iterative deeping.pdf - Depth-first Search 5 Iterative...

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1 1 5. Iterative Deepening and Move Ordering Jonathan Schaeffer [email protected] 9/9/02 2 Depth-first Search How do you know how deep to search? Search to terminal nodes? Could be too deep! Search to a fixed depth? Bad move ordering could make this a big search What if the search depth is set too large? What if the search depth is set too small? 9/9/02 3 Iterative Deepening Iterate on the search depth Search to depth 1, then 2, then 3, until resources run out The advantage is that you get the deepest possible search depth given the resource constraints Nice property for real-time search [1] The disadvantage is wasted search 9/9/02 4 Disadvantage: Extra Search? ID seems impractical because of all the repeated work: Assume a growth rate of b and search depth d Size = b + b 2 + b 3 + … + b d-1 + b d Aren’t all the early iterations wasted search?
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2 9/9/02 5 Advantages! Before searching to depth d+1 , order the moves at the root based on the scores returned from depth d This assumes that the best move at depth d is a good predictor of the best move at depth d+1 For most domains, this is true Increases the likelihood that the best move is searched first on the last (and most expensive) iteration 9/9/02 6 Example A 15 B 20 C 13 Depth d B 30 A 16 C 19 Depth d+1
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  • Fall '17
  • abc
  • Depth-first search, Iterative deepening depth-first search, Extra Search Revisited

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