# constraint : leave each city coming from another city
for i in N:
model += xsum(x[a] for a in IN[i]) == 1
# no subtours of size 2
for a in A:
if (a[1], a[0]) in A.keys():
model += x[a] + x[a[1], a[0]] <= 1
# computing farthest point for each point
F = []
G = nx.DiGraph()
for ((i, j), d) in A.items():
G.add_edge(i, j, weight=d)
for i in N:
P, D = nx.dijkstra_predecessor_and_distance(G, source=i)
DS = list(D.items())
DS.sort(key=lambda x: x[1])
F.append((i, DS[-1][0]))
model.lazy_constrs_generator = SubTourCutGenerator(F)
model.optimize()
print(model.status)
print('best route found has length {}'.format(model.objective_value))
arcs = [a for a in A.keys() if x[a].x >= 0.99]
print('optimal route : {}'.format(arcs))
for j in range(n) if i + j == k) <= 1
if cut.violation > 0.001:
model.add_cut(cut)
# number of queens
n = 50
queens = Model('queens', MAXIMIZE)
x = [[
queens.add_var('x({},{})'.format(i, j), var_type=BINARY) for j in range(n)
] for i in range(n)]
# one per row
for i in range(n):
queens += xsum(x[i][j] for j in range(n)) == 1, 'row({})'.format(i)
# one per column
for j in range(n):
queens += xsum(x[i][j] for i in range(n)) == 1, 'col({})'.format(j)
queens.lazy_constrs_generator = DiagonalCutGenerator()
queens.optimize()
stdout.write('\n')
for i, v in enumerate(queens.vars):
stdout.write('O ' if v.x >= 0.99 else '. ')
if i % n == n - 1:
stdout.write('\n')