All of the examples can be found in Jupyter notebook form here.

N queens

using Convex, GLPK, LinearAlgebra, SparseArrays, Test
aux(str) = joinpath(@__DIR__, "aux_files", str) # path to auxiliary files

n = 8

We encode the locations of the queens with a matrix of binary random variables.

x = Variable((n, n), :Bin)
size: (8, 8)
sign: real
vexity: affine
id: 128…152

Now we impose the constraints: at most one queen on any anti-diagonal, at most one queen on any diagonal, and we must have exactly one queen per row and per column.

# At most one queen on any anti-diagonal
constr = Constraint[sum(antidiag(x, k)) <= 1 for k in -n+2:n-2]
# At most one queen on any diagonal
constr += Constraint[sum(diag(x, k)) <= 1 for k in -n+2:n-2]
# Exactly one queen per row and one queen per column
constr += Constraint[sum(x, dims = 1)==1, sum(x, dims = 2)==1]
p = satisfy(constr)
solve!(p, GLPK.Optimizer)

Let us test the results:

for k in -n+2:n-2
    @test evaluate(sum(antidiag(x, k))) <= 1
    @test evaluate(sum(diag(x, k))) <= 1
@test all(evaluate(sum(x, dims = 1)) .≈ 1)
@test all(evaluate(sum(x, dims = 2)) .≈ 1)
Test Passed

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