using "if" in addVars()

Answered

Marina Csanady

• February 08, 2022 16:09

I'm working on a  problem with a lot of variables. In fact to much variables to run in reasonable time. My first approach was to create all possible varibles and delete the unnecessary ones (via model.remove()), but the runtime was still to long.

Therefore I want to eliminate the number of varibles before even creating them by using an "if" condition in addVars().
But now I have the problem of adressing the variables correctly. As error message i get

KeyError: (0,0)

Below you see a minimal example which represents my current approach:

import gurobipy as gp
from gurobipy import GRB

SetI=[i for i in range(3)]
SetJ=[i for i in range(2)]

m=gp.Model('example')

x=m.addVars([(i,j) for i in SetI for j in SetJ if i!=j], vtype=GRB.BINARY, name='x')

m.setObjective(sum(x[i,j] for i in SetI for j in SetJ ) , GRB.MINIMIZE)

m.addConstrs((sum(x[i,j] for j in SetJ )<=1 for i in SetI ), name='constr1') 


m.optimize()

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• 

  Jaromił Najman

  Gurobi Staff

  • February 09, 2022 09:49

  You can use the \(\texttt{if}\)-statement in other \(\texttt{for}\)-loops as well

  import gurobipy as gp
  from gurobipy import GRB
  
  SetI=[i for i in range(3)]
  SetJ=[i for i in range(2)]
  m=gp.Model('example')
  x=m.addVars([(i,j) for i in SetI for j in SetJ if i!=j], vtype=GRB.BINARY, name='x')
  m.setObjective(sum(x[i,j] for i in SetI for j in SetJ if i!=j) , GRB.MINIMIZE)
  m.addConstrs((sum(x[i,j] for j in SetJ if i!=j)<=1 for i in SetI), name='constr1') 
  m.optimize()

  Alternatively, you could generate an index list for your variables. However, this will not make the construction of your constraints easier due to the need of individually accessing the first or second index.

  import gurobipy as gp
  from gurobipy import GRB
  
  SetI=[i for i in range(3)]
  SetJ=[i for i in range(2)]
  varset = [(i,j) for i in SetI for j in SetJ if i!=j]
  m=gp.Model('example')
  x=m.addVars(varset, vtype=GRB.BINARY, name='x')
  m.setObjective(sum(x[i,j] for (i,j) in varset) , GRB.MINIMIZE)
  m.addConstrs((sum(x[i,j] for j in SetJ if i!=j)<=1 for i in SetI), name='constr1') 
  m.optimize()

  Best regards, 
  Jaromił

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• 

  Marina Csanady

  • February 09, 2022 10:34

  

  

  

  Thanks for your suggestions.
  
  I also tried to work with var.index, but also got the same error message. Why is that?

   

  import gurobipy as gp
  from gurobipy import GRB
  
  SetI=[i for i in range(3)]
  SetJ=[i for i in range(2)]
  m=gp.Model('example')
  x=m.addVars([(i,j) for i in SetI for j in SetJ if i!=j], vtype=GRB.BINARY, name='x')
  m.setObjective(sum(x[i,j] for i in SetI for j in SetJ if x[i,j].index!=-1 ) , GRB.MINIMIZE)
  m.addConstrs((sum(x[i,j] for j in SetJ if x[i,j].index!=-1)<=1 for i in SetI), name='constr1') 
  m.optimize()

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• 

  Jaromił Najman

  Gurobi Staff

  • February 09, 2022 12:17

  Working with var.index does not save you from trying to access key \(\texttt{(0,0)}\). You are still accessing the not  available variable \(\texttt{x[0,0]}\) to check its \(\texttt{.index}\) field.

  Best regards, 
  Jaromił

  0

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