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Unformatted text preview: 1 CMPSCI 383 September 20, 2011 Informed (Heuristic) Search 2 Tip for doing well Begin Assignments Early 3 Today’s lecture Informed (Heuristic) search methods • BestFirst Search • Greedy BestFirst Search • A* search • Heuristic functions 4 Review: Uninformed search strategies • Breadthfirst search • Uniformcost search • Depthfirst search • Depthlimited search • Iterative deepening depthfirst search 5 Review 6 Review • Breadthfirst search • Selects shallowest unexpanded node • FIFO queue • Uniformcost search • Selects node with lowest path cost • Priority queue by path cost • Depthfirst search • Selects deepest unexpanded node • LIFO queue (stack) • Depthlimited search • Depthfirst with nodes at depth limit treated as having no successors • Iterative deepening depthfirst search • Repeated depthlimited with increasing limit until goal found Note: step costs assumed nonnegative 7 Review: How do you evaluate a search strategy? • Completeness — Does it always find a solution if one exists? • Optimality — Does it find the best solution? • Time complexity • Space complexity 8 Informed (heuristic) search strategies Use problemspecific knowledge beyond what is given in the problem definition 9 BestFirst Search • Idea : use an evaluation function f(n) • For each node, gives an estimate of "desirability” Expand most desirable unexpanded node • Implementation : Order the nodes in fringe in decreasing order of desirability • Special cases : • greedy bestfirst search • A * search 10 Heuristic function • heuristic function h(n) — estimated cost of the cheapest path form the state at node n to a goal state. • “Heuristic” • “proceeding to a solution by trial and error or by rules that are only loosely defined.” * • “a technique designed to solve a problem that ignores whether the solution can be proven to be correct, but which usually produces a good solution or solves a simpler problem that...
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This note was uploaded on 11/29/2011 for the course COMPSCI 383 taught by Professor Andrewbarto during the Fall '11 term at UMass (Amherst).
 Fall '11
 AndrewBarto
 Artificial Intelligence

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