attribute error '__cindex__' using absolute value and square root

Answered

María Ciudad Alañón

• June 08, 2022 10:34

Hello!

I'm trying to implement a maximization of the following function: \sqrt(\abs(I)) + \sqrt(\abs(J)), where I and J are linear combinations of other variables. For that, I have add auxiliary variables for I and J and then I add also auxiliary variables for the absolute value (Iabs and Jabs) and for the square root of them (Isqrt and Jsqrt) and add the correspond constraints as follows:

m.addGenConstrAbs(Iabs,I)
m.addGenConstrAbs(Jabs,J)
m.addGenConstrPow(Iabs,Isqrt,0.5)
m.addGenConstrPow(Jabs,Jsqrt,0.5)

However I got the following error: AttributeError: 'gurobipy.LinExpr' object has no attribute '__cindex__'

Do you know what am I doing wrong? Thank you!

0

• 

  Marika Karbstein

  Gurobi Staff

  • June 08, 2022 13:28
  • Edited

  Hi María,

  How did you add your auxiliary variables?
  Doing like this

  import gurobipy as gp
  m = gp.Model("abs")
  
  x = m.addVars(5, vtype = gp.GRB.CONTINUOUS, name = "x")
  Iabs = m.addVar(vtype = gp.GRB.CONTINUOUS, lb = 0, name = "Iabs")
  I = gp.quicksum(x[i] for i in range(4))
  m.addGenConstrAbs(Iabs, I, "abs")

  would result in the error you mentioned.

  You need to add the auxiliary variable and the constraint also to the model:

  import gurobipy as gp
  m = gp.Model("abs")
  
  x = m.addVars(5, vtype = gp.GRB.CONTINUOUS, name = "x")
  Iabs = m.addVar(vtype = gp.GRB.CONTINUOUS, lb = 0, name = "y")
  I = m.addVar(vtype = gp.GRB.CONTINUOUS, name = "I")
  m.addConstr(I == gp.quicksum(x[i] for i in range(4)))
  m.addGenConstrAbs(Iabs, I, "abs")

  I hope this helps,
  Marika

  0
• 

  María Ciudad Alañón

  • June 08, 2022 14:21

  

  

  

  Thank you very much! It has been very helpful!

  0

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