Solution of optimization saved as array.
Answered
Hello everyone. I am new at programming and I've been struggling with this piece of code. I am working with a modified version of the Knapsack Problem.
- #Data
n = n. of items
weights =[...]
v = [...]
sigma =[...]
knapsack_model = Model('knapsack')
x = knapsack_model.addVars(n, vtype = GRB.BINARY, name="x")
# constraint on capacity
knapsack_model.addConstr(sum(weights[i]*x[i] for i in range(n)) <= C)
estimation = [ ]
for i in range(n):
values = np.random.normal(v[i], sigma[i])
estimation.append(values)
# Objective function
obj_fn = sum((estimation[i]*x[i]) for i in range(n))
knapsack_model.setObjective(obj_fn, GRB.MAXIMIZE)
knapsack_model.optimize()
print('Optimization is done. Objective Function Value: %.2f' % knapsack_model.objVal)
#Piece of code that gives me an error
total = [ ]
for v in knapsack_model.getVars():
total.append(v.x)
final_objective = sum((total[ i ]*v[ i ]) for i in range(n))
Here I get an error saying: 'Var' object is not subscriptable
I need to use the x that gurobi produced and multiply it with v[i], but I am having a hard time putting the x in an array that i can use.
If anyone could help me I would really appreciate it.
Thank you in advance!
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Official comment
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Gurobi Staff
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You are using the variable \(\texttt{v}\) in the \(\texttt{for}\)-loop
for v in knapsack_model.getVars():
total.append(v.x)but you have already defined and used \(\texttt{v}\) before. Using a different iterator in the above \(\texttt{for}\)-loop should solve the issue.
for var in knapsack_model.getVars():
total.append(var.x)Best regards,
Jaromił1 -
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