How to use the if statement with a variable?

Answered

Eduardo daniel perez

July 28, 2021 03:17

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Currently I'm implementing a python code, which is based on Xpress programming

To replicate this, I use:

Nevertheless when I'm programming there's an error:

AttributeError: Index out of range for attribute 'X'

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  June 23, 2025 02:50 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

  August 02, 2021 13:31

  Hi Eduardo,

  The X attribute is available only after the model has been optimized and at least one feasible solution has been found. In general, you cannot evaluate the value of a variable object because it does not have any value. You have to access it via its attributes.

  If the above does not help, could you post a minimal working example to make the issue reproducible?

  Best regards,
  Jaromił

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• 

  İrem Bahtiyar

  October 17, 2021 08:37

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  Hello, I have the same problem. x_j's are binary variables. But this does not work. how can I use variable in if statement?

  for j in range(T):
  sum8=[]
  sum9=[]
  if m.getattr(x[j])==0 :
  for i in range(C):
  if P[j] in max5[i]:
  sum8.append( 1/(v_0[i]+sum(max5[i]) - P[j]))
  elif P[j] not in max5[i]:
  sum8.append( 1/(v_0[i]+sum(max_sublist[i])))
  y_low0.append(sum8)

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• 

  Jose Vindel

  October 18, 2021 13:23 Edited

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  Hello Irem,

  I am also dealing with binary variables and this is what I do, I use binary variables to solve a knapsack problem where when \(y_s = 0\) is not in the knapsack. And get access to the values using \(model.getVarByName\)

   

  items = # list of items
  M = [i for i in range(len(items))]
  y = model.addVars(M,vtype=GRB.BINARY,name='y')
  
  # after the model has been solved
  
  if model.status == GRB.OPTIMAL:
     ys = [model.getVarByName('y[%d]'%j).x for j in M] # gets a list with all your binary solutions 
     for i in M:
        if ys[i] <.5:
           print('Item %s not in knapsack'%(item[i]))    

  Hope this helps.

  Alex.

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• 

  Jaromił Najman

  • Gurobi Staff

  October 19, 2021 06:49 Edited

  Hi İrem,

  The line

  if m.getAttr(x[j])==0 :

  should read

  if x[j].X==0 :

  Please note that the X attribute is only available after a previous optimization run found at least one feasible solution point.

  Jose's solution works as well.

  Best regards,
  Jaromił

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