There may be a bug for "addGenConstrLog"

回答済み

冠冠 李

2021年01月29日 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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• 正式なコメント

  

  Simranjit Kaur

  • Gurobi Staff

  2025年06月23日 03:00 編集済み

  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

  2021年01月29日 14:00 編集済み

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

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  冠冠 李

  2021年01月31日 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

  2021年02月11日 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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