how to remove and add back a constraint?

Answered

Mandar U

• June 21, 2019 15:10
• Edited

import gurobipy as grb

model = grb.read('model.lp')
model.optimize()

c = model.getConstrByName('budget')
c.__dict__

# this shows NULL object : {'_cmodel': <capsule object NULL at 0x1063e6420>, '_rowno': 0}
# is that a problem?

model.remove(c)

# looking a new lp I can confirm that this deletes the constraint.

 

Now how can I add back constraint `c`  ?

 

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• Official comment

  

  Simranjit Kaur

  Gurobi Staff

  • December 06, 2024 00:58

  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?.
• 

  Greg Glockner

  Gurobi Staff

  • June 21, 2019 16:20

  Gurobi does lazy updates to the model. You won't see that this constraint was deleted until the update is processed. To force the update to process immediately, call model.update().

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• 

  Mandar U

  • June 21, 2019 21:21

  

  

  

  I understand. My question is, how can I add this constraint back to the model? 

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• 

  Greg Glockner

  Gurobi Staff

  • June 21, 2019 22:54

  If you want to remove a constraint and add it later, you need to get the elements of a constraint. For a linear constraint, it looks like this:

  c = m.getConstrs()[0] # for example
  lhs, sense, rhs, name = m.getRow(c), c.Sense, c.RHS, c.ConstrName
  m.remove(c)
  
  # Optional, to show the current state
  m.update()
  m.printStats()
  
  # Add the constraint
  c = m.addConstr(lhs, sense, rhs, name)
  
  # Also optional, to show the current state
  m.update()
  m.printStats()

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• 

  Jia-Kai Kuo

  • September 19, 2020 07:01

  

  

  

  That's helpful, thanks.
  
  But if I want to add constraint back in a .NET environment, how can I do to get the lhs of the constraint c?
  I found that there's no attribute 'LHS' in a constraint of .NET environment so I don't know how to do that....
  
  Thanks!

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• 

  Jaromił Najman

  Gurobi Staff

  • September 21, 2020 08:48

  Hi,

  You can get the LHS by using the getRow() function.

  Best regards,
  Jaromił

  1
• 

  Fei Wang

  • May 07, 2023 00:58
  • Edited

  

  

  

  Hello,

  It seems

  c = m.addConstr(lhs, sense, rhs, name) 

  is no longer supported in the newest version of gurobi. What should I do to add back a constraint in this case?

   

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• 

  Riley Clement

  Gurobi Staff

  • May 08, 2023 00:20

  Hi Fei,

  Model.addLConstr should solve your problem.

  - Riley

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