MIT6_01F09_lec13

MIT6_01F09_lec13 - 6.01 Introduction to EECS 1 Week 13...

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Unformatted text preview: 6.01: Introduction to EECS 1 Week 13 December 1, 2009 1 6.01: Introduction to EECS I Optimal Search Algorithms Week 13 December 1, 2009 The story so far • Search domain — characterized by successors function, legal actions, start state, goal function. • Search tree — an explicit representation for the search space. • Depth-first search — explore search tree by expanding deepest node. • Breadth-first search — explore search tree by expanding shal- lowest node. • Dynamic programming — do not revisit nodes. Cost In many applications, actions have different costs, for example, dis- tance between cities can vary. S H B G F D A C E 2 1 3 2 4 1 1 6 2 2 3 4 Our algorithms thus far ignore this. Cost map1dist = {’S’ : [(’A’, 2), (’B’, 1)], ’A’ : [(’S’, 2), (’C’, 3), (’D’, 2)], ’B’ : [(’S’, 1), (’D’, 2), (’E’, 3)], ’C’ : [(’A’, 3), (’F’, 1)], ’D’ : [(’A’, 2), (’B’, 2), (’F’, 4), (’H’, 6)], ’E’ : [(’B’, 3), (’H’, 2)], ’F’ : [(’C’, 1), (’D’, 4), (’G’, 1)], ’H’ : [(’D’, 6), (’E’, 2), (’G’, 4)], ’G’ : [(’F’, 1), (’H’, 4)]} Path cost is the sum of the action costs along a path. Breadth-First Search Enumerates all length 1 paths, then length 2 paths, then length 3 paths, etc. S H B G F D A C E S B F D A C E D B F H A F H H 1 1 2 1 1 2 3 1 2 3 1 2 D G 1 2 Uniform-Cost Search Enumerate paths in order of their total path cost . Like breadth-first search, but: • The agenda is a priority queue (returns least cost entry)....
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MIT6_01F09_lec13 - 6.01 Introduction to EECS 1 Week 13...

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