Constraint satisfaction search
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1 CS 70 Foundations of AI Lecture 6 Constraint satisfaction search Milos Hauskrecht milos@cs.pitt.edu 539 Sennott Square Search problem A search problem: Search space (or state space): a set of objects among which we conduct the search; Initial state: an object we start to search from; Operators (actions): transform one state in the search space to the other; Goal condition: describes the object we search for Possible metric on the search space: measures the quality of the object with respect to the goal
2 Constraint satisfaction problem (CSP) wo types of search: path search (a path from the initial state to a state satisfying the goal condition) configuration search (a configuration satisfying goal conditions) Constraint satisfaction problem (CSP) = a configuration search problem where: A state is defined by a set of variables and their values Goal condition is represented by a set constraints on possible variable values Special properties of the CSP lead to special search procedures we can design to solve them Eample of a CSP: N-queens Goal: n queens placed in non-attacking positions on the board ariables: Represent queens, one for each column:,, 3, alues: Row placement of each queen on the board {,, 3, }, Constraints: i j i j i j wo queens not in the same row wo queens not on the same diagonal
3 Satisfiability () problem Determine whether a sentence in the conjunctive normal form (CNF) is satisfiable (can evaluate to true) Used in the propositional logic (covered later) ( P R) ( P R S) ( P ) ariables: Propositional symbols (P, R,, S) alues: rue, False Constraints: Every conjunct must evaluate to true, at least one of the literals must evaluate to true ( P R) rue,( P R S) rue, Other real world CSP problems Scheduling problems: E.g. telescope scheduling High-school class schedule Design problems: Hardware configurations LSI design More comple problems may involve: real-valued variables additional preferences on variable assignments the optimal configuration is sought 3
4 Eercise: Map coloring problem Color a map using k different colors such that no adjacent countries have the same color ariables:? ariable values:? Constraints:? Map coloring Color a map using k different colors such that no adjacent countries have the same color ariables: Represent countries A, B, C, D, E alues: K -different colors {Red, Blue, Green,..} Constraints:?
5 Map coloring Color a map using k different colors such that no adjacent countries have the same color ariables: Represent countries A, B, C, D, E alues: K -different colors {Red, Blue, Green,..} Constraints: A B, A C, C E, etc his is an eample of a problem with binary constraints Constraint satisfaction as a search problem A formulation of the search problem: States. Assignment (partial or complete) of values to variables. Initial state. No variable is assigned a value. Operators. Assign a value to one of the unassigned variables. Goal condition. All variables are assigned, no constraints are violated. Constraints can be represented: Eplicitly by a set of allowable values Implicitly by a function that tests for the satisfaction of constraints 5
6 Search strategies for solving CSP Unassigned:,, 3, Assigned: Unassigned:, Assigned: 3, Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, Search strategies for solving CSP Maimum depth of the tree (m):? Depth of the solution (d) :? Branching factor (b) :? Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, 6
7 Search strategies for solving CSP Maimum depth of the tree: Number of variables in the CSP Depth of the solution: Number of variables in the CSP Branching factor: if we fi the order of variable assignments the branch factor depends on the number of their values Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, Search strategies for solving CSP What search algorithm to use:? Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, 7
8 Search strategies for solving CSP What search algorithm to use:? Depth of the tree = Depth of the solution=number of vars Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, Search strategies for solving CSP What search algorithm to use: Depth first search!!! Since we know the depth of the solution We do not have to keep large number of nodes in queues Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, 8
9 Search strategies for solving CSP What search algorithm to use: Depth first search!!! Since we know the depth of the solution We do not have to keep large number of nodes in queues Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Depth-first search strategy for CSP is also referred to as backtracking Unassigned: 3, Assigned:, Constraint consistency uestion: When to check the constraints defining the goal condition? he violation of constraints can be checked: at the end (for the leaf nodes) for each node of the search tree during its generation or before its epansion Checking the constraints for intermediate nodes: More efficient: cuts branches of the search tree early 9
10 Constraint consistency Checking the constraints for intermediate nodes: More efficient: cuts branches of the search tree early Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, Constraint consistency Checking the constraints for intermediate nodes: More efficient: cuts branches of the search tree early Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, 0
11 Constraint consistency Checking the constraints for intermediate nodes: More efficient: cuts branches of the search tree early Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, Constraint consistency Another way to cut the search space and tree eploration Current variable assignments together with constraints restrict remaining legal values of unassigned variables he remaining legal and illegal values of variables may be inferred (effect of constraints propagates) Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:,
12 Constraint consistency Another way to cut the search space and tree eploration Current variable assignments together with constraints restrict remaining legal values of unassigned variables he remaining legal and illegal values of variables may be inferred (effect of constraints propagates) Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: Unassigned: 3, Assigned:, We know that Constraint consistency Another way to cut the search space and tree eploration Current variable assignments together with constraints restrict remaining legal values of unassigned variables he remaining legal and illegal values of variables may be inferred (effect of constraints propagates) Unassigned:,, Assigned: 3, Unassigned:, 3, Assigned: Unassigned:, 3, Assigned: hen node representing Unassigned: 3,, is not Assigned: even generated, We know that
13 Constraint consistency Another way to cut the search space and tree eploration Current variable assignments together with constraints restrict remaining legal values of unassigned variables he remaining legal and illegal values of variables may be inferred (effect of constraints propagates) o prevent blind eploration we can keep track of the remaining legal values, so we know when the constraints will be violated and when to terminate the search Constraint propagation A state (more broadly) is defined: by a set of assigned variables, their values and a list of legal and illegal assignments for unassigned variables Legal and illegal assignments can be represented: equations (value assignments) and disequations (list of invalid assignments) A Blue C Red Constraints + assignments can entail new equations and disequations A Red B Red Constraint propagation: the process of inferring of new equations and disequations from eisting equations and disequations 3
14 Constraint propagation Assign A=Red A=Red A B C D E F Red Blue Green - equations - disequations Constraint propagation Assign A=Red Red Blue Green A B C D E F E= A=Red - equations - disequations
15 Constraint propagation Assign E=Blue Red Blue Green A B C D E F E=Blue A=Red Constraint propagation Assign E=Blue Red Blue Green F=? E=Blue A=Red A B C D E F 5
16 Constraint propagation Assign F=Green Red Blue Green A B C D E F F=Green E=Blue A=Red Constraint propagation Assign F=Green Red Blue Green A B C D E F F=Green B=? A=Red E=Blue 6
17 Constraint propagation Assign F=Green Red Blue Green A B C D E F F=Green B=? A=Red E=Blue Conflict!!! No legal assignments available for B and C Constraint propagation We can derive remaining legal values through propagation A B C D E F Red Blue Green B=Green C=Green C=? B=? E=Blue A=Red 7
18 Constraint propagation We can derive remaining legal values through propagation A B C D E F Red Blue Green F=? C=? E=Blue B=? A=Red B=Green C=Green F=Red Constraint propagation We can derive remaining legal values through propagation A B C D E F Red Blue Green E=Blue B=Green C=Green F=Red A=Red B=Green C=Green F=Red 8
19 Constraint propagation hree known techniques for propagating the effects of past assignments and constraints on options left for other (yet to be assigned) variables Node propagation Arc consistency Forward checking Difference: Completeness of inferences ime compleity of inferences. Constraint propagation. Node consistency. Infers: equations (valid assignments) or disequations (invalid assignments) for an individual variable by applying a unary constraint. Arc consistency. Infers: disequations from the set of equations and disequations defining the partial assignment, and a constraint equations through the ehaustion of alternatives 3. Forward checking. Infers: disequations from equations defining the partial assignment, and a constraint Equations through the ehaustion of alternatives Restricted forward checking: uses only active constraints (active constraint only one variable unassigned in the constraint) 9
20 Eample Map coloring of Australia territories N Eample: node consistency Map coloring N Assume a constraint: Green vars N domain R G B R G B R G B R G B R G B R G B R G B inferred??????? 0
21 Map coloring Assume a constraint: Green Eample: node consistency Infer: invalid assignments from Green constraint vars N N domain R G B R G B R G B R G B R G B R G B R G B inferred R B R G B R G B R G B R G B R G B R G B Eample: forward checking Map coloring N Set: =Red vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R??????
22 Eample: forward checking Map coloring N Infer: invalid assignments from =Red + constraints vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B Eample: forward checking Map coloring N Set: =Green vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R? G????
23 Eample: forward checking Map coloring N Infer: invalid assignments from =Green + constraints vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B Eample: forward checking Map coloring N Infer: N=B Ehaustions of alternatives vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B Infer N R B G???? 3
24 Eample: forward checking Map coloring N Infer: invalid assignments from N=B vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B Infer N R B G R B R G B! R G B Eample: arc consistency Map coloring N Set: =Red Set: =Green vars N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B
25 Eample: arc consistency Map coloring N vars {B} Infer: invalid assignments from valid and invalid assignments N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B {R,B} Map coloring vars Eample: arc consistency {B} Infer: invalid assignments from valid and invalid assignments N N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B {R,B} =B =R Consistent assignment 5
26 Map coloring vars Eample: arc consistency {B} Infer: invalid assignments from valid and invalid assignments N N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B {R,B} =R =B Consistent assignment Map coloring vars Eample: arc consistency {B} Infer: invalid assignments from valid and invalid assignments N N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B {R,B} =B =! Inconsistent assignment 6
27 Map coloring vars Eample: arc consistency {B} Infer: invalid assignments from valid and invalid assignments N N domain R G B R G B R G B R G B R G B R G B R G B =Red R G B R G B R G B R G B G B R G B =Green R B G R B R G B B R G B R {R,B} =B =! Inconsistent assignment Heuristics for CSPs CSP searches the space in the depth-first manner. But we still can choose: Which variable to assign net? Which value to choose first? Heuristics Most constrained variable Which variable is likely to become a bottleneck? Least constraining value Which value gives us more fleibility later? 7
28 Eample: map coloring Heuristics for CSP Heuristics Most constrained variable? Least constraining value? Eamples: map coloring Heuristics for CSP Heuristics Most constrained variable Country E is the most constrained one (cannot use Red, Green) Least constraining value? 8
29 Eamples: map coloring Heuristics for CSP Heuristics Most constrained variable Country E is the most constrained one (cannot use Red, Green) Least constraining value Assume we have chosen variable C What color is the least constraining color? Eamples: map coloring Heuristics for CSP Heuristics Most constrained variable Country E is the most constrained one (cannot use Red, Green) Least constraining value Assume we have chosen variable C Red is the least constraining valid color for the future 9
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