Can't get constraint, like 0 <= 1, by name

Answered

ce jekl

• May 29, 2022 10:40
• Edited

I use

model.addConstr(x.sum(i, '*') <= 1, f"constr_{i}")

However, if x.select(i, '*') is empty, the constraint cannot be get by name.

m = Model()
c = m.addConstr(0 <= 1, "123")
m.update()
assert c is not None
cc = m.getConstrByName("123")
assert cc is None

How can I get this constraint? I want to keep the number of the constraints the same as my dict, so I can't just skip these trivial constraints.

PS: I found a workaround to this problem: just change the zero left hand side to LinExpr(0)

#c = m.addConstr(0 <= 1, "123")
c = m.addConstr(LinExpr(0) <= 1, "123")

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• 

  Eli Towle

  Gurobi Staff

  • May 31, 2022 20:52

  You could also pass the constraint name to Model.addConstr() using the \(\texttt{name}\) keyword argument instead of as a positional argument:

  c = m.addConstr(0 <= 1, name='c123')
  m.update()
  print(c.ConstrName)  # c123

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• 

  ce jekl

  • June 10, 2022 08:28

  

  

  

  I think your point is interesting, with name keyword, getConstrByName works ok, however, it doesn't work without the keyword.

  m = Model()
  c1 = m.addConstr(0 <= 1, "c1")
  c2 = m.addConstr(0 <= 1, name="c2")
  m.update()
  assert c1 is not None and c2 is not None
  cc1 = m.getConstrByName("c1")  # cannot get c1
  assert cc1 is None
  cc2 = m.getConstrByName("c2")  # can get c2. Why?
  assert cc2 is not None

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• 

  Eli Towle

  Gurobi Staff

  • June 11, 2022 02:58

  Right, I was suggesting a different workaround. We have logged this as a bug; thanks for reporting it!

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• 

  ce jekl

  • June 11, 2022 17:41

  

  

  

  Thanks a lot!

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