Adding new variables to existing model

Answered

Jonas Grundler

• June 20, 2022 14:24

Dear all,

I am working on the following model:

m = Model('Test')
first_set = list(range(3))
second_set = list(range(3))
x = m.addVars(first_set, second_set, vtype=GRB.CONTINUOUS) #9
y = m.addVars(second_set, vtype=GRB.BINARY) #3

m.addConstrs(x[i,j] <= y[j] for i in first_set for j in second_set)
m.addConstrs(quicksum(x[i,j] for j in second_set) >= 1 for i in first_set)

m.setObjective(quicksum(y[k] for k in second_set), GRB.MINIMIZE)
m.optimize()

However, if I 'simply' want to add some variables, the model becomes infeasible:

first_set.append(3)
for j in second_set:
    x[3,j] = m.addVar(3,j, vtype=GRB.CONTINUOUS)

m.update()
m.optimize()
for v in m.getVars():
    print(v.VarName, v.X)

Why can I observe this behaviour and how can I circumvent it? Thanks in advance!

Best wishes,
Jonas

0

• 

  Marika Karbstein

  Gurobi Staff

  • June 20, 2022 14:54

  Hi Jonas,

  The lines

  for j in second_set:
      x[3,j] = m.addVar(3,j, vtype=GRB.CONTINUOUS)

  add variables with lower bound 3 and upper bound j=0/1/2 which is infeasible.

  In general, the article How do I determine why my model is infeasible? might help.

  Best regards,
  Marika

  0
• 

  Marika Karbstein

  Gurobi Staff

  • June 20, 2022 15:02
  • Edited

  You probably wanted to do something like

  x[3] = m.addVars(second_set, vtype=GRB.CONTINUOUS)

  which adds multiple variables without defining specific bounds?

  0
• 

  Jonas Grundler

  • June 21, 2022 08:27

  

  

  

  Hi Marika,

  thanks, this helps!
  However, the new variables are not forced to fulfill the following constraint:

  m.addConstrs(quicksum(x[i,j] for j in second_set) >= 1 for i in first_set)

  How can I change this? Thanks!

  Best wishes,
  Jonas

  0
• 

  Marika Karbstein

  Gurobi Staff

  • June 21, 2022 08:42

  In your example, it was not clear that "3" is an index of first_set and that you need a variable for each pair of first_set, second_set.

  Did you try

  x = m.addVars(first_set, second_set, vtype=GRB.CONTINUOUS)

  ?

  Then your constraints should work.

  0
• 

  Jonas Grundler

  • June 22, 2022 07:03

  

  

  

  Yes, I tried, but unfortunately it does not work. Do I have to use the 'column' attribute here? Or is there a more convenient way?

  0
• 

  Marika Karbstein

  Gurobi Staff

  • June 22, 2022 07:40
  • Edited

  Ok, I see.
  Yes, you need to use the column argument like in the feasopt.py example (see also Modify a model) and add the variables individually with addVar().

  0
• 

  Jonas Grundler

  • June 22, 2022 08:02

  

  

  

  Okay, I see. Thanks a lot!

  0

Please sign in to leave a comment.