Introduction to Constraint Satisfaction Problems (CSPs) in Search Algorithms

1. Lecture 01 – Part CConstraint Satisfaction Problems Dr. Shazzad Hosain Department of EECS North South Universtiy shazzad@northsouth.edu

2. Constraint Satisfaction Problems (CSPs) • Standard search problem: • state is a "black box“ – any data structure that supports successor function, heuristic function, and goal test • CSP: • state is defined by variablesXi with values from domainDi • goal test is a set of constraints specifying allowable combinations of values for subsets of variables

3. Constraint satisfaction problem • A CSP is defined by • a set of variables • a domain of possible values for each variable • a set of constraints between variables • An assignment that does not violate any constraints is called a consistent or legal CONSISTENT assignment. • A complete assignment is one in which every variable is mentioned. • A solution to a CSP is • A complete assignment that satisfies all the constraints

4. Example: Map-Coloring • VariablesWA, NT, Q, NSW, V, SA, T • DomainsDi = {red, green, blue} • Constraints: adjacent regions must have different colors • e.g., WA ≠ NT

5. Example: Map-Coloring • Solutions are complete and consistent assignments, e.g., WA = red, NT = green, Q = red, NSW = green, V = red, SA = blue, T = green • A state may be incomplete e.g., just WA=red

6. T NSW WA NT SA Q V Constraint graph • It is helpful to visualize a CSP as a constraint graph • Binary CSP: each constraint relates two variables • Constraint graph: nodes are variables, arcs are constraints

7. Varieties of CSPs • Discrete variables • finite domains: • n variables, domain size d  O(dn) complete assignments • e.g., Boolean CSPs, incl. Boolean satisfiability (NP-complete) • infinite domains: • integers, strings, etc. • e.g., job scheduling, variables are start/end days for each job • need a constraint language, e.g., StartJob1 + 5 ≤ StartJob3 • Continuous variables • e.g., start/end times for Hubble Space Telescope observations • linear constraints solvable in polynomial time by linear programming

8. Varieties of constraints • Unary constraints involve a single variable, • e.g., SA ≠ green • Binary constraints involve pairs of variables, • e.g., SA ≠ WA • Higher-order constraints involve 3 or more variables, • e.g., cryptarithmetic column constraints

9. Constraint Satisfaction problem Backtracking Search

10. NxD WA WA WA NT T [NxD]x[(N-1)xD] WA NT WA NT WA NT NT WA Standard search formulation • Let’s try the standard search formulation. • We need: • Initial state: none of the variables has a value (color) • Successor state: one of the variables without a value will get some value. • Goal: all variables have a value and none of the constraints is violated. N layers N! x DN Equal! There are N! x DN nodes in the tree but only DN distinct states??

11. Backtracking search • Every solution appears at depth n with n variables use depth-first search • Depth-first search for CSPs with single-variable assignments is called backtracking search • Backtracking search is the basic uninformed algorithm for CSPs • Can solve n-queens for n ≈ 25

12. NT WA WA NT = Backtracking (Depth-First) search • Special property of CSPs: They are commutative: • This means: the order in which we assign variables • does not matter. • Better search tree: First order variables, then assign them values one-by-one. D WA WA WA WA NT D2 WA NT WA NT DN

13. Backtracking search

14. Backtracking example

15. Backtracking example

16. Backtracking example

17. Backtracking example

18. G F A B C E N D H I J K L M O Depth First Search (DFS) • Application: • Given the following state space (tree search), give the sequence of visited nodes when using DFS (assume that the nodeO is the goal state):

19. A B C E D Depth First Search • A,

20. A B C E D F G Depth First Search • A,B,

21. A B C E D F G Depth First Search • A,B,F,

22. A B C E D F G K L Depth First Search • A,B,F, • G,

23. A B C E D F G K L Depth First Search • A,B,F, • G,K,

24. A B C E D F G K L O Depth First Search • A,B,F, • G,K, • L,

25. A B C E D F G K L O Depth First Search • A,B,F, • G,K, • L, O: Goal State

26. A B C E D F G K L O Depth First Search • The returned solution is the sequence of operators in the path: A, B, G, L, O : assignments when using CSP !!!

27. Backtracking Example 1

28. Example of a csp A B E C D H F G 3 colour me!

29. {blue, green, red} {blue, green, red} A B {blue, green, red} E C {blue, green, red} D {blue, green, red} H {blue, green, red} F G {blue, green, red} {blue, green, red} Example of a csp 3 colour me!

30. A B E C D H F G Example of a csp 1 = red 2 = blue 3 = green

31. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

32. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

33. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

34. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

35. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

36. Example of a csp A B E C D H F G Dead end → backtrack 1 = red 2 = blue 3 = green

37. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

38. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

39. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

40. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

41. Example of a csp A B E C D H F G 1 = red 2 = blue 3 = green

42. Example of a csp A B E C D H Solution !!!! F G 1 = red 2 = blue 3 = green

43. Backpropagation Example 2

44. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G]

45. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G]

46. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G]

47. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G]

48. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G]

49. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G] Dead End → Backtrack

50. Backpropagation [R,B,G] [R,B,G] [R] [R,B,G] [R,B,G]