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Search Strategies and Constraint Satisfaction Search

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  1. Search 5주강의

  2. Generating action sequences • Generating a new set of states  expanding the state • Search strategy ::: the choice of which state to expand first • Search tree, Search graph • Data structures for search trees : Queue, Stack

  3. Search Strategies • Completeness ::: guaranteeing to find a solution where there is one • Time complexity • Space complexity • Optimality ::: to find the high quality solution ???

  4. Search Strategies • Uninformed (blind) search • Informed (heuristic) search

  5. Uninformed search I • Breadth-first search : branching factor : memory requirement • Uniform cost search : g(n). data structure : g(n) = depth(n)  breadth-first search

  6. Uninformed search II • Depth-first search : stack : ??? completeness  w+1 • Depth limit search • Iterative deepening search ::: depth를 증가하면서 depth limit search • Bi-directional Search

  7. 숙제 (기간:: 10월24일까지) • Missionary-cannibals 문제에 대해 (1) depth-first search로는 답을 구하지 못할 수 있음을 보여라 (2) depth-first search에 같은 sate가 다시 나타나면 제거하는 기능이 있으면 답을 구할 수 있다. 이를 프로그램하라. (3) breadth-first search로 구현하라. (4) iterative deepening 방법으로 구현하라. (5) (2), (3), (4)에 대해 space와 수행속도를 비교하라

  8. Comparing Search Strategies B: branching factor, d: depth of solution, m: maximum depth, l: depth limit

  9. Constraint Satisfaction Search • The states are defined by the values of a set of variables and the goal test specifies a set of constraints that must the values must obey • Cryptarithmetic, 8 queens problem, VLSI design • In CSP, the goal test is decomposed into a set of constraints on variables rather than being a blackbox

  10. Search in CSP • Depth-first search on a CSP wastes time searching when constraints have already been violated • Backtracking search • Forward checking ::: Each time a variable is instantiated, forward checking deletes from the domains of the as-yet-uninstantiated variables all of those values that conflict with the variables assigned • Constraints propagation

  11. 숙제 • 문제 3.2, 3.3, 3.5, 3.8, 3.11, 3.13, 3.14, 3.17, 3.20, 3.21

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