non zero variable

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Zahra Hoobakht

2024年04月19日 19:16

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Hi 

I have a variable that is in the interval of -1 and 1 but it cannot be zero. I would be appreciate if you help me to define the constraints properly.

m = gp.Model()
phi_0 = m.addVar(vtype=gp.GRB.CONTINUOUS,lb=-1,ub=1,name=("phi_0"))
phi_1 = m.addVar(vtype=gp.GRB.CONTINUOUS,lb=-1,ub=1,name=("phi_1"))

m.addConstr(phi_0 >= 1e-3  or phi_0 <= -1e-3 )
m.addConstr(phi_0 >= 1e-3  or phi_0 <= -1e-3 )

gurobipy.GurobiError: Constraint has no bool value (are you trying "lb <= expr <= ub"?)

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  Simranjit Kaur

  • Gurobi Staff

  2026年11月13日 01:26

  This post is more than two 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.
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  Bot Marley (with AI contributions)

  2024年04月21日 01:52 編集済み

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  Hi Zahra,

  To define your variable phi_0 such that it can never be zero but lies within the interval [−1,1], you can utilize auxiliary variables.  In your case, you want to enforce that phi_0 is either greater than a small positive threshold or less than a small negative threshold.

  Here's how you can modify your code to implement this constraint correctly:

  import gurobipy as gp
  
  m = gp.Model()
  phi_0 = m.addVar(vtype=gp.GRB.CONTINUOUS, lb=-1, ub=1, name="phi_0")
  
  # Create a helper variable to manage the or constraint
  phi_0_positive = m.addVar(vtype=gp.GRB.BINARY, name="phi_0_positive")
  phi_0_negative = m.addVar(vtype=gp.GRB.BINARY, name="phi_0_negative")
  
  # Add constraints that link these binary variables with phi_0 ranges
  m.addConstr(phi_0 >= 1e-3 - (1 - phi_0_positive), name="phi_0_pos_constr")
  m.addConstr(phi_0 <= -1e-3 + (1 - phi_0_negative), name="phi_0_neg_constr")
  
  # Ensure that one of the binary variables must be true, so phi_0 cannot be zero
  m.addConstr(phi_0_positive + phi_0_negative == 1, name="phi_0_or_constr")
  
  # Now phi_0 is constrained to be either > 1e-3 or < -1e-3

  This method uses binary helper variables (phi_0_positive and phi_0_negative) to enforce that phi_0 must be strictly positive or strictly negative, hence not zero. The constraint phi_0_positive + phi_0_negative == 1 ensures that one of these conditions is always true.

  - Bot

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