lecture 3 - Touring problems Start from Arad, visit each...

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Unformatted text preview: Touring problems Start from Arad, visit each city at least once. What is the state-space formulation? Start from Arad, visit each city exactly once. What is the state-space formulation? Two techniques for modeling constraints Invalidate states Precondition actions 1 2 Uninformed search strategies Uninformed search strategies use only the information available in the problem definition 3 Tree search algorithms Basic idea: offline, simulated exploration of state space by generating successors of already-explored states (a.k.a.~ expanding states) 4 Tree search example 5 Tree search example 6 Tree search example 7 Implementation: states vs. nodes A state is a (representation of) a physical configuration A node is a data structure constituting part of a search tree includes state , parent node , action , path cost g(x) , depth The Expand function creates new nodes, filling in the various fields and using the SuccessorFn of the problem to create the corresponding states. Two different nodes may refer to the same state (but on different paths)! 8 Implementation: general tree search 9 Breadth-first search Expand shallowest unexpanded node Implementation : fringe is a FIFO queue, i.e., new successors go at end 10 Breadth-first search Expand shallowest unexpanded node Implementation : fringe is a FIFO queue, i.e., new successors go at end 11 Breadth-first search Expand shallowest unexpanded node Implementation : fringe is a FIFO queue, i.e., new successors go at end 12 Breadth-first search Expand shallowest unexpanded node Implementation : fringe is a FIFO queue, i.e., new successors go at end 13 Repeated states Failure to detect repeated states can turn a linear problem into an exponential one!...
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lecture 3 - Touring problems Start from Arad, visit each...

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