are often useful too (why?)
Example: 8 Puzzle





What are the states?
How many states?
What are the actions?
What states can I reach from the start state?
What should the costs be?
Search State
8 Puzzle I
 Heuristic: Number of
tiles misplaced
 Why is it admissible?
Average nodes expanded when
optimal path has length…
 h(start) = 8
…4 steps …8 steps …12 steps
 This is a relaxedproblem heuristic
UCS
112
TILES 13
6,300
39
3.6 x 106
227
8 Puzzle II
 What if we had an
easier 8-puzzle where
any tile could slide any
direction at any time,
ignoring other tiles?
 Total Manhattan
distance
 Why admissible?
 h(start) =
3 + 1 + 2 + … TILES
= 18
MANHATTAN
Average nodes expanded when
optimal path has length…
…4 steps
…8 steps
…12 steps
13
12
39
25
227
73
[demo: eight-puzzle]
8 Puzzle III
 How about using the actual cost as a
heuristic?
 Would it be admissible?
 Would we save on nodes expanded?
 What’s wrong with it?
 With A*: a trade-off between quality of
estimate and work per node!
Trivial Heuristics, Dominance
 Dominance: ha ≥ hc if
 Heuristics form a semi-lattice:
 Max of admissible heuristics is admissible
 Trivial heuristics
 Bottom of lattice is the zero heuristic (what
does this give us?)
 Top of lattice is the exact heuristic
Other A* Applications







Pathing / routing problems
Resource planning problems
Robot motion planning
Language analysis
Machine translation
Speech recognition
…
Graph Search
Tree Search: Extra Work!
 Failure to detect repeated states can cause
exponentially more work. Why?
Graph Search
 In BFS, for example, we shouldn’t bother
expanding the circled nodes (why?)
S
e
d
b
c
a
a
e
h
p
q
q
c
a
h
r
p
f
q
G
p
q
r
q
f
c
a
G
Graph Search
 Very simple fix: never expand a state twice
Graph Search
 Idea: never expand a state twice
 How to implement:
 Tree search + list of expanded states (closed list)
 Expand the search tree node-by-node, but…
 Before expanding a node, check to make sure its state is new
 Python trick: store the closed list as a set, not a list
 Can graph search wreck completeness? Why/why not?
 How about optimality?
Consistency
1
A
h=4
1
h=1
S
h=6
C
1
B
2
h=1
 What went wrong?
 Taking a step must not reduce f value!
3
G
h=0
Consistency
 Stronger than admissability
 Definition:
 C(A→C)+h(C)≧h(A)
 C(A→C)≧h(A)-h(C)
 Consequence:
 The f value on a path never
decreases
 A* search is optimal
h=4
A
1
h=0
C
h=1
3
G
Optimality of A* Graph Search
Proof:
 New possible problem: nodes on path to
G* that would have been in queue aren’t,
because some worse n’ for the same
state as some n was dequeued and
expanded first (disaster!)
 Take the highest such n in tree
 Let p be the ancestor which was on the
queue when n’ was expanded
 Assume f(p) ≦ f(n) (consistency!)
 f(n) < f(n’) because n’ is suboptimal
 p would have been expanded before n’
 So n would have been expanded before
n’, too
 Contradiction!
Optimality
 Tree search:
 A* optimal if heuristic is admissible (and nonnegative)
 UCS is a special case (h = 0)
 Graph search:
 A* optimal if heuristic is consistent
 UCS optimal (h = 0 is consistent)
 Consistency implies admissibility
 In general, natural admissible heuristics tend to
be consistent
Summary: A*
 A* uses both backward costs and
(estimates of) forward costs
 A* is optimal with admissible heuristics
 Heuristic design is key: often use relaxed
problems