Heuristic Search Techniques {Artificial Intelligence}

Contents
• A framework for describing search methods is
provided and several general purpose search
techniques are discussed.
• All are varieties of Heuristic Search:
– Generate and test
– Hill Climbing
– Best First Search
– Problem Reduction
– Constraint Satisfaction
– Means-ends analysis

Generate-and-Test
• Algorithm:
1. Generate a possible solution. For some problems,
this means generating a particular point in the
problem space. For others it means generating a
path from a start state
2. Test to see if this is actually a solution by comparing
the chosen point or the endpoint of the chosen path
to the set of acceptable goal states.
3. If a solution has been found, quit, Otherwise return
to step 1.

Generate-and-Test
• It is a depth first search procedure since complete
solutions must be generated before they can be tested.
• In its most systematic form, it is simply an exhaustive
search of the problem space.
• Operate by generating solutions randomly.
• Also called as British Museum algorithm
• If a sufficient number of monkeys were placed in front of
a set of typewriters, and left alone long enough, then
they would eventually produce all the works of
shakespeare.
• Dendral: which infers the struture of organic compounds
using NMR spectrogram. It uses plan-generate-test.

Hill Climbing
• Is a variant of generate-and test in which
feedback from the test procedure is used to help
the generator decide which direction to move in
search space.
• The test function is augmented with a heuristic
function that provides an estimate of how close
a given state is to the goal state.
• Computation of heuristic function can be done
with negligible amount of computation.
• Hill climbing is often used when a good heuristic
function is available for evaluating states but
when no other useful knowledge is available.

Simple Hill Climbing
• Algorithm:
1. Evaluate the initial state. If it is also goal state, then
return it and quit. Otherwise continue with the initial
state as the current state.
2. Loop until a solution is found or until there are no new
operators left to be applied in the current state:
a. Select an operator that has not yet been applied to the current
state and apply it to produce a new state
b. Evaluate the new state
i. If it is the goal state, then return it and quit.
ii. If it is not a goal state but it is better than the current state, then
make it the current state.
iii. If it is not better than the current state, then continue in the loop.

Simple Hill Climbing
• The key difference between Simple Hill
climbing and Generate-and-test is the use
of evaluation function as a way to inject
task specific knowledge into the control
process.
• Is on state better than another ? For this
algorithm to work, precise definition of
better must be provided.

: Hill-climbing
This simple policy has three well-
known drawbacks:
1. Local Maxima: a local maximum
as opposed to global maximum.
2. Plateaus: An area of the search
space where evaluation function is
flat, thus requiring random walk.
3. Ridge: Where there are steep
slopes and the search direction is
not towards the top but towards the
side.
(a)
(b)
(c)
Figure 5.9 Local maxima, Plateaus and
ridge situation for Hill Climbing

Hill-climbing
• In each of the previous cases (local maxima, plateaus & ridge),
the algorithm reaches a point at which no progress is being
made.
• A solution is to do a random-restart hill-climbing - where
random initial states are generated, running each until it halts
or makes no discernible progress. The best result is then
chosen.
Figure 5.10 Random-restart hill-climbing (6 initial values) for 5.9(a)

Simulated Annealing
• A alternative to a random-restart hill-climbing when stuck on
a local maximum is to do a ‘reverse walk’ to escape the
local maximum.
• This is the idea of simulated annealing.
• The term simulated annealing derives from the roughly
analogous physical process of heating and then slowly
cooling a substance to obtain a strong crystalline structure.
• The simulated annealing process lowers the temperature by
slow stages until the system ``freezes" and no further
changes occur.

Simulated Annealing
Figure 5.11 Simulated Annealing Demo (http://www.taygeta.com/annealing/demo1.html)

Best First Search
• Combines the advantages of bith DFS and BFS
into a single method.
• DFS is good because it allows a solution to be
found without all competing branches having to
be expanded.
• BFS is good because it does not get branches
on dead end paths.
• One way of combining the tow is to follow a
single path at a time, but switch paths whenever
some competing path looks more promising than
the current one does.

BFS
• At each step of the BFS search process, we select the most
promising of the nodes we have generated so far.
• This is done by applying an appropriate heuristic function to each of
them.
• We then expand the chosen node by using the rules to generate its
successors
• Similar to Steepest ascent hill climbing with two exceptions:
– In hill climbing, one move is selected and all the others are rejected,
never to be reconsidered. This produces the straightline behaviour that
is characteristic of hill climbing.
– In BFS, one move is selected, but the others are kept around so that
they can be revisited later if the selected path becomes less promising.
Further, the best available state is selected in the BFS, even if that state
has a value that is lower than the value of the state that was just
explored. This contrasts with hill climbing, which will stop if there are no
successor states with better values than the current state.

OR-graph
• It is sometimes important to search graphs so that duplicate paths
will not be pursued.
• An algorithm to do this will operate by searching a directed graph in
which each node represents a point in problem space.
• Each node will contain:
– Description of problem state it represents
– Indication of how promising it is
– Parent link that points back to the best node from which it came
– List of nodes that were generated from it
• Parent link will make it possible to recover the path to the goal once
the goal is found.
• The list of successors will make it possible, if a better path is found
to an already existing node, to propagate the improvement down to
its successors.
• This is called OR-graph, since each of its branhes represents an
alternative problem solving path

Implementation of OR graphs
• We need two lists of nodes:
– OPEN – nodes that have been generated and have
had the heuristic function applied to them but which
have not yet been examined. OPEN is actually a
priority queue in which the elements with the highest
priority are those with the most promising value of the
heuristic function.
– CLOSED- nodes that have already been examined.
We need to keep these nodes in memory if we want
to search a graph rather than a tree, since whenver a
new node is generated, we need to check whether it
has been generated before.

BFS
Step 1 Step 2 Step 3
Step 4 Step 5
A A
B C D3 5 1
A
B C D3 5 1
E F4
6A
B C D3 5 1
E F4
6
G H6 5
A
B C D3 5 1
E F4
6
G H6 5
A A2 1

A* Algorithm
• BFS is a simplification of A* Algorithm
• Presented by Hart et al
• Algorithm uses:
– f’: Heuristic function that estimates the merits of each node we
generate. This is sum of two components, g and h’ and f’
represents an estimate of the cost of getting from the initial state
to a goal state along with the path that generated the current
node.
– g : The function g is a measure of the cost of getting from initial
state to the current node.
– h’ : The function h’ is an estimate of the additional cost of getting
from the current node to a goal state.
– OPEN
– CLOSED

A* Algorithm
1. Start with OPEN containing only initial node. Set that
node’s g value to 0, its h’ value to whatever it is, and its
f’ value to h’+0 or h’. Set CLOSED to empty list.
2. Until a goal node is found, repeat the following
procedure: If there are no nodes on OPEN, report
failure. Otherwise picj the node on OPEN with the
lowest f’ value. Call it BESTNODE. Remove it from
OPEN. Place it in CLOSED. See if the BESTNODE is
a goal state. If so exit and report a solution. Otherwise,
generate the successors of BESTNODE but do not set
the BESTNODE to point to them yet.

A* Algorithm ( contd)
• For each of the SUCCESSOR, do the following:
a. Set SUCCESSOR to point back to BESTNODE. These
backwards links will make it possible to recover the path once a
solution is found.
b. Compute g(SUCCESSOR) = g(BESTNODE) + the cost of getting
from BESTNODE to SUCCESSOR
c. See if SUCCESSOR is the same as any node on OPEN. If so call
the node OLD.
d. If SUCCESSOR was not on OPEN, see if it is on CLOSED. If so,
call the node on CLOSED OLD and add OLD to the list of
BESTNODE’s successors.
e. If SUCCESSOR was not already on either OPEN or CLOSED,
then put it on OPEN and add it to the list of BESTNODE’s
successors. Compute f’(SUCCESSOR) = g(SUCCESSOR) +
h’(SUCCESSOR)

Observations about A*
• Role of g function: This lets us choose
which node to expand next on the basis of
not only of how good the node itself looks,
but also on the basis of how good the path
to the node was.
• h’, the distance of a node to the goal.If h’
is a perfect estimator of h, then A* will
converge immediately to the goal with no
search.

AND-OR graphs
• AND-OR graph (or tree) is useful for representing the
solution of problems that can be solved by decomposing
them into a set of smaller problems, all of which must then
be solved.
• One AND arc may point to any number of successor
nodes, all of which must be solved in order for the arc to
point to a solution.
Goal: Acquire TV Set
Goal: Steal a TV Set Goal: Earn some money Goal: Buy TV Set

AND-OR graph examples
A
B C D5 3 4
A
B C D
F GE H I J
5 10 3 4 15 10
17 9 27
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Heuristic Search Techniques {Artificial Intelligence}

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