1. 1 Chapter 4 Search Methodologies

2. 2 Chapter 4 Contents l Brute force search l Depth-first search l Breadth-first search l Properties of search methods l Implementations l Depth first iterative deepening l Heuristics

3. 3 Chapter 4 Contents Continued l Hill climbing l Best first search l Beam search l Optimal paths l A* algorithms l Uniform cost search l Greedy search

4. 4 Brute Force Search l Search methods that examine every node in the search tree – also called exhaustive. l Generate and test is the simplest brute force search method: nGenerate possible solutions to the problem. nTest each one in turn to see if it is a valid solution. nStop when a valid solution is found. l The method used to generate possible solutions must be carefully chosen.

5. 5 Depth-First Search l An exhaustive search method. l Follows each path to its deepest node, before backtracking to try the next path.

6. 6 Breadth-First Search l An exhaustive search method. l Follows each path to a given depth before moving on to the next depth.

7. 7 Comparison of Depth-First and Breadth-First Search l It is important to choose the correct search method for a given problem. l In some situations, for example, depth first search will never find a solution, even though one exists.

8. 8 Properties of Search Methods l Complexity nHow much time and memory the method uses. l Completeness nA complete method is one that is guaranteed to find a goal if one exists. l Optimality & Admissibility nA method is optimal (or admissible) if it is guaranteed to find the best path to the goal. l Irrevocability nA method is irrevocable if, like hill climbing, it does not ever back-track.

9. 9 Implementations l Depth-first search can be implemented using a queue to store states nBut a stack makes more sense nAnd enables us to create a recursive depth-first search function l Breadth-first search implementations are almost identical to depth-first nExcept that they place states on the back of the queue instead of on the front.

10. 10 Depth-First Iterative Deepening l An exhaustive search method based on both depth-first and breadth-first search. l Carries out depth-first search to depth of 1, then to depth of 2, 3, and so on until a goal node is found. l Efficient in memory use, and can cope with infinitely long branches. l Not as inefficient in time as it might appear, particularly for very large trees, in which it only needs to examine the largest row (the last one) once.

11. 11 Heuristics l Heuristic: a rule or other piece of information that is used to make methods such as search more efficient or effective. l In search, often use a heuristic evaluation function, f(n): nf(n) tells you the approximate distance of a node, n, from a goal node. l f(n) may not be 100% accurate, but it should give better results than pure guesswork.

12. 12 Heuristics – how informed? l The more informed a heuristic is, the better it will perform. l Heuristic h is more informed than j, if: h(n)  j(n) for all nodes n. l A search method using h will search more efficiently than one using j. l A heuristic should reduce the number of nodes that need to be examined.

13. 13 Hill Climbing l An informed, irrevocable search method. l Easiest to understand when considered as a method for finding the highest point in a three dimensional search space: l Check the height one foot away from your current location in each direction; North, South, East and West. l As soon as you find a position whose height is higher than your current position, move to that location, and restart the algorithm.

14. 14 Foothills nCause difficulties for hill-climbing methods. nA foothill is a local maximum.

15. 15 Plateaus nCause difficulties for hill-climbing methods. nFlat areas that make it hard to find where to go next.

16. 16 Ridges nCause difficulties for hill-climbing methods nB is higher than A. n At C, the hill- climber can’t find a higher point North, South, East or West, so it stops.

17. 17 Best-First Search l Works rather like hill-climbing: l Picks the most likely path (based on heuristic value) from the partially expanded tree at each stage. l Tends to find a shorter path than depth- first or breadth-first search, but does not guarantee to find the best path.

18. 18 Beam Search l Breadth-first method. l Only expands the best few paths at each level. l Thus has the memory advantages of depth-first search. l Not exhaustive, and so may not find the best solution. l May not find a solution at all.

19. 19 Optimal Paths l An optimal path through a tree is one that is the shortest possible path from root to goal. l In other words, an optimal path has the lowest cost (not necessarily at the shallowest depth, if edges have costs associated with them). l The British Museum procedure finds an optimal path by examining every possible path, and selecting the one with the least cost. l There are more efficient ways.

20. 20 A* Algorithms l Uses the cost function: nf(node) = g(node) + h(node). l g(node) is the cost of the path so far leading to the node. l h(node) is an underestimate of how far node is from a goal state. l f is a path-based evaluation function. l An A* algorithm expands paths from nodes that have the lowest f value.

21. 21 A* Algorithms (continued) l A* algorithms are optimal: nThey are guaranteed to find the shortest path to a goal node. l Provided h(node) is always an underestimate. (h is an admissible heuristic). l A* methods are also optimally efficient – they expand the fewest possible paths to find the right one. l If h is not admissible, the method is called A, rather than A*.

22. 22 Uniform Cost Search l Also known as branch and bound. l Like A*, but uses: nf(node) = g(node). ng(node) is the cost of the path leading up to node. l Once a goal node is found, the method needs to continue to run in case a preferable solution is found.

23. 23 Greedy Search l Like A*, but uses: nf(node) = h(node). l Hence, always expands the node that appears to be closest to a goal, regardless of what has gone before. l Not optimal, and not guaranteed to find a solution at all. l Can easily be fooled into taking poor paths.