Lecture-03-04-Uninformed_Search

Finding a solution solution is a sequence of

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Finding a solution Solution: is a sequence of operators that bring you from current state to the goal state. Strategy: The search strategy is determined by ??? CS561 - Lectures 3-4 - Macskassy - Fall 2010 function T REE -S EARCH ( problem , strategy ) returns a solution , or failure initialize the search tree using the initial state problem loop do if there are no candidates for expansion then return failure choose a leaf node for expansion according to strategy if the node contains a goal state then return the corresponding solution else expand the node and add resulting nodes to the search tree end
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Finding a solution Solution: is a sequence of operators that bring you from current state to the goal state. Strategy: The search strategy is determined by the order in which the nodes are expanded. CS561 - Lectures 3-4 - Macskassy - Fall 2010 function T REE -S EARCH ( problem , strategy ) returns a solution , or failure initialize the search tree using the initial state problem loop do if there are no candidates for expansion then return failure choose a leaf node for expansion according to strategy if the node contains a goal state then return the corresponding solution else expand the node and add resulting nodes to the search tree end
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Example: Traveling from Arad To Bucharest CS561 - Lectures 3-4 - Macskassy - Fall 2010
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Search example CS561 - Lectures 3-4 - Macskassy - Fall 2010
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Search example CS561 - Lectures 3-4 - Macskassy - Fall 2010
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Search example CS561 - Lectures 3-4 - Macskassy - Fall 2010
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State Graphs vs. Search Trees CS561 - Lectures 3-4 - Macskassy - Fall 2010 b a d s p c e q h r f G s d e p q h r f c G a p q q b a c a e h r f c G a p q q Each NODE in the search tree is an entire PATH in the state graph (note how many nodes in the graph appear multiple times in the search tree) We almost always construct both on demand and we construct as little as possible
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Encapsulating state information in nodes CS561 - Lectures 3-4 - Macskassy - Fall 2010
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Implementation: General Search CS561 - Lectures 3-4 - Macskassy - Fall 2010
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CS561 - Lectures 3-4 - Macskassy - Fall 2010 Implementation: General Search The operations on fringe are queuing functions which inserts and removes elements into the fringe and determines the order of node expansion . Varieties of the queuing functions produce varieties of the search algorithm.
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Evaluation of search strategies A search strategy is defined by picking the order of node expansion. Search algorithms are commonly evaluated according to the following four criteria: Completeness: does it always find a solution if one exists? Time complexity: how long does it take as function of num. of nodes? Space complexity: how much memory does it require? Optimality: does it guarantee the least-cost solution? Time and space complexity are measured in terms of: b max branching factor of the search tree d depth of the least-cost solution m max depth of the search tree (may be infinity) CS561 - Lectures 3-4 - Macskassy - Fall 2010
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Uninformed search strategies Use only information available in the problem formulation Breadth-first Uniform-cost Depth-first Depth-limited Iterative deepening CS561 - Lectures 3-4 - Macskassy - Fall 2010
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