How to code subtour constraint in cplex python

Question

I am quite new in python cplex. I tried to model VRP and needed to eliminate subtours for feasible solution, but I cannot do that with the following code:

from docplex.mp.model import Model
import numpy as np
n = 10
Q = 20
N = [i for i in range(1, n+1)]
V = [0] + N
rnd = np.random
rnd.seed(0)
q = {i: rnd.randint(1, 10) for i in N}
loc_x = rnd.rand(len(V))*200
loc_y = rnd.rand(len(V))*100
A = [(i, j) for i in V for j in V if i != j]
c = {(i, j): np.hypot(loc_x[i]-loc_x[j], loc_y[i]-loc_y[j]) for i, j in A}
mdl = Model('CVRP')
x = mdl.binary_var_dict(A, name='x')
u = mdl.continuous_var_dict(N, lb=0, name='u')
mdl.minimize(mdl.sum(c[i, j]*x[i, j] for i, j in A))
mdl.add_constraints(mdl.sum(x[i, j] for j in V if j != i) == 1 for i in N)
mdl.add_constraints(mdl.sum(x[i, j] for i in V if i != j) == 1 for j in N)
mdl.add_constraints(mdl.add(u[i] - u[j] + n*(x[i,j]) <=  n - 1  for i,j in N if i!=j))
mdl.parameters.timelimit = 15
solution = mdl.solve()
print(solution)

When I run it, I get the following error:

File "C:\Users\user.spyder-py3\TSP\cplexExp.py", line 25, in mdl.add_constraints(mdl.add(u[i] - u[j] + n*(x[i,j]) <= n - 1 for i,j in N if i!=j))

TypeError: cannot unpack non-iterable int object

I really appreciate for any help. thanks!!

You have stored arcs as (i,j) 2-tuples in list `A` so I guess you should iterate on A as `for i,j in A` — Philippe Couronne, Aug 11 '21 at 20:06

score 0 · Answer 1 · answered Aug 13 '21 at 08:31

Thanks guys, the problem is that N is one dimensional array, but I considered it as two dimensional by using i,j in N. I figured that out. By the way, do you know any good resource to learn CPLEX Python? I am new and do not know where to start. Thanks!!

score 0 · Answer 2 · answered Aug 13 '21 at 08:56

0

Have a look at the Docplex examples here: https://github.com/IBMDecisionOptimization/docplex-examples

answered Aug 13 '21 at 08:56

Philippe Couronne

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How to code subtour constraint in cplex python

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