TypeError: object of type 'gurobipy.LinExpr' has no len()

Answered

Hernani Delgado Chantre

March 26, 2021 18:32

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Hi there, I am new in  Gurobi, so I wonder if you could help me on a problem.

I have this constraint

\[\psi_{uik} = \max_{v \in V, 1 \leq k\leq K }\{\tau_v x_{uvik}\} \quad \forall u \in U, 1 \leq i \leq C_u \]

where \(x_{u,v,i,k}\) is Binary decision variable and \(\psi_{uik}\) is Continuous decision variables and \(\tau_{v}\) is an array of constant.

I wrote the constraint as follow:

for u in U:
  for i in range(C_u[u]):
    for v in V:
      for k in range(1,K):
        model.addConstr(psi_ui[u,i,k] == max_(tau_v[v]*x[u,v,i,k]))

And I got this Error : TypeError: object of type 'gurobipy.LinExpr' has no len()

 would like to get some insights how to solve or write the constraint properly.

Thanks

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  June 23, 2025 02:57 Edited

  This post is more than three years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum. Or why not try our AI Gurobot?.
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  Matthias Miltenberger

  • Gurobi Staff

  March 29, 2021 13:00

  Hi Hernani,

  The argument in the \(\texttt{max_()}\) call is just a single element because both \(\tau_v\) and \(x_{uvik}\) are scalar elements. You should use list comprehension to pass the entire array:

  model.addConstr(psi_ui[u,i,k] == max_([tau_v[v]*x[u,v,i,k] for v in V]))

  You also need to remove the for-loop over \(\texttt{V}\) as a consequence.

  Cheers,
  Matthias

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• 

  Hernani Delgado Chantre

  March 30, 2021 11:55

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  Hi Matthias, thank you in advance for you time,

  I wrote the problem as follow

  for u in U:
      for i in range(C_u[u]):
          for k in range(K):
               model.addConstr(psi_ui[u,i,k] == max_([tau_v[v]*x[u,v,i,k] for v in V]))

   

  And got this error: line 185, in model1NKpsv1qs
  model.addConstr(psi_ui[u,i,k] == max_([tau_v[v]*x[u,v,i,k] for v in V]))
  File "model.pxi", line 2943, in gurobipy.Model.addConstr
  File "model.pxi", line 3287, in gurobipy.Model.addGenConstrMax
  gurobipy.GurobiError: Invalid data in vars array

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• 

  Matthias Miltenberger

  • Gurobi Staff

  March 30, 2021 12:19 Edited

  Hi Hermani,

  You need to rewrite your constraints and probably add new auxiliary variables The \(\texttt{max_()}\) constraint only takes a list of variables - you cannot pass coefficients or weights for the variables.

  Please excuse the confusion.

  Cheers,
  Matthias

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• 

  Hernani Delgado Chantre

  March 31, 2021 16:23

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  Hi Matthias, thank you for the your advise it solved my problem this is how i solve the problem,

  adding new auxiliary variables and than passing it to the model.addGenConstrMax(psi_ui[u,i,k],maxVarsList1)

  for u in U:

          for i in range(C_u[u]): 

              for v in V:

  
  

                  for k in range(1,K):

                      model.addConstr(tpsi_ui[u,i,k] == tau_v[v]*x[u,v,i,k])

          maxVarsList1 = [tpsi_ui[u,i,k] for i in range(C_u[u]) for k in range(1,K)]

          model.addGenConstrMax(psi_ui[u,i,k] ,maxVarsList1)

          

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