There may be a bug for "addGenConstrLog"

Answered

冠冠 李

January 29, 2021 13:09

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I establish a model that contain constrains:

model.addGenConstrLog(X[i], Y[i])

model.addConstrs((X[i]==Cost_Non[i]-(Cost_Tra[i]+C_e[i]) for i in range(N_i) ),"con_X") 

model.addConstrs(Cost_Non[i]-(Cost_Tra[i]+C_e[i])>=0 for i in range(N_i) )

There is a GurobiError: 'lower bound of x variable is less than zero for LOG function' if X[i] is free.

I confuse that, X[i]>=0 according to the constrain "model.addConstrs(Cost_Non[i]-(Cost_Tra[i]+C_e[i])>=0 for i in range(N_i) )".

Why this error occurs? What is the principle of solution method for "addGenConstrLog"? 

 

 

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  June 23, 2025 03:00 Edited

  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 why not try our AI Gurobot?.
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  Silke Horn

  • Gurobi Staff

  January 29, 2021 14:00 Edited

  The logarithm function only takes positive arguments. Hence the x-variable may not be negative here.

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  冠冠 李

  January 31, 2021 01:54

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  Thanks for your answer.

  Although x is free, I have a constrait

  model.addConstrs(Cost_Non[i]-(Cost_Tra[i]+C_e[i])>=0 for i in range(N_i) )

  to satisfy x>=0. However, it doesn't work on "x". I'm confused on it.

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  Jaromił Najman

  • Gurobi Staff

  February 11, 2021 12:07

  Hi,

  The check whether the variable \(x\) in \(\log(x)\) is \(\geq 0\) is done before the presolving step. Thus, it is required to explicitly set the lower bound of the \(x\) variable to 0. Moreover, it is a good modelling style to provide all bounds they are known.

  Best regards,
  Jaromił

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