N = 8 -- size of the board


-- check whether position (n, c) is free from attacks 
function is_place_ok (a, n, c)
    for i = 1, n - 1 do -- leaving the inc/dec value empty defaults to +1 
        if (a[i] == c) or 
            (a[i] - i == c - n) or 
            (a[i] + i == c + n) then
            return false
        end
    end
    return true
end 

function print_solution (a)
    for i = 1, N, 1 do
        for j = 1, N, 1 do 
            io.write(a[i] == j and "X" or "-", " ")
        end
        io.write("\n")
    end 
    io.write("\n")
end