Why Variable is not in model after model.copy()

Answered

Runfeng Yu

February 07, 2023 13:01

• 
• 
• 

I have built a model called m and a linear expression lin_exp with variables x.
And I used the function m.copy() to create another model n.

Now, I want to modify the objective function of n. 
I tried to call the function n.setObjective(lin_exp).
Unfortunately, gurobi told me "Variable not in model". 

Besides, I tried to simplify the model m with function ps_m = m.presolve().
The function ps_m.setObjective(lin_exp) is still errored with "Variable not in model".

A simplified code block is as shown as follows:

m = gp.Model() # variables, constraints, objective function are defined

lin_exp = gp.LinExpr(x) # a long linear expression
m.update()

n = m.copy()
n.setObjective(lin_exp) # ERROR "Variable not in model"

ps_m = m.presolve()
ps_m.setObjective(lin_exp) # ERROR "Variable not in model"

Related to

• Error

0

• 

  Jonasz Staszek

  • Community Moderator

  February 07, 2023 13:21

  • 
  • 
  • 

  Hi Runfeng,

  in Gurobi a variable is an object which is tied to a model instance. Whenever you create a new model instance (e.g. by copying or creating a presolved model) you also generate "new" variables.

  One way to go about this could be, for example, trying to recreate the LinExpr by storing the names of the variables which are included in it in a list and then rebuilding it by, say:

  LinExprN = gp.LinExpr([1 for _ in var_names_to_include], [n.getVarByName(i) for i in var_names_to_include])

  Perhaps someone from Gurobi team can offer a more efficient approach?

  Best regards
  Jonasz

  0
• 

  Runfeng Yu

  February 07, 2023 14:54

  • 
  • 
  • 

  Hi Jonasz,

  Thank you for your plain explanation and timely reply!
  And I will try this method to recreate my codes.

  Best regards,
  Runfeng

  0
• 

  Elisabeth Rodríguez Heck

  • Gurobi Staff

  February 07, 2023 17:12

  Hi Runfeng, hi Jonasz,

  Perhaps someone from Gurobi team can offer a more efficient approach?

  Jonasz reply is correct, a Gurobi Var object is always associated with a particular model. It is not possible to create variables (or constraints) only once and use them on multiple models.

  Rebuilding the LinExpr as Jonasz suggests is the way to go.

  Best regards,

  Elisabeth

  0
• 

  Runfeng Yu

  February 08, 2023 03:15

  • 
  • 
  • 

  Hi Elisabeth,

  Thank for your reply! And it really helps. 

  Best regards,
  Runfeng

  0

Please sign in to leave a comment.