Min GenExpr and multiplying by -1

Answered

Phillip Meng

• October 05, 2020 07:38

We're trying to model and implement a constraint that relies on the "end of an event" in its schedule. 
To do so,  we're trying to do (in python):
g = min_(0, -w) or alternatively, g = - min_(0,w); w is an auxiliary variable in {-1,0,1} and we want to detect the case of w = -1.

However we get an error that the gen expr can't be multiplied or invalid array multiplication.

Is there any functionality to implement like this? Our alternative is to use additional auxiliary variables that are in essence the reverse of w, but it would be nice if this function could be possible.

0

• 

  Matthias Miltenberger

  Gurobi Staff

  • October 05, 2020 07:51

  Hi Phillip,

  Could you please post the (relevant section of) your source code so we can better help you understand the issue?

  Cheers,
  Matthias

  1
• 

  Phillip Meng

  • October 05, 2020 08:32
  • Edited

  

  

  

  Hi Matthias, we had:

  self.scheme.g_w_relationship = {}
          for b in self.list_of_b:
              self.scheme.g_w_relationship[b] = {}
              for event in self.events:
                  self.scheme.g_w_relationship[b][event] = {}
                  for season in self.seasons:
                      ret_name = 'f_w_rel' + '_' + str(b.id)  + '_' + str(event.id) + '_' + str(season.id)
                      lhs = self.scheme.g[b][event][season]
                      if not season.last:
                          rhs = self.model.max_([0, -self.scheme.w[b][event][season.successor]])
                      else:
                          rhs = 1 - self.scheme.a[b][event][season]
                      self.scheme.g_w_relationship[b][event][season] = self.model.add_constraint(full_expression=lhs == rhs, name=ret_name)

  It returns specifically:
  return self._model.addConstr(full_expression, name=name)
  File "model.pxi", line 3218, in gurobipy.Model.addConstr
  File "model.pxi", line 3939, in gurobipy.Model.addGenConstrMax
  gurobipy.GurobiError: Invalid data in vars array
  
  I hope that is useful.
  
  Regards, Phillip

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• 

  Matthias Miltenberger

  Gurobi Staff

  • October 05, 2020 10:58

  Hi Phillip,

  I'm sorry but this is still not very helpful. There is no add_constraint method in Gurobi; it appears the model object wraps the Gurobi Model. I also cannot tell what kind of data you are using to construct the max constraint and where the error really comes from.

  Please try to post a minimal reproducible example.

  Cheers,
  Matthias

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• 

  Phillip Meng

  • March 09, 2022 06:00

  

  

  

  Thought I would follow this one up whilst logged in - turns out our modelling was prohibitive for the constraint behaviour we were trying to design. Remodelling seemed to find a way around the issue.

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