**Informed search algorithms**Chapter 4**Material**• Chapter 4 Section 1 - 3 • Exclude memory-bounded heuristic search**Outline**• Best-first search • Greedy best-first search • A* search • Heuristics • Local search algorithms • Hill-climbing search • Simulated annealing search • Local beam search • Genetic algorithms**Review: Tree search**• \input{\file{algorithms}{tree-search-short-algorithm}} • A search strategy is defined by picking the order of node expansion**Best-first search**• Idea: use an evaluation functionf(n) for each node • estimate of "desirability" • Expand most desirable unexpanded node • Implementation: Order the nodes in fringe in decreasing order of desirability • Special cases: • greedy best-first search • A* search**Greedy best-first search**• Evaluation function f(n) = h(n) (heuristic) • = estimate of cost from n to goal • e.g., hSLD(n) = straight-line distance from n to Bucharest • Greedy best-first search expands the node that appears to be closest to goal**Properties of greedy best-first search**• Complete? No – can get stuck in loops, e.g., Iasi Neamt Iasi Neamt • Time?O(bm), but a good heuristic can give dramatic improvement • Space?O(bm) -- keeps all nodes in memory • Optimal? No**A* search**• Idea: avoid expanding paths that are already expensive • Evaluation function f(n) = g(n) + h(n) • g(n) = cost so far to reach n • h(n) = estimated cost from n to goal • f(n) = estimated total cost of path through n to goal**Admissible heuristics**• A heuristic h(n) is admissible if for every node n, h(n) ≤ h*(n), where h*(n) is the true cost to reach the goal state from n. • An admissible heuristic never overestimates the cost to reach the goal, i.e., it is optimistic • Example: hSLD(n) (never overestimates the actual road distance) • Theorem: If h(n) is admissible, A* using TREE-SEARCH is optimal**Optimality of A* (proof)**• Suppose some suboptimal goal G2has been generated and is in the fringe. Let n be an unexpanded node in the fringe such that n is on a shortest path to an optimal goal G. • f(G2) = g(G2) since h(G2) = 0 • g(G2) > g(G) since G2 is suboptimal • f(G) = g(G) since h(G) = 0 • f(G2) > f(G) from above**Optimality of A* (proof)**• Suppose some suboptimal goal G2has been generated and is in the fringe. Let n be an unexpanded node in the fringe such that n is on a shortest path to an optimal goal G. • f(G2) > f(G) from above • h(n) ≤ h^*(n) since h is admissible • g(n) + h(n) ≤ g(n) + h*(n) • f(n) ≤ f(G) Hence f(G2) > f(n), and A* will never select G2 for expansion**Consistent heuristics**• A heuristic is consistent if for every node n, every successor n' of n generated by any action a, h(n) ≤ c(n,a,n') + h(n') • If h is consistent, we have f(n') = g(n') + h(n') = g(n) + c(n,a,n') + h(n') ≥ g(n) + h(n) = f(n) • i.e., f(n) is non-decreasing along any path. • Theorem: If h(n) is consistent, A* using GRAPH-SEARCH is optimal**Optimality of A***• A* expands nodes in order of increasing f value • Gradually adds "f-contours" of nodes • Contour i has all nodes with f=fi, where fi < fi+1**Properties of A$^*$**• Complete? Yes (unless there are infinitely many nodes with f ≤ f(G) ) • Time? Exponential • Space? Keeps all nodes in memory • Optimal? Yes**Admissible heuristics**E.g., for the 8-puzzle: • h1(n) = number of misplaced tiles • h2(n) = total Manhattan distance (i.e., no. of squares from desired location of each tile) • h1(S) = ? • h2(S) = ?**Admissible heuristics**E.g., for the 8-puzzle: • h1(n) = number of misplaced tiles • h2(n) = total Manhattan distance (i.e., no. of squares from desired location of each tile) • h1(S) = ? 8 • h2(S) = ? 3+1+2+2+2+3+3+2 = 18**Dominance**• If h2(n) ≥ h1(n) for all n (both admissible) • then h2dominatesh1 • h2is better for search • Typical search costs (average number of nodes expanded): • d=12 IDS = 3,644,035 nodes A*(h1) = 227 nodes A*(h2) = 73 nodes • d=24 IDS = too many nodes A*(h1) = 39,135 nodes A*(h2) = 1,641 nodes**Relaxed problems**• A problem with fewer restrictions on the actions is called a relaxed problem • The cost of an optimal solution to a relaxed problem is an admissible heuristic for the original problem • If the rules of the 8-puzzle are relaxed so that a tile can move anywhere, then h1(n) gives the shortest solution • If the rules are relaxed so that a tile can move to any adjacent square, then h2(n) gives the shortest solution**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**Example: n-queens**• Put n queens on an n × n board with no two queens on the same row, column, or diagonal**Hill-climbing search**• "Like climbing Everest in thick fog with amnesia"**Hill-climbing search**• Problem: depending on initial state, can get stuck in local maxima**Hill-climbing search: 8-queens problem**• h = number of pairs of queens that are attacking each other, either directly or indirectly • h = 17 for the above state**Hill-climbing search: 8-queens problem**• A local minimum with h = 1**Simulated annealing search**• Idea: escape local maxima by allowing some "bad" moves but gradually decrease their frequency**Properties of simulated annealing search**• 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**Local beam search**• Keep track of k states rather than just one • Start with k randomly generated states • At each iteration, all the successors of all k states are generated • If any one is a goal state, stop; else select the k best successors from the complete list and repeat.**Genetic algorithms**• A successor state is generated by combining two parent states • Start with k randomly generated states (population) • A state is represented as a string over a finite alphabet (often a string of 0s and 1s) • Evaluation function (fitness function). Higher values for better states. • Produce the next generation of states by selection, crossover, and mutation**Genetic algorithms**• Fitness function: number of non-attacking pairs of queens (min = 0, max = 8 × 7/2 = 28) • 24/(24+23+20+11) = 31% • 23/(24+23+20+11) = 29% etc