Get variables indexes

Answered

José Rodrigues

• July 26, 2022 09:19

Hi everyone, 

I have a binary variable, named X, with two indexes (X_i,j). After running the optimization, I want to know for what index in i, is the binary variable X equal to one. I've written the script as in below, however, this way, the v.index is the multiplication of i times j. I would also like to know, for each i (where X_i,j == 1), what are the j's for which X_i,j == 1.  If I print, the binary variable, it looks like this: X[221,8759] (value 0.0)>}.

In short, what I want is a list, with the i indexes for which X_i,j ==1, but not with repeated i's and another list with the j indexes corresponding to each i for wich X_i,j == 1.

X_index = []
for v in X.values():
    if v.X == 1:
        X_index.append(v.index)

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• Python

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• 

  Simon Bowly

  Gurobi Staff

  • July 27, 2022 07:35

  Hi José,

  "v.index" will give you the index of the variable in the model. This is different from your indices i and j, as it is an internal Gurobi variable counter.

  I believe what you want is the following (assuming that X is a tupledict created using model.addVars()):

  import collections
  
  nonzeros_ij = collections.defaultdict(list)
  any_nonzero_i = set()
  
  for (i, j), v in X.items():
      if v.X > 0.9:  # best to allow for numerical tolerances
          nonzeros_ij[i].append(j)
          any_nonzero_i.add(i)

  "nonzeros_ij" will then be a mapping from an index i to a list of j indices such that X[i,j] is nonzero, while "any_nonzero_i" will be a set of i indices for which X[i,j] is nonzero for some j

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• 

  José Rodrigues

  • August 10, 2022 16:36

  

  

  

  Hi Simon, 

  Thank you so much for your reply, it is exactly what I was looking for. Nevertheless, there is something still troubling me. 

  I have another variable with the same i and j, HP which is a tupledict created using model.addVars() and this time it's continuous. In this case, besides doing the same as you wrote above, I would also like to get the value of the variable HP, in different lists according to each i at every j. Actually, it's quite similar to getting the nonzeros_ij dictionary that you wrote but this time appending the variable value instead of the j. 

   

  Could you write down how to do it since I'm not being able to?

  Thank you once more, 

  José Rodrigues 

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• 

  Alison Cozad

  Gurobi Staff

  • January 20, 2023 15:01

  You can retrieve variable values after optimizing with the X variable attribute.  For example, Simon used this attribute check with the variable value \(\texttt{v.X}\) is greater than 0.9 in the conditional \(\texttt{if v.X > 0.9}\).  For more details, see How do I retrieve variable values after optimizing?

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