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SP10 cs188 lecture 3 -- a-star search (2PP)

SP10 cs188 lecture 3 -- a-star search (2PP) - CS 188...

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1 CS 188: Artificial Intelligence Fall 2009 Lecture 3: A* Search 9/3/2009 Pieter Abbeel – UC Berkeley Many slides from Dan Klein Announcements square4 Assignments: square4 Project 0 (Python tutorial): due Thursday 1/28 square4 Written 1 (Search): due Thursday 1/28 square4 Project 1 (Search): to be released today, due Thursday 2/4 square4 You don’t need to submit answers to P1 discussion questions square4 5 slip days for projects; up to two per deadline square4 Try pair programming, not divide-and-conquer square4 Study materials square4 Slides, Section materials, Assignments square4 Book
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2 Office hours, Section square4 Drop-in lab times: Wed 1/26 4-5pm in 271 Soda square4 Office hours posted on the course website square4 Sections starting this week: square4 Working though exercises are key for your understanding square4 Section handout contains several exercises similar to written 1 square4 Solutions will be posted Wed 1pm (after last section) square4 Section 101: Tue 3-4pm square4 Section 104: Tue 4-5pm square4 Section 102: Wed 11-noon square4 Section 103: Wed noon-1pm Today square4 Iterative deepening square4 Uniform cost search square4 A* Search square4 Heuristic Design
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3 Recap: Search square4 Search problem: square4 States (configurations of the world) square4 Successor function: a function from states to lists of (state, action, cost) triples; drawn as a graph square4 Start state and goal test square4 Search tree: square4 Nodes: represent plans for reaching states square4 Plans have costs (sum of action costs) square4 Search Algorithm: square4 Systematically builds a search tree square4 Chooses an ordering of the fringe (unexplored nodes) DFS DFS and BFS Algorithm Complete Optimal Time Space DFS w/ Path Checking BFS Y N O( b m ) O( bm ) b 1 node b nodes b 2 nodes b m nodes s tiers Y N* O( b s+1 ) O( b s+1 ) b s nodes BFS m tiers
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4 Iterative Deepening Iterative deepening uses DFS as a subroutine: 1. Do a DFS which only searches for paths of length 1 or less. 2. If “1” failed, do a DFS which only searches paths of length 2 or less.
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