2.Problems Problem Spaces and Search.ppt

19ECB336
ARTIFICIAL INTELLIGENCE
1

Problems, Problem Spaces and Search:
To build a AI system to solve a particular problem, we need to do
the following four things;
1. Define the problem precisely.
2. Analyse the problem.
3. Isolate and represent the task knowledge that is necessary to
solve the problem.
4.Choose the best problem-solving techniques and apply it to
the particular problem.
 The state space representation forms the basis of most of the AI
methods.
2

Defining the problem as State Space Search
Its structure corresponds to the structure of problem solving in
two important ways:
 It allows for a formal definition of a problem as the need to
convert some given situation into some desired situation using
a set of permissible operations.
 It permits us to define the process of solving a particular
problem as a combination of known techniques (each
represented as a rule defining a single step in the space) and
search, the general technique of exploring the space to try to
find some path from current state to a goal state.
Search is a very important process in the solution of hard
problems for which no more direct techniques are available. 3

Example: Playing Chess
 To build a program that could “play chess”, we could first have to
specify the starting position of the chess board, the rules that
define the legal moves, and the board positions that represent a
win for one side or the other.
 In addition, we must make explicit the previously implicit goal of
not only playing the legal game of chess but also winning the
game, if possible,
 The starting position can be described as an 8x8 array where
each position contains a symbol for appropriate piece.
 We can define as our goal to check mate position.
4

 The legal moves provide the way of getting from initial state to a
goal state.
 They can be described easily as a set of rules consisting of two
parts:
 A left side that serves as a pattern to be matched against the
current board position.
 And a right side that describes the change to be made to reflect
the move.
 However, this approach leads to large number of rules 10^120
board positions!!
5

 Using so many rules poses problems such as:
 No person could ever supply a complete set of such rules.
 No program could easily handle all those rules. Just storing so
many rules poses serious difficulties.
 In order to minimize such problems, We need to write the rules
describing the legal moves in as general a way as possible.
 For example:
6

 In general, we can describe the rules we need, the less work we will
have to do to provide them and more efficient the program.
7

Water Jug Problem
 The state space for this problem can be described as the set of
ordered pairs of integers (x, y) such that x = 0, 1,2, 3 or 4 and y = 0,1,2 or
3;
 How can we get two gallons of water in the first jug?
 x represents the number of gallons of water in the 4-gallon jug
 and y represents the quantity of water in 3-gallon jug.
 The start state is (0,0) and the goal state is (2,n), where n may be any
but it is limited to three holding from 0 to 3 gallons of water or empty.
8

Production rules for Water Jug Problem
9

Production rules for Water Jug Problem
10

To solve the water jug problem
 Required a control structure that loops through a simple cycle in
which some rule whose left side matches the current state is chosen,
the appropriate change to the state is made as described in the
corresponding right side, and the resulting state is checked to see if it
corresponds to goal state.
 Shortest such sequence will have a impact on the choice of
appropriate mechanism to guide the search for solution.
11

Summarizing the formal description of the problem
1. Define a state space that contains all the possible configurations of
the relevant objects.
2. Specify one or more states within that space which describe possible
situations from which the problem solving process may start ( initial
state).
3. Specify one or more states that would be acceptable as solutions to
the problem ( goal states).
4. Specify a set of rules that describe the actions ( operations)
available.
12

Production Systems
 A production system is a model of computation that provides pattern-
directed control of a problem – solving process and consists of a set
of production rules, a working memory, and a recognize – act control
cycle.
 All of these systems provide the overall architecture of a production
systems and allow the programmer to write rules that define
particular problems to be solved.
13

Production Systems
A production system consists of:
 A set of rules, each consisting of a left side that determines the
applicability of the rule and a right side that describes the operation to
be performed if that rule is applied.
 One or more knowledge/database(KB) that contain whatever
information is appropriate for the particular task. Some parts of the
database may be permanent, while other parts of it may pertain only
to the solution of the current problem.
 A control strategy that specifies the order in which the rules will be
compared to the database and a way of resolving the conflicts that
arise when several rules match at once.
 A rule applier; Check current state with LHS of rules in KB and find
appropriate rule to apply. 14

Production Systems
 One can often represent the expertise that someone uses to do an expert
task as rules.
 A rule means a structure which has an if component and a then
component.
 The statement, or set of statements, after the word if represents some pattern
which you may observe.
 The statement, or set of statements, after the word then represents some
conclusion that you can draw, or some action that you should take.
 IF some condition(s) exists THEN perform some action(s)
• IF-THEN
• Test-Action
15

Production Systems
 Therefore production system can be described as set of production rules,
together with software that can reason with them.
 Production System are sometimes known as Rule Based Expert System.
 A rule-based system consists of
• a set of IF-THEN rules
• a set of facts
• an interpreter controlling the application of the rules, given the facts.
16

Production Systems
In order to solve a problem:
 We must reduce it to one for which a precise statement can be given.
This can be done by defining the problem’s state space ( start and goal
states) and a set of operators for moving that space.
 The problem can then be solved by searching for a path through the
state space from an initial state to a goal state.
 The process of solving the problem can usefully be modelled as a
production system.
17

Example 8-Puzzle problem
 Search process contains:
1. Expanding a node
2. Generating a node
3. Comparing
4. Track the path and path cost
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Control Strategies
 How to decide which rule to apply next during the process of
searching for a solution to a problem?
 The two requirements of good control strategy are
 it should cause motion.
 It should be systematic
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Algorithm: Breadth First Search
1. Create a variable called NODE-LIST and set it to initial state
2. Until a goal state is found or NODE-LIST is empty
do:
a. Remove the first element from NODE-LIST and call it E. If NODE-
LIST was empty, quit.
b. For each way that each rule can match the state described in E
do:
i. Apply the rule to generate a new state
ii. If the new state is a goal state, quit and return this state
iii.Otherwise, add the new state to the end of NODE-LIST
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Algorithm: Depth First Search
1. If the initial state is a goal state, quit and return success
2. Otherwise, do the following until success or failure is signalled:
do:
a. Generate a successor, E, of initial state. If there are no more
successors, signal failure.
b. Call Depth-First Search, with E as the initial state
c. If success is returned, signal success. Otherwise continue in this
loop.
22

Depth First Search(Cont.…)
 In this search, we pursue a single branch of the tree until it yields a
solution or until a decision to terminate the path is made.
 It makes sense to terminate a path if it reaches dead end, produces a
previous state. In such a state backtracking occurs.
 Chronological Backtracking: Order in which steps are undone
depends only on the temporal sequence in which steps were initially
made.
 Specifically most recent step is always the first to be undone.
 This is also simple backtracking.
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Advantages
Advantages of DFS
 DFS requires less memory since only the nodes on the current path
are stored.
 By chance, DFS may find a solution without examining much of the
search space at all.
Advantages of BFS
 BFS will not get trapped exploring a blind alley.
 If there is a solution, BFS is guaranteed to find it.
 If there are multiple solutions, then a minimal solution will be found.
24

Traveling Salesman Problem (TSP)
 A salesman has a list of cities, each of which he must visit exactly once.
There are direct roads between each pair of cities on the list.
 Find the route the salesman should follow for the shortest possible
round trip that both starts and finishes at any one of the cities.
 A simple motion causing and systematic control structure could solve
this problem.
 Simply explore all possible paths in the tree and return the shortest
path.
 If there are N cities, then number of different paths among them is
1.2….(N-1) or (N-1)!.
 The time to examine single path is proportional to N!.
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 So the total time required to perform this search is proportional to
N!. For 10 cities, 10! = 3,628,800 which is very large number.
 The salesman could normally visits nearly 25 cities.
 This phenomenon is called Combinatorial explosion.
 So above approach will take more time to solve the TSP.
 To beat the Combinatorial explosion technique, a new strategy
known as branch and bound technique came into existence.
26

Branch and Bound:
 Begin generating complete paths, keeping track of the shortest
path found so far.
 Give up exploring any path as soon as its partial length becomes
greater than the shortest path found so far.
 Using this algorithm, we are guaranteed to find the shortest path.
 It still requires exponential time.
 The time it saves depends on the order in which paths are explored.
 But it is still inadequate for solving large problems.
27

Heuristic Search
 It is technique designed to solve a problem quickly.
 A Heuristic is a technique that improves the efficiency of a search
process, possibly by sacrificing claims of completeness.
 Heuristics are like tour guides.
 They are good to the extent that they point in generally interesting
directions;
 They are bad to the extent that they may miss points of interest to
particular individuals.
 On the average, they improve the quality of the paths that are
explored.
28

Heuristic Search
 Using Heuristics, we can hope to get good ( though possibly non-
optimal ) solutions to hard problems such TSP in non exponential
time.
 There are good general purpose heuristics that are useful in a wide
variety of problem domains.
 Special purpose heuristics exploit domain specific knowledge.
 It gives good solution. The solution may be optimal but there is no
guarantee.
 Try to solve the problem from non-polynomial to polynomial time
complexity.
29

Heuristic Search
 One of the good purpose heuristic that is useful for variety of
combinatorial problems is the Nearest Neighbour Heuristic which
works by selecting the locally superior alternative at each step.
 Applying it to TSP; The following procedure should be followed;
1. Arbitrarily select a starting city
2. To select the next city, look at all cities not yet visited and select
the one closest to the current city. Go to next step.
3. Repeat step 2 until all cities have been visited.
 This procedure executes in time proportional to N^2
 It is possible to prove an upper bound on the error it incurs.
 This provides reassurance that one is not paying too high price in
accuracy for speed. 30

Heuristic Function
 This is a function that maps from problem state descriptions to
measures of desirability, usually represented as numbers.
 Which aspects of the problem state are considered,
 How those aspects are evaluated, and
 The weights given to individual aspects are chosen in such a way
that
 The value of the heuristic function at a given node in the search
process gives as good an estimate as possible of whether that node
is on the desired path to a solution.
 Well designed heuristic functions can play an important part in
efficiently guiding a search process toward a solution.
31

Examples: Simple Heuristic functions
 Chess : The material advantage of our side over opponent.
 TSP: the sum of distances so far.
 Tic-Tac-Toe: 1 for each row in which we could win and in we already
have one piece plus 2 for each such row in we have two pieces.
32

Problem Characteristics
 In order to choose the most appropriate method for a particular
problem, it is necessary to analyze the problem along several key
dimensions:
 Is the problem decomposable into a set of independent smaller
or easier subproblems?
 Can solution steps be ignored or at least undone if they prove
unwise?
 Is the problem’s universe predictable?
 Is a good solution to the problem obvious without comparison to
all other possible solutions?
 Is the desired solution a state of the world or a path to a state?
33

Problem Characteristics
 Is a large amount of knowledge absolutely required to solve the
problem or is knowledge important only to constrain the
search?
 Can a computer that is simply given the problem return the
solution or will the solution of the problem require interaction
between the computer and a person?
34

Is the problem Decomposable?
 Whether the problem can be decomposed into smaller problems?
 Using the technique of problem decomposition, we can often solve
very large problems easily.
 Suppose, We want to solve the problem of computing the
expression;
35

Blocks World Problem
 The blocks-world problem is known as Sussman Anomaly.
 Noninterleaved planners of the early 1970s were unable to solve
this problem, hence it is considered as anomalous.
 When two subgoals G1 and G2 are given, a noninterleaved planner
produces either a plan for G1 concatenated with a plan for G2, or
vice-versa.
 In blocks-world problem, three blocks labeled as 'A', 'B', 'C' are
allowed to rest on the flat surface/table. The given condition is that
only one block can be moved at a time to achieve the goal.
36

Blocks World Problem
 Following operators are available:
37

Can Solution Steps be ignored or undone?
 Suppose we are trying to prove a mathematical theorem. We can
prove a lemma. If we find the lemma is not of any help, we can still
continue.
 8-puzzle problem.
 Chess: A move cannot be taken back.
 Important classes of problems:
 Ignorable ( theorem proving)
 Recoverable ( 8-puzzle)
 Irrecoverable ( Chess)
38

Can Solution Steps be ignored or undone?
 The recoverability of a problem plays an important role in
determining the complexity of the control structure necessary for
the problem’s solution.
 Ignorable problems can be solved using a simple control
structure that never backtracks
 Recoverable problems can be solved by a slightly more
complicated control strategy that does sometimes make
mistakes
 Irrecoverable problems will need to be solved by systems that
expends a great deal of effort making each decision since
decision must be final.
39

Is the universe Predictable?
 Certain Outcome ( ex: 8-puzzle).
 Uncertain Outcome ( ex: Bridge, controlling a robot arm).
 For solving certain outcome problems, open loop approach (
without feedback) will work fine.
 For uncertain-outcome problems, planning can at best generate a
sequence of operators that has a good probability of leading to a
solution.
 We need to allow for a process of plan revision to take place.
40

Is a good solution absolute or relative?
 Any path problem
 Best path problem
 Any path problems can often be solved in a reasonable amount of
time by using heuristics that suggest good paths to explore.
 Best path problems are computationally harder than any path
problems.
41

Is a good solution absolute or relative?
 For example: The salesman starts from Boston. The following
figure
42

Is the solution a state or a path?
 Consider the problem of finding a consistent interpretation for the
sentence;
“The bank president ate a dish of pasta salad with the fork”.
 We need to find the interpretation but not the record of the
processing.
 Water jug Problem: Here it is not sufficient to report that we have
solved, but the path that we found to the state (2,0). Thus a
statement of a solution to this problem must be a sequence of
operations ( Plan) that produces the final state.
43

What is the role of knowledge?
 Two examples; Chess: Knowledge is required to constrain the search
for a solution.
 Newspaper story understanding: Lot of knowledge is required even to
be able to recognize a solution.
 Consider a problem of scanning daily newspapers to decide which are
supporting the democrats and which are supporting the republicans in
some election. We need lots of knowledge to answer such questions as:
 The names of the candidates in each party.
 The facts that if the major thing you want to see done is have taxes
lowered, you are probably supporting the republicans.
 The fact that if the major thing you want to see done is improved
education for minority students, you are probably supporting the
democrats.
44

Does the task require Interaction with a person?
 Sometimes, it is useful to program computers to solve a problems
in ways that the majority of people would not be able to
understand.
 That is fine if the level o interaction between the computer and its
human users is problem-in solution-out.
 But increasingly, we are building programs that require
intermediate interaction with people for additional inputs and to
provided reassurance to the user.
45

Does the task require Interaction with a person?
 Thus we must distinguish between two types of programs:
• Solitary, in which computer is given a problem description and
produces an answer with no intermediate communication and
with no demand for an explanation of the reasoning process.
• Conversational, in which there is intermediate communication
between person and the computer, either to provide additional
assistance to the computer or to provide additional information
to the user or both.
• Decision on using one of these approaches will be important in the
choice of problem solving method.
46

Problem Classification
 There are several broad classes into which the problems fall.
 These classes can each be associated with generic control
strategy that is appropriate for solving the problems:
 Classification : medical diagnostics,diagnosis of faults in
mechanical devices.
 Propose and Refine: design and planning
47

Production System Characteristics
 We have seen that the production systems are good way to
describe the operations that can be performed in search for a
solution to a problem. Two questions, we might reasonably ask,
are:
1. Can production systems, like problems, be described by a set of
characteristics that shed some light on how they can easily be
implemented?
2. If so, what relationships are there between problem types and
the types of production systems best suited to solving the
problems?
 The answer of the first question is yes.
48

 Consider the following definitions of classes of Production systems:
 Monotonic Production System: The application of a rule never
prevents the later application of another rule that could also have
been applied at the time the first rule was selected. i.e., rules are
independent.
 Non-Monotonic Production system is one in which this is not true.
 Partially commutative Production system: property that if application
of a particular sequence of rules transforms state x to state y, then
permutation of those rules allowable, also transforms state x into
state y.
 A commutative Production system is the production system that is
both monotonic and partially commutative.
49

 Partially Commutative, Monotonic Production Systems:
These production systems are useful for solving ignorable
problems. Example: Theorem Proving
 They can be implemented without the ability to backtrack to
previous states when it is discovered that an incorrect path has
been followed.
 This often results in a considerable increase in efficiency,
particularly because since the database will never have to be
restored
 It is not necessary to keep track of where in the search process
every change was made.
 They are good for problems where things do not change; new
things get created. 50

Non Monotonic, Partially Commutative Production
Systems:
 Useful for problems in which changes occur but can be reversed
and in which order of operations is not critical.
 Example: Robot Navigation, 8-puzzle, blocks world
 Suppose the robot has the following ops: go North (N), go East (E),
go South (S), go West (W). To reach its goal, it does not matter
whether the robot executes the N-N-E or N-E-N.
51

Non Partially Commutative Production Systems:
 Problems in which irreversible change occurs. Example: chemical
synthesis.
 The ops can be : Add chemical x to the pot, change the temperature
to t degrees.
 These ops may cause irreversible changes to the portion being
brewed.
 The order in which they are performed can be very important in
determining the final output.
 (X+y) +z is not the same as (z+y) +x
 Non partially commutative production systems are less likely to
produce the same node many times in search process.
52

Non Partially Commutative Production Systems:
 When dealing with ones that describe irreversible processes, it is
partially important to make correct decisions the first time,
although if the universe is predictable, planning can be used to
make that less important.
Four Categories of Production System
53

Issues in the design of search programs
 The direction in which to conduct the search ( forward versus
backward reasoning).
 How to select applicable rules ( Matching) •
 How to represent each node of the search process ( knowledge
representation problem)
54

Summary
 Four steps for designing a program to solve a problem:
1. Define the problem precisely.
2. Analyse the problem
3. Identify and represent the knowledge required by the task
4. Choose one or more techniques for problem solving and apply
those techniques to the problem.
55

2.Problems Problem Spaces and Search.ppt

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2.Problems Problem Spaces and Search.ppt