Sort Decision variables' value in Gurobi Python

Answered

ahmad alanaqreh

September 13, 2021 10:12

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I have a decision variables list as follows :

vars=[ <gurobi.Var x1,5,>, <gurobi.Var x5,1,>, <gurobi.Var x1,22,>, <gurobi.Var x22,1,>]

if I want to sort the elements based on the indices of each var as follows :

vars=[ <gurobi.Var x1,5,>,, <gurobi.Var x1,22,>, <gurobi.Var x5,1,>, <gurobi.Var x22,1,>]

I am using the following :

vars.sort(key=lambda k: x[k], reverse=False)

but it gives an error message 

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  June 23, 2025 02:47 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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  Jaromił Najman

  • Gurobi Staff

  September 14, 2021 08:24 Edited

  Hi Ahmad,

  Please always provide a minimal working example to make your issue reproducible and post the error message you get.

  Before optimization has been performed you can only really sort your variables w.r.t. their VarName attribute. Unless you want to sort the variables w.r.t. the indices you used.

  Best regards,
  Jaromił

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  ahmad alanaqreh

  September 14, 2021 08:12

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  Hi Jaromil 

  I am so sorry if the issue was not clear, let me give more details :

  I have a model where the variable x is defined as follow :

  x = {}
  for (i, j) in E:
    x[i, j] = model.addVar(vtype=GRB.BINARY, name="x%d,%d," % (i, j))

  where E is a set of edges, now in a specific step of my work I want to get the x by using 

  variables = model.getVars()

  and I want the vars to be sorted based on their indices (as I mentioned in the previous example) by 

  modelvars = variables.sort(key=lambda k: x[k], reverse=False)

  but I got the following error message 

   File "E:/Induced path/Benders/LIP2_callback.py", line 300, in <lambda>
  modelvars = variables.sort(key=lambda k: x[k], reverse=False)
  KeyError: <gurobi.Var x[1,2]>

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  Jaromił Najman

  • Gurobi Staff

  September 14, 2021 09:19

  Hi Ahmad,

  Since you are naming your variables appropriately, one easy way to sort the variables list would be

  variables.sort(key=lambda x: x.VarName)

  Best regards,
  Jaromił

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  ahmad alanaqreh

  September 14, 2021 11:04

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  Hi Jaromil 

  your suggestion kind of works, the problem that by using the x.VarName it takes x,1,11 to be before x,1,2 in the sorted list !!

   

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  Jaromił Najman

  • Gurobi Staff

  September 14, 2021 12:12 Edited

  Hi Ahmad,

  In this case, you have to go a bit further and provide a tuple for sorting consisting of numbers

  variables.sort(key=lambda x: (float(x.VarName.split(",")[0][1:]),float(x.VarName.split(",")[1])))

  The VarName is spit at "," providing 3 strings \(\texttt{xi}\) , \(\texttt{j}\) and an empty character.

  The tuple for the \(\texttt{lambda}\) function consists of two entries, the first index and the second one in VarName.

  Best regards,
  Jaromił

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