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AI Spring 2010 Lecture 3-5

AI Spring 2010 Lecture 3-5 - Artificial Intelligence...

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Click to edit Master subtitle style Artificial Intelligence Lectures 3-5: Problem Solving and Search Spring 2010 Instructor: Paul S. Rosenbloom
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22 This 1.5 Week’s Lectures £ Problem solving £ Search £ Blind search r Search strategies r With repeated states £ Knowledge in Search l How can knowledge help search? l Best-first search and its variants l More on heuristic functions
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33 Problem Solving in AI £ Fundamentally about achieving a goal l Being rich l Winning a game of chess l Getting to a particular location l Saving people from a burning building l Getting an A in this course £ May take on a variety of variant formulations l Find any state that matches goal l Find all states that match goal l Prove that no state matches goal l Find best state that matches goal l Find best path to a goal state
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44 Core Activity Determine Action to Perform Next £ May have knowledge that tells you what to do l This is the predominant mode for human experts l They directly “see” what to do next l But will look at knowledge later in course £ Can search over sequences of actions to find one that achieves goal and then pick first action in sequence l Search is fundamentally a response to a lack of knowledge l But it itself requires knowledge in the form of a world model o Could search in world instead of in a model, but this essentially involves making a random next move Generally referred to as exploration or online search Can lead to learning a world model for later use in internal search
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55 Problem Formulation £ Given goal, first step is to formulate the problem for search £ First Step: Specify an appropriate world model as a problem space comprising l A set of states representing situations l A set of actions which move among states Then will need to instantiate problem in space
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66 Route Finding in Romania Traveling Salesman Problem is one variation where must visit each city exactly once and want lowest cost path
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77 Specifying States £ Actions must be specified explicitly, but states may be specified l Explicitly as a list l Implicitly as a cross-product of features l UB: [((B, W) F (P, R, N, B, K, Q)) + Blank]Sq = 1364 (~1071) l Implicitly via an initial state and a set of actions
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88 Instantiating Problem in Space £ Given problem space, second step is to instantiate problem in space l Current/initial state representing starting state for problem solving l Goal state(s) representing situations that meet goal l Can be explicit as a list of one or more states l Can be implicit as a binary test on states Checkmate: King under attack and with no ability to move to a safe square, block attack or take attacker
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99 Eight Puzzle £ Initial State: A configuration of Tiles £ Goal State: Another configuration £ Operators: Slide a tile into space l Or “move” space l Can actually only reach half of all configurations £ Search spaces get dramatically larger as get to fifteen puzzle, twenty four puzzle, etc.
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1010 Formulating the Mutilated Checkerboard £ Can formulate as: l
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