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## What we will try to do ...

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**Constraint Satisfaction Problems (CSP)(Where we postpone**making difficult decisions until they become easy to make)R&N:Chap. 5**What we will try to do ...**• Search techniques make choices in an often arbitrary order. Often little information is available to make each of them • In many problems, the same states can be reached independent of the order in which choices are made (“commutative” actions) • Can we solve such problems more efficiently by picking the order appropriately? Can we even avoid making any choice?**Constraint Propagation**• Place a queen in a square • Remove the attacked square from future consideration**Constraint Propagation**5555567 6 6 5 5 5 5 6 • Count the number of non-attacked squares in every row and column • Place a queen in a row or column with minimum number • Remove the attacked squares from future consideration**Constraint Propagation**433345 3 4 4 3 3 5 • Repeat**Constraint Propagation**33343 4 3 2 3 4**Constraint Propagation**3331 4 2 2 1 3**Constraint Propagation**221 2 2 1**Constraint Propagation**12 2 1**What do we need?**• More than just a successor function and a goal test • We also need: • A means to propagate the constraints imposed by one queen’s position on the the positions of the other queens • An early failure test • Explicit representation of constraints • Constraint propagation algorithms**Constraint Satisfaction Problem (CSP)**• Set ofvariables {X1, X2, …, Xn} • Each variable Xi has a domain Di of possible values. Usually, Di is finite • Set of constraints{C1, C2, …, Cp} • Each constraint relates a subset of variables by specifying the valid combinations of their values • Goal: Assign a value to every variable such that all constraints are satisfied**NT**Q WA SA NT NSW Q V WA SA T NSW V T Map Coloring • 7 variables {WA,NT,SA,Q,NSW,V,T} • Each variable has the same domain: {red, green, blue} • No two adjacent variables have the same value:WANT, WASA, NTSA, NTQ, SAQ, SANSW, SAV,QNSW, NSWV**All constraints are binary**8-Queen Problem • 8 variables Xi, i = 1 to 8 • The domain of each variable is: {1,2,…,8} • Constraints are of the forms: • Xi = k Xj k for all j = 1 to 8, ji • Similar constraints for diagonals**2**3 4 1 5 Street Puzzle Ni = {English, Spaniard, Japanese, Italian, Norwegian} Ci= {Red, Green, White, Yellow, Blue} Di = {Tea, Coffee, Milk, Fruit-juice, Water} Ji = {Painter, Sculptor, Diplomat, Violinist, Doctor} Ai = {Dog, Snails, Fox, Horse, Zebra} The Englishman lives in the Red house The Spaniard has a Dog The Japanese is a Painter The Italian drinks Tea The Norwegian lives in the first house on the left The owner of the Green house drinks Coffee The Green house is on the right of the White house The Sculptor breeds Snails The Diplomat lives in the Yellow house The owner of the middle house drinks Milk The Norwegian lives next door to the Blue house The Violinist drinks Fruit juice The Fox is in the house next to the Doctor’s The Horse is next to the Diplomat’s Who owns the Zebra? Who drinks Water?**2**3 4 1 5 left as an exercise Street Puzzle Ni = {English, Spaniard, Japanese, Italian, Norwegian} Ci= {Red, Green, White, Yellow, Blue} Di = {Tea, Coffee, Milk, Fruit-juice, Water} Ji = {Painter, Sculptor, Diplomat, Violinist, Doctor} Ai = {Dog, Snails, Fox, Horse, Zebra} (Ni = English) (Ci = Red) The Englishman lives in the Red house The Spaniard has a Dog The Japanese is a Painter The Italian drinks Tea The Norwegian lives in the first house on the left The owner of the Green house drinks Coffee The Green house is on the right of the White house The Sculptor breeds Snails The Diplomat lives in the Yellow house The owner of the middle house drinks Milk The Norwegian lives next door to the Blue house The Violinist drinks Fruit juice The Fox is in the house next to the Doctor’s The Horse is next to the Diplomat’s (Ni = Japanese) (Ji = Painter) (N1 = Norwegian) (Ci = White) (Ci+1 = Green) (C5 White) (C1 Green)**2**3 4 1 5 Street Puzzle Ni = {English, Spaniard, Japanese, Italian, Norwegian} Ci= {Red, Green, White, Yellow, Blue} Di = {Tea, Coffee, Milk, Fruit-juice, Water} Ji = {Painter, Sculptor, Diplomat, Violinist, Doctor} Ai = {Dog, Snails, Fox, Horse, Zebra} (Ni = English) (Ci = Red) The Englishman lives in the Red house The Spaniard has a Dog The Japanese is a Painter The Italian drinks Tea The Norwegian lives in the first house on the left The owner of the Green house drinks Coffee The Green house is on the right of the White house The Sculptor breeds Snails The Diplomat lives in the Yellow house The owner of the middle house drinks Milk The Norwegian lives next door to the Blue house The Violinist drinks Fruit juice The Fox is in the house next to the Doctor’s The Horse is next to the Diplomat’s (Ni = Japanese) (Ji = Painter) (N1 = Norwegian) (Ci = White) (Ci+1 = Green) (C5 White) (C1 Green) unaryconstraints**2**3 4 1 5 i,j[1,5], ij, Ni Nj i,j[1,5], ij, Ci Cj ... Street Puzzle Ni = {English, Spaniard, Japanese, Italian, Norwegian} Ci= {Red, Green, White, Yellow, Blue} Di = {Tea, Coffee, Milk, Fruit-juice, Water} Ji = {Painter, Sculptor, Diplomat, Violinist, Doctor} Ai = {Dog, Snails, Fox, Horse, Zebra} The Englishman lives in the Red house The Spaniard has a Dog The Japanese is a Painter The Italian drinks Tea The Norwegian lives in the first house on the left The owner of the Green house drinks Coffee The Green house is on the right of the White house The Sculptor breeds Snails The Diplomat lives in the Yellow house The owner of the middle house drinks Milk The Norwegian lives next door to the Blue house The Violinist drinks Fruit juice The Fox is in the house next to the Doctor’s The Horse is next to the Diplomat’s**2**3 4 1 5 Street Puzzle Ni = {English, Spaniard, Japanese, Italian, Norwegian} Ci= {Red, Green, White, Yellow, Blue} Di = {Tea, Coffee, Milk, Fruit-juice, Water} Ji = {Painter, Sculptor, Diplomat, Violinist, Doctor} Ai = {Dog, Snails, Fox, Horse, Zebra} The Englishman lives in the Red house The Spaniard has a Dog The Japanese is a Painter The Italian drinks Tea The Norwegian lives in the first house on the left N1 = Norwegian The owner of the Green house drinks Coffee The Green house is on the right of the White house The Sculptor breeds Snails The Diplomat lives in the Yellow house The owner of the middle house drinks Milk D3 = Milk The Norwegian lives next door to the Blue house The Violinist drinks Fruit juice The Fox is in the house next to the Doctor’s The Horse is next to the Diplomat’s**2**3 4 1 5 Street Puzzle Ni = {English, Spaniard, Japanese, Italian, Norwegian} Ci= {Red, Green, White, Yellow, Blue} Di = {Tea, Coffee, Milk, Fruit-juice, Water} Ji = {Painter, Sculptor, Diplomat, Violinist, Doctor} Ai = {Dog, Snails, Fox, Horse, Zebra} The Englishman lives in the Red house C1 Red The Spaniard has a Dog A1 Dog The Japanese is a Painter The Italian drinks Tea The Norwegian lives in the first house on the left N1 = Norwegian The owner of the Green house drinks Coffee The Green house is on the right of the White house The Sculptor breeds Snails The Diplomat lives in the Yellow house The owner of the middle house drinks Milk D3 = Milk The Norwegian lives next door to the Blue house The Violinist drinks Fruit juice J3 Violinist The Fox is in the house next to the Doctor’s The Horse is next to the Diplomat’s**T1**T2 T4 T3 Task Scheduling • Four tasks T1, T2, T3, and T4 are related by time constraints: • T1 must be done during T3 • T2 must be achieved before T1 starts • T2 must overlap with T3 • T4 must start after T1 is complete • Are the constraints compatible? • What are the possible time relations between two tasks? • What if the tasks use resources in limited supply? How to formulate this problem as a CSP?**3-SAT**• n Boolean variables u1, ..., un • p constrains of the form ui* uj* uk*= 1where u* stands for either u or u • Known to be NP-complete**Finite vs. Infinite CSP**• Finite CSP: each variable has a finite domain of values • InfiniteCSP: some or all variables have an infinite domainE.g., linear programming problems over the reals: • We will only consider finite CSP**CSP as a Search Problem**• n variables X1, ..., Xn • Valid assignment: {Xi1 vi1, ..., Xik vik}, 0 k n, such that the values vi1, ..., vik satisfy all constraints relating the variables Xi1, ..., Xik • Complete assignment: one where k = n [if all variable domains have size d, there are O(dn) complete assignments] • States: valid assignments • Initial state: empty assignment {}, i.e. k = 0 • Successor of a state: {Xi1vi1, ..., Xikvik} {Xi1vi1, ..., Xikvik, Xik+1vik+1} • Goal test: k = n**The order in which variables are assigned values has no**impact on the assignment reached A Key property of CSP: Commutativity Hence: One can expand a node N by first selecting one variable X not in the assignment A associated with N and then assigning every value v in the domain of X [ big reduction in branching factor]**4 variables X1, ..., X4**• Let the valid assignment of N be: A = {X1 v1, X3 v3} • For example pick variable X4 • Let the domain of X4 be {v4,1, v4,2, v4,3} • The successors of A are all the valid assignments among: {X1 v1, X3 v3 , X4 v4,1 } {X1 v1, X3 v3 , X4 v4,2 } {X1 v1, X3 v3 , X4 v4,2 }**The order in which variables are assigned values has no**impact on the assignment reached A Key property of CSP: Commutativity Hence: One can expand a node N by first selecting one variable X not in the assignment A associated with N and then assigning every value v in the domain of X [ big reduction in branching factor] One need not store the path to a node Backtracking search algorithm**Backtracking Search**Essentially a simplified depth-first algorithm using recursion**Backtracking Search(3 variables)**Assignment = {}**Backtracking Search(3 variables)**X1 v11 Assignment = {(X1,v11)}**Backtracking Search(3 variables)**X1 v11 X3 v31 Assignment = {(X1,v11), (X3,v31)}**Then, the search algorithm**backtracks to the previous (X3) variable and tries another value Backtracking Search(3 variables) X1 v11 X3 v31 X2 Assume that no value of X2 leads to a valid assignment Assignment = {(X1,v11), (X3,v31)}**Backtracking Search(3 variables)**X1 v11 X3 v31 v32 X2 Assignment = {(X1,v11), (X3,v32)}**Backtracking Search(3 variables)**The search algorithm backtracksto the previous variable (X3) and tries another value. But assume that X3 has only two possible values. The algorithm backtracks to X1 X1 v11 X3 v31 v32 X2 X2 Assume again that no value of X2 leads to a valid assignment Assignment = {(X1,v11), (X3,v32)}**Backtracking Search(3 variables)**X1 v11 v12 X3 v31 v32 X2 X2 Assignment = {(X1,v12)}**Backtracking Search(3 variables)**X1 v11 v12 X3 X2 v31 v32 v21 X2 X2 Assignment = {(X1,v12), (X2,v21)}**The algorithm need not consider**the variables in the same order in this sub-tree as in the other Backtracking Search(3 variables) X1 v11 v12 X3 X2 v31 v32 v21 X2 X2 Assignment = {(X1,v12), (X2,v21)}**Backtracking Search(3 variables)**X1 v11 v12 X3 X2 v31 v32 v21 X2 X2 X3 v32 Assignment = {(X1,v12), (X2,v21), (X3,v32)}**Backtracking Search(3 variables)**X1 v11 v12 X3 X2 v31 v32 v21 The algorithm need not consider the values of X3 in the same order in this sub-tree X2 X2 X3 v32 Assignment = {(X1,v12), (X2,v21), (X3,v32)}**Backtracking Search(3 variables)**X1 v11 v12 X3 X2 v31 v32 v21 Since there are only three variables, the assignment is complete X2 X2 X3 v32 Assignment = {(X1,v12), (X2,v21), (X3,v32)}**Backtracking Algorithm**CSP-BACKTRACKING(A) • If assignment A is complete then return A • X select a variable not in A • D select an ordering on the domain of X • For each value v in D do • Add (Xv) to A • If A is valid then • result CSP-BACKTRACKING(A) • If result failure then return result • Return failure Call CSP-BACKTRACKING({}) [This recursive algorithm keeps too much data in memory. An iterative version could save memory (left as an exercise)]**{}**NT Q WA WA=red WA=green WA=blue SA NSW V WA=red NT=green WA=red NT=blue T WA=red NT=green Q=red WA=red NT=green Q=blue Map Coloring**Critical Questions for the Efficiency of CSP-Backtracking**CSP-BACKTRACKING(A) • If assignment A is complete then return A • X selecta variable not in A • D select an ordering on the domain of X • For each value v in D do • Add (Xv) to A • If a is valid then • result CSP-BACKTRACKING(A) • If result failure then return result • Return failure**Critical Questions for the Efficiency of CSP-Backtracking**• Which variable X should be assigned a value next?The current assignment may not lead to any solution, but the algorithm still does know it. Selecting the right variable to which to assign a value may help discover the contradiction more quickly • In which order should X’s values be assigned?The current assignment may be part of a solution. Selecting the right value to assign to X may help discover this solution more quickly More on these questions in a short while ...**Critical Questions for the Efficiency of CSP-Backtracking**• Which variable X should be assigned a value next?The current assignment may not lead to any solution, but the algorithm does not know it yet. Selecting the right variable X may help discover the contradiction more quickly • In which order should X’s values be assigned?The current assignment may be part of a solution. Selecting the right value to assign to X may help discover this solution more quickly More on these questions in a short while ...**Critical Questions for the Efficiency of CSP-Backtracking**• Which variable X should be assigned a value next?The current assignment may not lead to any solution, but the algorithm does not know it yet. Selecting the right variable X may help discover the contradiction more quickly • In which order should X’s values be assigned?The current assignment may be part of a solution. Selecting the right value to assign to X may help discover this solution more quickly More on these questions in a short while ...**Critical Questions for the Efficiency of CSP-Backtracking**• Which variable X should be assigned a value next?The current assignment may not lead to any solution, but the algorithm does not know it yet. Selecting the right variable X may help discover the contradiction more quickly • In which order should X’s values be assigned?The current assignment may be part of a solution. Selecting the right value to assign to X may help discover this solution more quickly More on these questions very soon ...**1**2 3 4 5 6 7 8 X1X2X3X4X5X6X7X8 Forward Checking A simple constraint-propagation technique: Assigning the value 5 to X1leads to removing values from the domains of X2, X3, ..., X8**NT**Q WA T NSW SA V Forward Checking in Map Coloring