Chapter 7: Constraint satisfaction problems

DIT411/TIN175, Artificial Intelligence

Peter Ljunglöf

30 January, 2018

Table of contents

CSP: Constraint satisfaction problems (R&N 7.1)

Formulating a CSP

Constraint graph

Constraint satisfaction problems (CSP)

  • Standard search problem:

    • the state is a “black box”, any data structure that supports:
      • goal test, cost evaluation, successor
  • CSP is a more specific search problem:

    • the state is defined by variables \(X_{i}\), taking values from the domain \(\mathbf{D}_{i}\)

    • the goal test is a set of constraints specifying allowable combinations
      of values for subsets of variables

  • Since CSP is more specific, it allows useful algorithms with more power than standard search algorithms

States and variables

Just a few variables can describe many states: 

\(n\) binary variables can describe \(2^{n}\)   states
10 binary variables can describe \(2^{10}\)  = 1,024
20 binary variables can describe \(2^{20}\)  = 1,048,576
30 binary variables can describe \(2^{30}\)  = 1,073,741,824
100 binary variables can describe \(2^{100}\) = 1,267,650,600,228,229,

Hard and soft constraints

  • Given a set of variables, assign a value to each variable that either

    • satisfies some set of constraints:
      • satisfiability problems — “hard constraints”
    • or minimizes some cost function,
      where each assignment of values to variables has some cost:
      • optimization problems — “soft constraints” — “preferences”
    • many problems are a mix of hard constraints and preferences:
      • constraint optimization problems
  • In this course we will focus on satisfiability problems
  • Differences between CSP and general search problems:

    • The path to a goal isn’t important, only the solution is.

    • There are no predefined starting nodes.

    • Often these problems are huge, with thousands of variables,
      so systematically searching the space is infeasible.

    • For optimization problems, there are no well-defined goal nodes.

Formulating a CSP

  • A CSP is characterized by

    • A set of variables \(X_{1},X_{2},\ldots,X_{n}\).

    • Each variable \(X_{i}\) has an associated domain \(\mathbf{D}_{i}\) of possible values.

    • There are hard constraints \(C_{X_i,\ldots,X_j}\) on various subsets of the variables
      which specify legal combinations of values for these variables.

    • A solution to the CSP is an assignment of a value to each variable
      that satisfies all the constraints.

Example: Scheduling activities

Variables: \(A, B, C, D, E\) representing starting times of various activities.
(e.g., courses and their study periods)
Domains: \(\mathbf{D}_{A}=\mathbf{D}_{B}=\mathbf{D}_{C}=\mathbf{D}_{D}=\mathbf{D}_{E}=\{1,2,3,4\}\)
Constraints: \((B\neq3), (C\neq2), (A\neq B), (B\neq C), (C<D), (A=D),\)
\((E<A), (E<B), (E<C), (E<D), (B\neq D)\)

Example: Crossword puzzle

  • Words: ant, big, bus, car, has, book, buys, hold, lane, year, beast, ginger, search, symbol, syntax, …

Dual representations

Many problems can be represented in different ways as a CSP, e.g., the crossword puzzle:

  • One representation:
    • each variable represent one word
    • the domain is all words in the lexicon
    • constraints specify that the letters
      on the intersections must be the same
    • 5 variables, 5 constraints, \(|\mathbf{D}|\) ≈ 100,000
  • Dual representation:
    • each variable represent an individual square
    • the domain is the letters in the alphabet
    • constraints specify that letter combinations must
      be in the lexicon
    • 15 variables, 5 constraints, \(|\mathbf{D}|\) = 26

Example: Map colouring

Variables: \(\mathit{WA}, NT, Q, \mathit{NSW}, V, \mathit{SA}, T\)
Domains: \(\mathbf{D}_{i}=\{red,green,blue\}\)
Constraints: adjacent regions must have different colors, i.e.,
\(\mathit{WA}\neq NT,\mathit{WA}\neq\mathit{SA},NT\neq\mathit{SA},NT\neq Q,\ldots\)

Example: Map colouring

  • Solutions are assignments satisfying all constraints, e.g.,

Constraint graph

  • Binary CSP: each constraint relates at most two variables
    (note: this does not say anything about the domains)

  • Constraint graph: every variable is a node, every binary constraint is an arc

  • CSP algorithms can use the graph structure to speed up search,
    e.g., Tasmania is an independent subproblem.

Example: Cryptarithmetic puzzle

Variables: \(F,T,U,W,R,O,X_{1},X_{2},X_{3}\)
Domains: \(\{0,1,2,3,4,5,6,7,8,9\}\)
Constraints: \(\mathit{Alldiff}(F,T,U,W,R,O)\),   \(O+O=R+10\cdot X_{1}\),   etc.
Note: This is not a binary CSP!
The graph is a constraint hypergraph

Example: Sudoku

Variables: \(A_1\ldots A_9,B_1,\ldots,E_5,\ldots,I_9\)
Domains: \(\{1,2,3,4,5,6,7,8,9\}\)
Constraints: \(\mathit{Alldiff}(A_1,\ldots,A_9)\), …, \(\mathit{Alldiff}(A_5,\ldots,I_5)\), …, \(\mathit{Alldiff}(D_1,\ldots,F_3)\), …, \(B_1=9\), …, \(F_6=8\), …, \(I_7=3\)

Example: n-queens

Variables: \(Q_1, Q_2,\ldots, Q_n\)
Domains: \(\{1,2,3,\ldots,n\}\)
Constraints: \(\mathit{Alldiff}(Q_1,Q_2,\ldots,Q_n)\),
\(Q_i-Q_j \neq |i-j|\)   (\(1\leq i<j\leq n\))

CSP Varieties

  • Discrete variables, finite domains:
    • \(n\) variables, domain size \(d\)   \(\Rightarrow\)   \(O(d^{n})\) complete assignments
    • this is what we discuss in this course
  • Discrete variables, infinite domains (integers, strings, etc.)
    • e.g., job scheduling — variables are start/end times for each job
    • we need a constraint language for formulating the constraints
      (e.g., \(T_{1}+d_1\leq T_{2}\))
    • linear constraints are solvable — nonlinear are undecidable
  • Continuous variables:
    • e.g., scheduling for Hubble Telescope observations and manouvers
    • linear constraints (linear programming) — solvable in polynomial time!

Different kinds of constraints

  • Unary constraints involve a single variable:
    • e.g., \(SA\neq green\)
  • Binary constraints involve pairs of variables:
    • e.g., \(SA\neq WA\)
  • Global constraints (or higher-order) involve 3 or more variables:
    • e.g., \(\mathit{Alldiff}(WA,NT,SA)\)
    • all global constraints can be reduced to a number of binary constraints
      (but this might lead to an explosion of the number of constraints)
  • Preferences (or soft constraints):
    • “constraint optimization problems”
    • often representable by a cost for each variable assignment
    • not discussed in this course

CSP as a search problem (R&N 7.3–7.3.2)

Heuristics: Improving backtracking efficiency

Generate-and-test algorithm

  • Generate the assignment space \(\mathbf{D} = \mathbf{D}_{V_{1}}\times\mathbf{D}_{V_{2}}\times\cdots\times\mathbf{D}_{V_{n}}\)
    Test each assignment with the constraints.

  • Example:

    \(\mathbf{D}\) = \(\mathbf{D}_{A}\times\mathbf{D}_{B}\times\mathbf{D}_{C}\times\mathbf{D}_{D}\times\mathbf{D}_{E}\)
      = \(\{1,2,3,4\}\times\cdots\times\{1,2,3,4\}\)
      = \(\{(1,1,1,1,1),(1,1,1,1,2),\ldots,(4,4,4,4,4)\}\)
  • How many assignments need to be tested for \(n\) variables,
    each with domain size \(d=|\mathbf{D_i}|\)?

CSP as a search problem

  • Let’s start with the straightforward, dumb approach.
    (But still not as stupid as generate-and-test…)

  • States are defined by the values assigned so far:
    • Initial state: the empty assignment, { }
    • Successor function: assign a value to an unassigned variable
      that does not conflict with current assignment
      \(\Longrightarrow\) fail if there are no legal assignments
    • Goal test: the current assignment is complete
  • Every solution appears at depth \(n\) (assuming \(n\) variables)
    \(\Longrightarrow\) we can use depth-first-search, no risk for infinite loops

  • At search depth \(k\), the branching factor is \( b=(n-k)d \), (where \( d=|\mathbf{D}_i| \)
    is the domain size and \(n-k\) is the number of unassigned variables)
    \(\Longrightarrow\) hence there are \(n!d^{n}\) leaves

Backtracking search

  • Variable assignments are commutative:

    • \(\{WA=red,NT=green\}\) is the same as \(\{NT=green,WA=red\}\)
  • It’s unnecessary work to assign \(WA\) followed by \(NT\) in one branch,
    and \(NT\) followed by \(WA\) in another branch.

  • Instead, at each depth level, we can decide on one single variable to assign:

    • this gives branching factor \(b=d\), so there are \(d^{n}\) leaves (instead of \(n!d^{n}\))
  • Depth-first search with single-variable assignments is called backtracking search:

    • backtracking search is the basic uninformed CSP algorithm
    • it can solve \(n\)-queens for \(n\approx25\)
  • Why not use breadth-first search?

Simple backtracking example

Variables: \(A, B, C\)
Domains: \(\mathbf{D}_{A}=\mathbf{D}_{B}=\mathbf{D}_{C}=\{1,2,3,4\}\)
Constraints: \((A<B), (B<C)\)

Example: Australia map colours

  • Assign variable: WA (Western Australia) NT (Northern Territory) Q (Queensland)
  • function BacktrackingSearch(csp):
    • return Backtrack(csp, { })
  • function Backtrack(csp, assignment):
    • if assignment is complete then return assignment
    • var := SelectUnassignedVariable(csp, assignment)
    • for each value in OrderDomainValues(csp, var, assignment):
      • if value is consistent with assignment:
        • inferences := Inference(csp, var, value)
        • if inferences ≠ failure:
          • result := Backtrack(csp, assignment \(\cup\) {var=value} \(\cup\) inferences)
          • if result ≠ failure then return result
    • return failure

Heuristics: Improving backtracking efficiency

  • The general-purpose algorithm gives rise to several questions:

    • Which variable should be assigned next?
      • SelectUnassignedVariable(csp, assignment)
    • In what order should its values be tried?
      • OrderDomainValues(csp, var, assignment)
    • What inferences should be performed at each step?
      • Inference(csp, var, value)
    • Can the search avoid repeating failures?
      • Conflict-directed backjumping, constraint learning, no-good sets
        (R&N 7.3.3, not covered in this course)

Selecting unassigned variables

  • Heuristics for selecting the next unassigned variable:

    • Minimum remaining values (MRV):
      \(\Longrightarrow\) choose the variable with the fewest legal values

    • Degree heuristic (if there are several MRV variables):
      \(\Longrightarrow\) choose the variable with most constraints on remaining variables

Ordering domain values

  • Heuristics for ordering the values of a selected variable:

    • Least constraining value:
      \(\Longrightarrow\) prefer the value that rules out the fewest choices for the neighboring variables in the constraint graph

Inference: Forward checking

  • Forward checking is a simple form of inference:
    • Keep track of remaining legal values for unassigned variables
      — terminate when any variable has no legal values left
    • When a new variable is assigned, recalculate the legal values for its neighbors

  • Colour WA red \(\Longrightarrow\) NT and SA cannot be red
  • Colour Q green \(\Longrightarrow\) NT, NSW and SA cannot be green
  • Colour V blue \(\Longrightarrow\) NSW and SA cannot be blue
    \(\Longrightarrow\) SA has no legal values left

Inference: Constraint propagation

  • Forward checking propagates information from assigned to
    unassigned variables, but doesn’t detect all failures early:
    NT and SA cannot both be blue, but forward checking doesn’t notice that!

    • Forward checking enforces local constraints
    • Constraint propagation enforces local constraints,
      repeatedly until reaching a fixed point

Constraint propagation (R&N 7.2–7.2.2)

Arc consistency

Maintaining arc consistency

Constraint propagation: Arc consistency

  • The simplest form of propagation is to make the graph arc consistent:

    • \(X\rightarrow Y\) is arc consistent iff:
      for every value \(x\) of \(X\), there is some allowed value \(y\) in \(Y\)

    • for every \(x\) in SA, there is an allowed \(y\) in NSW
      \(\Longrightarrow\) the arc SA \(\rightarrow\) NSW is consistent
    • if NSW is blue, there is no allowed color \(y\) in SA
      \(\Longrightarrow\) remove blue from NSW
    • if V is red, there is no allowed color \(y\) in NSW
      \(\Longrightarrow\) remove red from V
    • If \(X\) loses a value, neighbors of \(X\) need to be rechecked
      • i.e., the arc SA \(\rightarrow\) NSW must be rechecked
    • Arc consistency detects failure earlier than forward checking


  • Different variants of constistency:

    • A variable is node-consistent if all values in its domain satisfy
      its own unary constraints

    • a variable is arc-consistent if every value in its domain satisfies
      the variable’s binary constraints

    • Generalised arc-consistency is the same, but for \(n\)-ary constraints

    • Path consistency is arc-consistency, but for 3 variables at the same time

    • \(k\)-consistency is arc-consistency, but for \(k\) variables

    • …and there are consistency checks for several global constraints,
      such as \(\mathit{Alldiff}\) or \(\mathit{Atmost}\)

  • A graph is \(X\)-consistent if every variable is \(X\)-consistent with every other variable.

Scheduling example (again)

Variables: \(A, B, C, D, E\) representing starting times of various activities.
Domains: \(\mathbf{D}_{A}=\mathbf{D}_{B}=\mathbf{D}_{C}=\mathbf{D}_{D}=\mathbf{D}_{E} = \{1,2,3,4\}\)
Constraints: \((B\neq3), (C\neq2), (A\neq B), (B\neq C), (C<D), (A=D),\)
\((E<A), (E<B), (E<C), (E<D), (B\neq D)\)
  • Is this example node consistent?

    • \(\mathbf{D}_{B}=\{1,2,3,4\}\) is not node consistent,
      since \(B=3\) violates the constraint \(B\neq3\)
      \(\Longrightarrow\) reduce the domain \(\mathbf{D}_{B}=\{1,2,4\}\)

    • \(\mathbf{D}_{C}=\{1,2,3,4\}\) is not node consistent,
      since \(C=2\) violates the constraint \(C\neq2\)
      \(\Longrightarrow\) reduce the domain \(\mathbf{D}_{C}=\{1,3,4\}\)

Scheduling example as a constraint graph

If we reduce the domains for \(B\) and \(C\), then the constraint graph is node consistent.

Arc consistency

  • A variable \(X\) is binary arc-consistent with respect to another variable \(Y\) if:
    • For each value \(x\in\mathbf{D}_{X}\), there is some \(y\in\mathbf{D}_{Y}\)
      such that the binary constraint \(C_{XY}(x,y)\) is satisfied.
  • A variable \(X\) is generalised arc-consistent with respect to variables \((Y,Z,\dots)\) if:
    • For each value \(x\in\mathbf{D}_{X}\), there is some assignment \(y,z,\dots\in\mathbf{D}_{Y},\mathbf{D}_{Z},\dots\) such that \(C_{XYZ\ldots}(x,y,z,\dots)\) is satisfied.
  • What if \(X\) is not arc consistent to \(Y\)?
    • All values \(x\in\mathbf{D}_{X}\) for which there is no corresponding \(y\in\mathbf{D}_{Y}\)
      can be deleted from \(\mathbf{D}_{X}\) to make \(X\) arc consistent.
  • Note! The arcs in a constraint graph are directed:
    • \((X,Y)\) and \((Y,X)\) are considered as two different arcs,
    • i.e., \(X\) can be arc consistent to \(Y\), but \(Y\) not arc consistent to \(X\).

Arc consistency algorithm

  • Keep a set of arcs to be considered: pick one arc \((X,Y)\) at the time and make it consistent (i.e., make \(X\) arc consistent to \(Y\)).

    • Start with the set of all arcs \(\{(X,Y),(Y,X),(X,Z),(Z,X),\ldots\}\).
  • When an arc has been made arc consistent, does it ever need to be checked again?

    • An arc \((Z,X)\) needs to be revisited if the domain of \(X\) is changed.
  • Three possible outcomes when all arcs are made arc consistent:
    (Is there a solution?)

    • One domain is empty \(\Longrightarrow\) no solution
    • Each domain has a single value \(\Longrightarrow\) unique solution
    • Some domains have more than one value \(\Longrightarrow\) maybe a solution, maybe not

Quiz: Arc consistency

  • The variables and constraints are in the constraint graph:

  • Assume the initial domains are \(\mathbf{D}_{A}=\mathbf{D}_{B}=\mathbf{D}_{C}=\{1,2,3,4\}\)

  • How will the domains look like after making the graph arc consistent?

The arc consistency algorithm AC-3

  • function AC-3(inout csp):
    • initialise queue to all arcs in csp
    • while queue is not empty:
      • (X, Y) := RemoveOne(queue)
      • if Revise(csp, X, Y):
        • if   \(\mathbf{D}_X=\emptyset\)   then return false
        • for each Z in X.neighbors–{Y}:
          • add (Z, X) to queue
    • return true
  • function Revise(inout csp, X, Y):
    • revised := false
    • for each x in \(\mathbf{D}_X\):
      • if there is no value y in \(\mathbf{D}_Y\) satisfying the csp constraint \(C_{XY}(x,y)\):
        • delete x from \(\mathbf{D}_X\)
        • revised := true
    • return revised
  • Note: This algorithm destructively updates the domains of the CSP!
    You might need to copy the CSP before calling AC-3.

Maintaining arc-consistency (MAC)

  • What if some domains have more than one element after AC?

  • We can always resort to backtracking search:

    • Select a variable and a value using some heuristics
      (e.g., minimum-remaining-values, degree-heuristic, least-constraining-value)
    • Make the graph arc-consistent again
    • Backtrack and try new values/variables, if AC fails
    • Select a new variable/value, perform arc-consistency, etc.
  • Do we need to restart AC from scratch?

    • no, only some arcs risk becoming inconsistent after a new assignment
    • restart AC with the queue \(\{(Y_i,X) | X\rightarrow Y_i\}\),
      i.e., only the arcs \((Y_i,X)\) where \(Y_i\) are the neighbors of \(X\)
    • this algorithm is called Maintaining Arc Consistency (MAC)

Domain splitting (not in R&N)

  • What if some domains are very big?

    • Instead of assigning every possible value to a variable, we can split its domain

    • Split one of the domains, then recursively solve each half, i.e.:

      • perform AC on the resulting graph, then split a domain,
        perform AC, split a domain, perform AC, split, etc.
    • It is often good to split a domain in half, i.e.:

      • if \(\mathbf{D}_{X}=\{1,\dots,1000\}\), split into \(\{1,\dots500\}\) and \(\{501,\dots,1000\}\)