I've been trying to implement a mTSP in a normal TSP using the already made code in MIP Link
So this is the code I have so far in python and its throwing me an error which I don't understand:
places = ['Antwerp', 'Bruges', 'C-Mine', 'Dinant', 'Ghent',
'Grand-Place de Bruxelles', 'Hasselt', 'Leuven',
'Mechelen', 'Mons', 'Montagne de Bueren', 'Namur',
'Remouchamps', 'Waterloo']
salesman=['Salesman1','Salesman2']
# distances in an upper triangular matrix
dists = [[83, 81, 113, 52, 42, 73, 44, 23, 91, 105, 90, 124, 57],
[161, 160, 39, 89, 151, 110, 90, 99, 177, 143, 193, 100],
[90, 125, 82, 13, 57, 71, 123, 38, 72, 59, 82],
[123, 77, 81, 71, 91, 72, 64, 24, 62, 63],
[51, 114, 72, 54, 69, 139, 105, 155, 62],
[70, 25, 22, 52, 90, 56, 105, 16],
[45, 61, 111, 36, 61, 57, 70],
[23, 71, 67, 48, 85, 29],
[74, 89, 69, 107, 36],
[117, 65, 125, 43],
[54, 22, 84],
[60, 44],
[97],
[]]
# number of nodes and list of vertices
n, V, S = len(dists), set(range(len(dists))), set(range(len(salesman)))
# distances matrix
c = [[0 if i == j
else dists[i][j-i-1] if j > i
else dists[j][i-j-1]
for j in V] for i in V]
model = Model()
# binary variables indicating if arc (i,j) is used on the route or not
x = [[[model.add_var(var_type=BINARY) for j in V] for i in V] for s in S]
# objective function: minimize the distance
model.objective = minimize(xsum(c[i][j]*x[i][j][s] for i in V for j in V for s in S))
The error is:
IndexError Traceback (most recent call last)
<ipython-input-52-8550246fcd90> in <module>
48
49 # objective function: minimize the distance
---> 50 model.objective = minimize(xsum(c[i][j]*x[i][j][s] for i in V for j in V for s in S))
51
52
~/opt/anaconda3/lib/python3.7/site-packages/mip/model.py in xsum(terms)
1453 """
1454 result = mip.LinExpr()
-> 1455 for term in terms:
1456 result.add_term(term)
1457 return result
<ipython-input-52-8550246fcd90> in <genexpr>(.0)
48
49 # objective function: minimize the distance
---> 50 model.objective = minimize(xsum(c[i][j]*x[i][j][s] for i in V for j in V for s in S))
51
52
IndexError: list index out of range
And it doesn't make sense to me since I created just another summation. Thank you very much in advance.
This is a Python issue, not a Gurobi issue. You don't completely understand how nested list-comprehensions work.
We can reproduce this with:
x = [[["x%s%s%s" % (i,j,k) for i in range(2)] for j in range(2)] for k in range(3)]
i = 0
j = 0
k = 2
x[i][j][k]
The x is not x[i][j][k] but rather x[k][j][i]. So in the above example we see:
[[['x000', 'x100'], ['x010', 'x110']], [['x001', 'x101'], ['x011', 'x111']], [['x002', 'x102'], ['x012', 'x112']]]
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-13-a2e90cc605ea> in <module>()
4 j = 0
5 k = 2
----> 6 x[i][j][k]
IndexError: list index out of range
If we would have entered:
i = 0
j = 0
k = 2
print(x[k][j][i])
we would see:
x002
Conclusion: try:
x = [[[model.add_var(var_type=BINARY) for s in S] for j in V] for i in V]
...
x[i][j][s]