Sudoku solver

In case you have missed out on the Sudoku hype, the goal is to fill in unspecified elements in a matrix with numbers between 1 to 9, keeping elements in all rows and columns different, and keeping all elements in the 9 3x3 blocks different. Unspecified elements are indicated by zeros here.

S = [0,0,1,9,0,0,0,0,8;6,0,0,0,8,5,0,3,0;0,0,7,0,6,0,1,0,0;...
     0,3,4,0,9,0,0,0,0;0,0,0,5,0,4,0,0,0;0,0,0,0,1,0,4,2,0;...
     0,0,5,0,7,0,9,0,0;0,1,0,8,4,0,0,0,7;7,0,0,0,0,9,2,0,0];

ans =
0  0  1  9  0  0  0  0  8
6  0  0  0  8  5  0  3  0
0  0  7  0  6  0  1  0  0
0  3  4  0  9  0  0  0  0
0  0  0  5  0  4  0  0  0
0  0  0  0  1  0  4  2  0
0  0  5  0  7  0  9  0  0
0  1  0  8  4  0  0  0  7
7  0  0  0  0  9  2  0  0

High-level model

In this example, we will first use the logic constraint alldifferent to pose and solve this problem. Note that this operator introduces (a lot of) binary variables, hence you need to have an efficient integer linear programming solver installed.

We begin by creating our 9x9 integer decision variable and the basic constraint structure.

M = intvar(9,9,'full');

fixed = find(S);
F = [1 <= M <= 9, M(fixed) == S(fixed)];

We add the logic constraints using some MATLAB indexing tricks, add some redundant cuts, and solve the problem (The solution time depends highly on your MILP solver. CPLEX, GUROBI and MOSEK solve this problem in roughly 0 seconds, while GLPK and LPSOLVE fail to solve the problem in reasonable time.)

for i = 1:3
 for j = 1:3
  block = M((i-1)*3+(1:3),(j-1)*3+(1:3))
  F = [F, alldifferent(block)];
 end
end

for i = 1:9
 F = [F, alldifferent(M(i,:))];
 F = [F, alldifferent(M(:,i))];
end

F = [F, sum(M,1) == 45, sum(M,2) == 45];
optimize(F);

Note that this model of the Sudoku game is pretty weak due to the simple implementation of the alldifferent operator in YALMIP!

Binary model

An alternative model can be created by using the support for multi-dimensional sdpvar variables. We will use a binary three-dimensional variable A(i,j,k) to indicate that element (i,j) has value k.

This model is much stronger, in the sense that it is easily solved using any MILP solver (even YALMIPs native solver BNB solves the problem in no time, indicating how simple this problem actually is). The drawback is that the model is much less intuitive, since it doesn’t use the simple alldifferent operator, but instead relies on pure binary constraints.

We begin by creating the variable, and define the basic Sudoku constraints (unique values in each row and column)

S = [0,0,1,9,0,0,0,0,8;6,0,0,0,8,5,0,3,0;0,0,7,0,6,0,1,0,0;...
     0,3,4,0,9,0,0,0,0;0,0,0,5,0,4,0,0,0;0,0,0,0,1,0,4,2,0;...
     0,0,5,0,7,0,9,0,0;0,1,0,8,4,0,0,0,7;7,0,0,0,0,9,2,0,0];

p = 3;
A = binvar(p^2,p^2,p^2,'full');
F = [sum(A,1) == 1, sum(A,2) == 1, sum(A,3) == 1];

Setting up the constraints for each 3x3 block is a bit messier.

for m = 1:p
    for n = 1:p
        for k = 1:p^2
            s = sum(sum(A((m-1)*p+(1:p),(n-1)*p+(1:p),k)));             
            F = [F, s == 1];
        end
    end
end

Define constraints for the specified elements

for i = 1:p^2
    for j = 1:p^2
        if S(i,j)
            F = [F, A(i,j,S(i,j)) == 1];
        end
    end
end

Note that the loop alternatively could have been simplified to a vectorized expression.

[i,j,k] = find(S);
F = [F, A(sub2ind([p^2 p^2 p^2],i,j,k)) == 1];

We are ready to invoke the solver.

diagnostics = optimize(F);

The integer solution is finally recovered from the binary indicators.

Z = 0;
for i = 1:p^2
      Z = Z  + i*value(A(:,:,i));
end
Z

The loop can alternatively be vectorized.

Z = value(reshape(A,p^2,p^4)*kron((1:p^2)',eye(p^2)))

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