For a quick introduction we will write a Sudoku solver. Sudokus are popular puzzles that are often found in Newspapers. The task is to fill in numbers in a (already partly filled) 9x9 grid, such that no number is present twice in a row, column or one of the 9 3x3 blocks.

Sudokus are constraint satisfaction problems. The 81 fields of the grid are the variables, their domain is the set of numbers from 1 to 9, and the constraints are the number placement rules.


The first step is to model the space our problem lives in. For that we need the variables and their domains. We can make use of python to do this very compactly for all 81 variables

from constrainingorder.sets import DiscreteSet
from constrainingorder.variables import DiscreteVariable

numbers = range(1,10)
domain = DiscreteSet(numbers)
variables = {}
for i in numbers:
    for j in numbers:
        name = 'x%d%d' % (i,j)
        variables[name] = DiscreteVariable(name,domain=domain)

A DiscreteSet is a datastructure representing a set of discrete elements, very similar to pythons built-in set. But it can also represent the “set of everything”, which is sometimes convenient. For details see .. todo:: Add DiscreteSet reference

A DiscreteVariable is a variable that can take on values from a DiscreteSet. Each variable has a name. The variables x11 is the number in the first row and first column, the variable x12 the one in the first row and second column and so on. We store them in a dictionary, so that we can easily to refer to them by name when building the constraints. This is often convenient, but not always necessary.


The constraints model the requirements, that no number is allowed to occur twice in a row, column or block. Or equivalently, that all numbers in a row, column or block are different (as there are exactly nine different numbers). Luckily constraining order already comes with a constraint of this type, so we just have to use it:

from constrainingorder.constraints import AllDifferent

cons = []
#row constraints
for i in numbers:
    cons.append(AllDifferent([variables['x%d%d'%(i,j)] for j in numbers]))
#column constraints
for i in numbers:
    cons.append(AllDifferent([variables['x%d%d'%(j,i)] for j in numbers]))
#block constraints
for i in range(0,3):
    for j in range(0,3):
        #assemble list of parameternames for this block
        names = []
        for k in range(0,3):
            for l in range(0,3):
                names.append('x%d%d' % (3*i + k + 1,3*j + l + 1))
        #create constraint
        cons.append(AllDifferent([variables[n] for n in names]))

If we wanted to find all possible completely filled sudokus, we could now try to enumerate all solutions to this problem (see below), but this would take a very, very long while, as there are 6.67 10^21 different ones see [Felgenhauer].

In the sudokus found in newspapers some numbers are already given, in such a way that there is only one solution. We can add these filled in numbers by adding additional constraints that restrict certain variables to just a single value. Again this kind of constraint is already included in Constraining Order:

from constrainingorder.constraints import FixedValue



With the variables and the constraints we can set up a Space. A Space collects all the variables and constraints, and keeps track of the possible values (the domain) for each variable. We print the domain for the first few variables.

from constrainingorder import Space

space = Space(variables.values(),cons)
for vname, domain in sorted(space.domains.items())[:15]:
    print vname, domain

This outputs

x11 {1,2,3,4,5,6,7,8,9}
x12 {1,2,3,4,5,6,7,8,9}
x13 {1,2,3,4,5,6,7,8,9}
x14 {1,2,3,4,5,6,7,8,9}
x15 {1,2,3,4,5,6,7,8,9}
x16 {1,2,3,4,5,6,7,8,9}
x17 {1,2,3,4,5,6,7,8,9}
x18 {1,2,3,4,5,6,7,8,9}
x19 {1,2,3,4,5,6,7,8,9}
x21 {1,2,3,4,5,6,7,8,9}
x22 {1,2,3,4,5,6,7,8,9}
x23 {1,2,3,4,5,6,7,8,9}
x24 {1,2,3,4,5,6,7,8,9}
x25 {1,2,3,4,5,6,7,8,9}
x26 {1,2,3,4,5,6,7,8,9}

A space can also tell us if a labelling (a dictionary with parameter names and values) is consistent with the constraints or satisfies them.


With the Space set up, we can now solve the CSP with backtracking, i.e. by filling in a number into a field and then checking if this is consistent with the constraints. If it is put a number into another field, if not, try another number, or if all numbers have been tried, go back to the previous field and try another number there.

This procedure can take a long time, as there are 9^81 possibilities that have to be tried. One possibility to speed this up is to reduce the problem space. For some fields possible numbers can be eliminated, as they are not consistent with the posed constraints. For example if the value of a field is fixed to 3, then its value can not be something else, and also the 3 can be removed from the domain of the fields in the same row, column and block.

In the constraint satisfaction literature this is called problem reduction. Constraining Order has an algorithm included for problem reduction called ac3, that does that.

from constrainingorder.solver import ac3

for vname, domain in sorted(space.domains.items())[:15]:
    print vname, domain

Which now yields

x11 {1}
x12 {2,4,5,7}
x13 {4,5,7}
x14 {8}
x15 {2,4,7,9}
x16 {2,4,7,9}
x17 {2,3,4,7}
x18 {3,4,6}
x19 {2,4,6}
x21 {6}
x22 {3}
x23 {4,7}
x24 {1,4,7}
x25 {5}
x26 {1,2,4,7}

We can see that the domains of the variables have been reduces dramatically, which will speed up backtracking by a huge factor. Another thing that has a big impact on the performance is the order in which the variables are tried. In general one wants find conflicts as early as possible, as this eliminates whole branches of the search tree at once. For the case of sudoku a columns wise ordering (or row or blockwise) has proven to be effective.

Finally we can solve the sudoku by backtracking. The solve function is a generator which iterates over all found solutions. In this case we only want one, so break out of the loop after the first one is found.

from constrainingorder.solver import solve

#column wise ordering
ordering = []
for i in numbers:
    for j in numbers:
        ordering.append('x%d%d' % (i,j))

#find first solution and print it, then stop
for solution in solve(space,method='backtrack',ordering=ordering):
    for i in numbers:
        for j in numbers:
            print solution['x%d%d' % (i,j)],

The output of the solution should look like this

1 2 5 8 9 4 7 3 6
6 3 7 1 5 2 9 4 8
8 9 4 6 7 3 5 1 2
4 5 1 2 8 7 6 9 3
3 6 2 9 4 5 1 8 7
9 7 8 3 1 6 2 5 4
2 4 6 5 3 9 8 7 1
7 8 9 4 6 1 3 2 5
5 1 3 7 2 8 4 6 9


[Felgenhauer]Bertram Felgenhauer and Frazer Jarvis. Enumerating possible sudoku grids. Technical report, 2005