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

回答済み

Runfeng Yu

2023年02月07日 13:01

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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"

関連

• Error

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• 正式なコメント

  

  Simranjit Kaur

  • Gurobi Staff

  2026年11月13日 01:49

  This post is more than two 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 try Gurobot, our chatbot interface offering instant, expert-level support.
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  Jonasz Staszek

  • Community Moderator

  2023年02月07日 13:21

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  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

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  Runfeng Yu

  2023年02月07日 14:54

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  Hi Jonasz,

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

  Best regards,
  Runfeng

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  Elisabeth Rodríguez Heck

  • Gurobi Staff

  2023年02月07日 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

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  Runfeng Yu

  2023年02月08日 03:15

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  Hi Elisabeth,

  Thank for your reply! And it really helps. 

  Best regards,
  Runfeng

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