Artificial Intelligence Searching Techniques

Searching Techniques
Dr. C.V. Suresh Babu
(CentreforKnowledgeTransfer)
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• Problem solving
• Problem formulation
• Search Techniques for Artificial Intelligence
• Classification of AI searching Strategies
• What is Search strategy ?
• Defining a Search Problem
• State Space Graph versus Search Trees
• Graph vs. Tree
• Problem Solving by Search
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 Problem formalization.
 Problem Space (state, state space ,search
tree, search node, goal, action and
successor function )
 Uninformed Search :Depth First Search
(DFS), Breadth First Search (BFS), Depth
First Iterative Deeping Search (DFIDS).
 Heuristic Search: Best First Search, Hill
Climbing, Constraint Satisfaction
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Problem solving
We want:
 To automatically solve a problem.
We need:
 A representation of the problem.
 Algorithms that use some strategy to solve the problem defined in that
representation.
Searching technique
Using artificial intelligence
(using intelligence similar to human intelligence or using Intelligent Agents )
Formalization
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Problem Formulation
Goal formulation
 Based on the current situation and the agent’s performance
measure, is the first step in problem solving.
 Goal is a set of states:
 The agent’s task is to find out which sequence of actions will get it
to a goal state.
 Problem formulation :
 is the process of deciding what sorts of actions and states to
consider, given a goal .
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Search Techniques for Artificial Intelligence
 Searching strategies is a central topic in Artificial Intelligence.
 This part of the course will show why search is such an important
topic, present a general approach to representing problems to do
with search.
 Introduce several search algorithms, and demonstrate how to
implement these algorithms in Prolog.
 Problem solving strategies
 Representing problem solution.
 Basic search strategies.
 Informed search strategies.
 Uninformed search strategies .
Search in AI.
 Automated reasoning
 Theorem proving
 Game playing
 Navigation
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Classification of AI
searching Strategies
Searching
Strategies in AI
Iterative
Deepening
Depth-
first (DFS)
Depth-
Limited
(DLS)
Un-informed
(Blind Search)
Breadth-
First (BFS)
Cost-
First (CFS)
Informed
(Heuristic Search)
Hill
Climbing
Constraint
Satisfaction
Best-First
Search (BFS)
A* Search
A very large number of AI problems are formulated as search problems.
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What is Search strategy ?
Search
Search is the systematic examination of states to find path from the
start/root state to the goal state.
 Search usually results from a lack of knowledge.
 Search explores knowledge alternatives to arrive at the best
answer.
 Search algorithm output is a solution, ie, a path from the initial state
to a state that satisfies the goal test.
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Defining a Search Problem
A well-defined problem can be described by
 State space S : all possible configurations(situations) of the domain of
interest .
All states reachable from initial by any sequence of actions.
 Is a graph whose nodes are the set of all states, and whose links are actions
that transform one state into another.
Each state is an abstract representation of the environment.
The state space is discrete.
 An initial (start) state: ∈ S
 Usually the current state .
 Sometimes one or several hypothetical states (“what if …”)
 Goal states G ⊂ S :
The set of end states –Often defined by a goal test rather than enumerating a
set of states.
s0
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A well-defined problem can be described by
Operators (Action) A :
 The actions available–often defined in terms of a mapping from a state to its
successor.
 Is something that the agent can choose to do.
Search tree :
 (A graph with no undirected loops) in which the root node is the start state
and the set of children for each node consists of the states reachable by
taking any action.
Search node :
 Is a node in the search tree.
Goal :
Is a state that the agent (solution) is trying to reach.
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Defining a Search Problem: A well-defined problem can be described by
 Path :
 A sequence of states and operators.
 Sequence through state space.
 Path cost:
A number associated with any path.
Measures the quality of the path.
Sum of costs of individual actions along the path.
Usually the smaller, the better.
 Solution of a search problem:
 Is a path from to some ∈ G.
Optimal solution:
Any path with minimum cost.
 Goal test :
 Test to determine if at goal state.
s0
Sg
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State Space Graph versus Search Trees
State Space Graph
 Graph of states with arrows pointing to successors.
 State graph shows the possible states of a system.
 A state is a node in which a system can be at any given time.
 May contain a loop.
State Space Graph
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State Space Tree
 Tree is a special case of a graph.
 The topmost node in a tree is called the root node; at root node all operations on the tree begin.
 Each NODE in in the search tree is an entire PATH in the problem graph.
 Represent a plan (sequence of actions) which results in the node’s state .
State Space Tree
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Graph vs. Tree
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Problem Solving by Search
 An important aspect of intelligence is goal-based problem solving.
 The solution of many problems can be described by finding a
sequence of actions that lead to a desirable goal.
 Each action changes the state and the aim is to find the sequence of
actions and states that lead from the initial (start) state to a final (goal)
state.
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Example (1) : Vacuum Cleaner world state space graph
States? Discrete: dirt and robot location
Initial state? Any
Actions? Left, right, suck
Goal test? No dirt at all locations
Path cost? 1 per action
Solution Optimal sequence of operations(actions)
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Example (1) : Vacuum Cleaner world state space
 Observable, start in #5.
Solution? [Right , Suck]
Solution? [Suck , Left , Suck]
Solution? [Suck , Left]
Solution? [Suck , Right , Suck]
 Unobservable, start in {1,2,3,4,5,6,7,8}
Solution? [Right , Suck , Left , Suck]
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Example (1) :
Vacuum Cleaner
world state space
graph
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Example (1) :
Vacuum Cleaner
world state space
tree
L R
S
S
8
3 ,7
7
4 ,8
L
3 ,7
7
S
1 ,3,5,7
S
5 ,7
R
6,8
S
8
4 ,6,8
4 ,8
S
4 ,8
3 ,7
7
S
L
S
5,7
R
S
6 ,8
8
2,4,6 ,8
1,3,5,7 4,5 ,7,8
S
L
R
L
S
R
5 ,7
R
6,8
S
2,4,6 ,8
S
L
4 ,8
1 ,3,5,7
S
S
5,7
R
6 ,8
8
R
Any State
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Example(2): The 8-puzzle Problem
state space
Initial State: any configuration Goal State : tiles in a specific order
States? Locations of tiles
Initial State? Given
Actions? Move blank left, right, up, down
Goal test? Goal state (given)
Path cost? 1 per move
Solution: Optimal sequence of operators
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Example(2): The 8-puzzle Problem
State space tree
Up
Left Down Right
U
L R D L R U D
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Example (3) :Route Planning: Dongla Khartoum
 On holiday in Khartoum; currently in Dongla.
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Example (3) :Route Planning: Dongla Khartoum
States? Various cities
Initial State? Dongla
Actions? Drive between cities or choose next city
Goal test? Be in Khartoum
Path cost? Distance in km
Solution: Sequence of cities, e. g. Dongla, Karima
, Abu-hammed, Khartoum.
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Karima
Dongla
Old-Dongla
Karmma
Wadi-Halfa Marawi Korti
Abu-Hammed
Khartoum
Sodri Qoz-Abu-Dulu
Khartoum
118
75
140
71 80 97
211
101
97
146
Qoz-Abu-Dulu
Um-Badir
Khartoum
Abu-Urug
Um-Badir
Sodri
Marawi Qoz-Abu-Dulu
Khartoum
Qoz-Abu-Dulu
Khartoum
111
70
75
120
120
138
101
146 118
101
97
101
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Example (4): Robotic assembly
• states?: real-valued coordinates of robot joint angles
parts of the object to be assembled
• initial state?: rest configuration
• actions?: continuous motions of robot joints
• goal test?: complete assembly
• path cost?: time to execute
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Uninformed search
 Uninformed Search (also called blind search) is a search that has
no information about its domain.
Use only the information available in the problem definition.
Generates the search tree without using any domain specific
knowledge.
The only thing that a blind search can do is distinguish a non-goal
state from a goal state.
Brute-force algorithms search, through the search space, all
possible candidates for the solution checking whether each
candidate satisfies the problem's statement.
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Uninformed search
Breadth-first search (BFS)
Depth-first search (DFS)
Uniform-cost search
Depth-limited search
Iterative deepening search
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Informed Search
Informed Search algorithms use heuristic functions, that
are specific to the problem, apply them to guide the search
through the search space to try to reduce the amount of
time spent in searching.
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Breadth-first search (BFS)
A Search strategy, in which the highest layer of a decision tree is searched
completely before proceeding to the next layer is called Breadth-first search
(BFS).
When a state is examined, all of its siblings are examined before any
of its children.
The space is searched level-by-level, proceeding all the way across
one level before doing down to the next level.
In this strategy, no viable solution is omitted and therefore guarantee
that optimal solution is found.
This strategy is often not feasible when the search space is large.
BFS explores nodes nearest to the root before exploring nodes that
are father or further away.
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Node are explored in the order :
A B C D E F G H I J K L M N O P Q
■ After searching A, then B, then C, the search proceeds with D,E, F, G, . . . . . . . . .
■ The goal node J will be found before the goal node N.
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Depth-first search (DFS)
 A search strategy that extends the current path as far as possible before backtracking to
the last choice point and trying the next alternative path is called Depth-first search
(DFS).
 When a state is examined, all of its children and their descendants are examined
before any of its siblings.
 DFS goes deeper in to the search space when ever this is possible only when no
further descendants of a state can found.
 This strategy does not guarantee that the optimal solution has been found.
 In this strategy, search reaches a satisfactory solution more rapidly than breadth first,
an advantage when the search space is large.
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The Breadth-first search (BFS) and depth-first search (DFS)
are the foundation for all other search techniques.
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Node are explored in the order :
A B D E H L M N I O P C F G J K Q
■ After searching node A, then B, then D, the search backtracks and tries another
path from node B .
■ The goal node N will be found before the goal node J.
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DFS vs BFS Algorithm
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Artificial Intelligence Searching Techniques

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