attribute error '__cindex__' using absolute value and square root

Answered

María Ciudad Alañón

June 08, 2022 10:34

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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!

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 16, 2025 23:35

  This post is more than three years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
• 

  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

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  María Ciudad Alañón

  June 08, 2022 14:21

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  Thank you very much! It has been very helpful!

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