1	Solving problems by searching Chapter 3
2	Outline Problem-solving agents Problem types Problem formulation Example problems Basic search algorithms
3	Problem-solving agents
4	Example: Romania On holiday in Romania; currently in Arad. Flight leaves tomorrow from Bucharest Formulate goal: be in Bucharest Formulate problem: states: various cities actions: drive between cities Find solution: sequence of cities, e.g., Arad, Sibiu, Fagaras, Bucharest
5	Example: Romania
6	Problem types Deterministic, fully observable à single-state problem Agent knows exactly which state it will be in; solution is a sequence Non-observable à sensorless problem (conformant problem) Agent may have no idea where it is; solution is a sequence Nondeterministic and/or partially observable à contingency problem percepts provide new information about current state often interleave} search, execution Unknown state space à exploration problem
7	Example: vacuum world Single-state, start in #5. Solution?
8	Example: vacuum world Single-state, start in #5. Solution? [Right, Suck] Sensorless, start in {1,2,3,4,5,6,7,8} e.g., Right goes to {2,4,6,8} Solution?
9	Example: vacuum world Sensorless, start in {1,2,3,4,5,6,7,8} e.g., Right goes to {2,4,6,8} Solution? [Right,Suck,Left,Suck] Contingency Nondeterministic: Suck may dirty a clean carpet Partially observable: location, dirt at current location. Percept: [L, Clean], i.e., start in #5 or #7 Solution?
10	Example: vacuum world Sensorless, start in {1,2,3,4,5,6,7,8} e.g., Right goes to {2,4,6,8} Solution? [Right,Suck,Left,Suck] Contingency Nondeterministic: Suck may dirty a clean carpet Partially observable: location, dirt at current location. Percept: [L, Clean], i.e., start in #5 or #7 Solution? [Right, if dirt then Suck]
11	Single-state problem formulation A problem is defined by four items: initial state e.g., "at Arad" actions or successor function S(x) = set of action–state pairs e.g., S(Arad) = {<Arad à Zerind, Zerind>, … } goal test, can be explicit, e.g., x = "at Bucharest" implicit, e.g., Checkmate(x) path cost (additive) e.g., sum of distances, number of actions executed, etc. c(x,a,y) is the step cost, assumed to be ≥ 0 A solution is a sequence of actions leading from the initial state to a goal state
12	Selecting a state space Real world is absurdly complex à state space must be abstracted for problem solving (Abstract) state = set of real states (Abstract) action = complex combination of real actions e.g., "Arad à Zerind" represents a complex set of possible routes, detours, rest stops, etc. For guaranteed realizability, any real state "in Arad“ must get to some real state "in Zerind" (Abstract) solution = set of real paths that are solutions in the real world Each abstract action should be "easier" than the original problem
13	Vacuum world state space graph states? actions? goal test? path cost?
14	Vacuum world state space graph states? integer dirt and robot location actions? Left, Right, Suck goal test? no dirt at all locations path cost? 1 per action
15	Example: The 8-puzzle states? actions? goal test? path cost?
16	Example: The 8-puzzle states? locations of tiles actions? move blank left, right, up, down goal test? = goal state (given) path cost? 1 per move [Note: optimal solution of n-Puzzle family is NP-hard]
17	Example: robotic assembly states?: real-valued coordinates of robot joint angles parts of the object to be assembled actions?: continuous motions of robot joints goal test?: complete assembly path cost?: time to execute
18	Tree search algorithms Basic idea: offline, simulated exploration of state space by generating successors of already-explored states (a.k.a.~expanding states)
19	Tree search example
20	Tree search example
21	Tree search example
22	Implementation: general tree search
23	Implementation: states vs. nodes A state is a (representation of) a physical configuration A node is a data structure constituting part of a search tree includes state, parent node, action, path cost g(x), depth The Expand function creates new nodes, filling in the various fields and using the SuccessorFn of the problem to create the corresponding states.
24	Search strategies A search strategy is defined by picking the order of node expansion Strategies are evaluated along the following dimensions: completeness: does it always find a solution if one exists? time complexity: number of nodes generated space complexity: maximum number of nodes in memory optimality: does it always find a least-cost solution? Time and space complexity are measured in terms of b: maximum branching factor of the search tree d: depth of the least-cost solution m: maximum depth of the state space (may be ∞)
25	Uninformed search strategies Uninformed search strategies use only the information available in the problem definition Breadth-first search Uniform-cost search Depth-first search Depth-limited search Iterative deepening search
26	Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end
27	Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end
28	Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end
29	Breadth-first search Expand shallowest unexpanded node Implementation: fringe is a FIFO queue, i.e., new successors go at end
30	Properties of breadth-first search Complete? Yes (if b is finite) Time? 1+b+b²+b³+… +b^d + b(b^d-1) = O(b^d+1) Space? O(b^d+1) (keeps every node in memory) Optimal? Yes (if cost = 1 per step) Space is the bigger problem (more than time)
31	Uniform-cost search Expand least-cost unexpanded node Implementation: fringe = queue ordered by path cost Equivalent to breadth-first if step costs all equal Complete? Yes, if step cost ≥ ε Time? # of nodes with g ≤ cost of optimal solution, O(b^ceiling(C/^ε⁾) where C^ is the cost of the optimal solution Space? # of nodes with g ≤ cost of optimal solution, O(b^ceiling(C*/^ε⁾) Optimal? Yes – nodes expanded in increasing order of g(n)
32	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
33	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
34	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
35	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
36	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
37	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
38	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
39	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
40	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
41	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
42	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
43	Depth-first search Expand deepest unexpanded node Implementation: fringe = LIFO queue, i.e., put successors at front
44	Properties of depth-first search Complete? No: fails in infinite-depth spaces, spaces with loops Modify to avoid repeated states along path à complete in finite spaces Time? O(b^m): 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! Optimal? No
45	Depth-limited search = depth-first search with depth limit l, i.e., nodes at depth l have no successors Recursive implementation:
46	Iterative deepening search
47	Iterative deepening search l =0
48	Iterative deepening search l =1
49	Iterative deepening search l =2
50	Iterative deepening search l =3
51	Iterative deepening search Number of nodes generated in a depth-limited search to depth d with branching factor b: N_DLS = b⁰ + b¹ + b² + … + b^d-2 + b^d-1 + b^d Number of nodes generated in an iterative deepening search to depth d with branching factor b: N_IDS = (d+1)b⁰ + d b^¹ + (d-1)b^² + … + 3b^d-2 +2b^d-1 + 1b^d For b = 10, d = 5, N_DLS= 1 + 10 + 100 + 1,000 + 10,000 + 100,000 = 111,111 N_IDS = 6 + 50 + 400 + 3,000 + 20,000 + 100,000 = 123,456 Overhead = (123,456 - 111,111)/111,111 = 11%
52	Properties of iterative deepening search Complete? Yes Time? (d+1)b⁰ + d b¹ + (d-1)b² + … + b^d = O(b^d) Space? O(bd) Optimal? Yes, if step cost = 1
53	Summary of algorithms
54	Repeated states Failure to detect repeated states can turn a linear problem into an exponential one!
55	Graph search
56	Summary Problem formulation usually requires abstracting away real-world details to define a state space that can feasibly be explored Variety of uninformed search strategies Iterative deepening search uses only linear space and not much more time than other uninformed algorithms