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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• 

  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()

  2
• 

  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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