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
include(aux("antidiag.jl"))

n = 8
8

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

x = Variable((n, n), :Bin)
Variable
size: (8, 8)
sign: real
vexity: affine
id: 374…871

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 = -n+2:n-2]
# At most one queen on any diagonal
constr += Constraint[sum(diag(x, k)) <= 1 for k = -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 = -n+2:n-2
	@test evaluate(sum(antidiag(x, k))) <= 1
	@test evaluate(sum(diag(x, k))) <= 1
end
@test all(evaluate(sum(x, dims=1)) .≈ 1)
@test all(evaluate(sum(x, dims=2)) .≈ 1)
Test Passed

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