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1 Lecture 3: 18/4/1435 Uninformed search strategies Lecturer/ Kawther Abas k.albasheir@sau.edu.sa 363CS – Artificial Intelligence

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2 Uninformed search strategies Uninformed: While searching you have no clue whether one non-goal state is better than any other. Your search is blind. Various blind strategies: Breadth-first search Uniform-cost search Depth-first search Iterative deepening search

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3 Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a first-in-first-out (FIFO) queue, i.e., new successors go at end of the queue. Is A a goal state?

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4 Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end Expand: fringe = [B,C] Is B a goal state?

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5 Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end Expand: fringe=[C,D,E] Is C a goal state?

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6 Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end Expand: fringe=[D,E,F,G] Is D a goal state?

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7 Example BFS

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8 Properties of breadth-first search Complete? Yes it always reaches goal (if b is finite) Time? 1+b+b 2 +b 3 +… +b d + (b d+1 -b)) = O(b d+1 ) (this is the number of nodes we generate) Space? O(b d+1 ) (keeps every node in memory, either in fringe or on a path to fringe). Optimal? Yes (if we guarantee that deeper solutions are less optimal, e.g. step-cost=1). Space is the bigger problem (more than time)

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9 Depth-first search Expand deepest unexpanded node Implementation: fringe = Last In First Out (LIPO) queue, i.e., put successors at front Is A a goal state?

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10 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[B,C] Is B a goal state?

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11 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[D,E,C] Is D = goal state?

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12 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[H,I,E,C] Is H = goal state?

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13 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[I,E,C] Is I = goal state?

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14 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[E,C] Is E = goal state?

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15 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[J,K,C] Is J = goal state?

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16 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[K,C] Is K = goal state?

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17 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[C] Is C = goal state?

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18 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[F,G] Is F = goal state?

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19 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[L,M,G] Is L = goal state?

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20 Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front queue=[M,G] Is M = goal state?

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21 Example DFS

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22 Properties of depth-first search Complete? No: fails in infinite-depth spaces Can modify to avoid repeated states along path Time? O(b m ) with m=maximum depth terrible if m is much larger than d but if solutions are dense, may be much faster than breadth-first Space? O(bm), i.e., linear space! (we only need to remember a single path + expanded unexplored nodes) Optimal? No (It may find a non-optimal goal first)

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23 Iterative deepening search To avoid the infinite depth problem of DFS, we can decide to only search until depth L, i.e. we don’t expand beyond depth L. Depth-Limited Search What of solution is deeper than L? Increase L iteratively. Iterative Deepening Search As we shall see: this inherits the memory advantage of Depth-First search.

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24 Iterative deepening search L=0

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25 Iterative deepening search L=1

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26 Iterative deepening search lL=2

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27 Iterative deepening search lL=3

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28 Bi-directional search Alternate searching from the start state toward the goal and from the goal state toward the start. Stop when the frontiers intersect. Works well only when there are unique start and goal states. Requires the ability to generate “predecessor” states. Can (sometimes) lead to finding a solution more quickly.

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29 Simulated annealing search Idea: escape local maxima by allowing some "bad" moves but gradually decrease their frequency Properties One can prove: If T decreases slowly enough, then simulated annealing search will find a global optimum with probability approaching 1 Widely used in VLSI layout, airline scheduling, etc

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30 Local search algorithms In many optimization problems, the path to the goal is irrelevant; the goal state itself is the solution State space = set of "complete" configurations Find configuration satisfying constraints, e.g., n-queens In such cases, we can use local search algorithms keep a single "current" state, try to improve it. Very memory efficient (only remember current state)

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