A star search in A.I

1. Informed search
12
A* Search

2. Informed (Heuristic) Search:
 Informed search strategy is one that
uses problem-specific knowledge
beyond the definition of the problem
itself and can find solutions more
efficiently.
 The general approach we consider is
called best-first search.
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3. A* Search
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The most widely known form of best-first search is called
A*. It evaluates nodes by combining g(n) and h(n):
g(n): The actual cost from start node to n.
h(n): The estimation cost from the n to the goal state:
f(n) = estimated cost of the cheapest solution through n .
f(n) = g(n) + h(n) .

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A
C
B
H
F
E
D
G
13
8
9
6
5
4
5
0
1
4
3
7
5 8
6
2
7 1
Expand A:
(A,B)f=(1+5)=6
(A,C)f=(3+6)=9
Expand B:
(A,C)f=(3+6)=9
(A,B,D) f={(1+7)+8}=16
(A,B,E) f={(1+4)+5}=10
Expand C:
(A,B,D) f={(1+7)+8}=16
(A,B,E) f={(1+4)+5}=10
(A,C,F) f={(3+7)+4}=14
(A,C,H) f={(3+1)+9}=13
Expand E
(A,B,E,G) f= {(1+4+2)+0}=7
ABEG is best path with actual cost 7
f(n)=g(n)+h(n)

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Space Complexity O(bd )
Time Complexity O(bd )
Optimality Yes
Completeness Yes

6. Thanks For
Watching
Next Topic: Hill-climbing search
Reference:
Artificial Intelligence
A Modern Approach Third Edition
Peter Norvig and Stuart J. Russell
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