Lecture06HO - CS440/ECE448: Intro to Articial Intelligence!...

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Lecture 6: More on constraint satisfaction problems Prof. Julia Hockenmaier juliahmr@illinois.edu http://cs.illinois.edu/fa11/cs440 CS440/ECE448: Intro to ArtiFcial Intelligence Tuesday ʼ s key concepts Constraint satisfaction problems: Given: a set of n variables X 1 ..X n , with domains (sets of possible values) D 1 …D n , and a set of m constraints C 1 … C m Task: assign a value from D i for each X i subjects to the constraints. CSP 1: Map coloring (Binary constraints) Map coloring: a solution for N=3 4 CS440/ECE448: Intro AI
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Constraint satisfaction problems are deFned by… - a set of variables X : {WA, NT, QLD, NSW, VA, SA, TA} - a set of domains D i (possible values for variable x i ): D WA = {red, blue, green} - a set of constraints C : { ʪ (WA,NT) , WA NT ʫ , (WA,QLD), WA QLD ,…} scope relation 5 CS440/ECE448: Intro AI States and solutions Each state is a complete or partial assignment of values to variables: state35 = {WA=red, NT=blue, QLD= green, NSW= red, VA= green, SA= blue, TA= red}; state23 = {WA = red} Legal assignments don ʼ t violate any constraints. Solutions are complete legal assignments 6 CS440/ECE448: Intro AI Binary constraints: constraint graph QLD NT NSW VA TA SA WA CS440/ECE448: Intro AI 7 Consistency Node consistency: X is node-consistent iff each element in D X satisFes unary constraints on X Arc consistency: X is arc-consistent iff for each C(X, Y) and for each x ˥ D X there is a y ˥ D Y such that the assignment {X=x, Y=y} satisFes C(X,Y). Path consistency: {X,Y} are path consistent wrt. Z iff for every x ˥ D X and y ˥ D X there is a z ˥ D Z such that the assignment {X=x,Y=y,Z=z} satisFes C(X,Z) and C(Y,Z) 8 CS440/ECE448: Intro AI
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AC-3 // Is the CSP c arc-consistent? function AC3(CSP c) input: CSP c = (X,D,C) local: queue q ˡ all arcs C(X,Y) in c while q () do: // Can C(X,Y) be satisfied? (X,Y) = pop(q);
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This note was uploaded on 10/13/2011 for the course CS 440 taught by Professor Levinson,s during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lecture06HO - CS440/ECE448: Intro to Articial Intelligence!...

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