Pool Solution

Ongoing

mohammad hemmati

February 19, 2025 02:01

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Problem with value of variables I try to solve my model with 10 alternatives solutions. Niw while I see the model has 10 different objectives values, all of the variables remain unchanged in 19 solutions and that makes me wonder!

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  Ronald van der Velden

  • Gurobi Staff

  February 19, 2025 06:27

  Hi Mohammad,

  One cause might be that you look at the X attribute instead of the Xn attribute for retrieving the solutions. X will always contain the best solution irrespective of what you do with SolutionNumber. The values of Xn depend on how you set SolutionNumber.

  If that's not the issue, then do you have a small reproducible piece of code? 

  Kind regards,
  Ronald

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  mohammad hemmati

  February 19, 2025 10:00

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  Thank Ronald for your reply.

  My model is large MILP optimization. I already solved it with Gurobi, now I am going to provide some alternative solutions as follows:

  opt = SolverFactory("gurobi_persistent")
  opt.set_instance(model)

  opt.set_gurobi_param("PoolSolutions", 10)
  opt.set_gurobi_param("PoolSearchMode", 2)
  opt.set_gurobi_param("TimeLimit", 14400)
  opt.set_gurobi_param("MIPGap", 0.05)
  # حل مدل
  opt.solve()

  nSolutions = opt.get_model_attr("SolCount")
  print('Number of solutions found: ' + str(nSolutions))
  for e in range(nSolutions):
      opt.set_gurobi_param("SolutionNumber", e)
      print('%g ' % opt.get_model_attr("PoolObjVal"), end='')
  print('')

   

  I can see all 10 objectives, however, I don't know how I see the value of all variables in each solution. I use the following for this purpose:

  for e in range(nSolutions):  
      opt.set_gurobi_param("SolutionNumber", e)  

      cec_value = model.PipeOC.value 

      print(f'Solution {e + 1}: PipeOC = {cec_value}')

   

  But, in the output for all variables (in this example PipeOC) I see the same value in all solutions: 

  Solution 1: PipeOC = 147071.35583463698
  Solution 2: PipeOC = 147071.35583463698
  Solution 3: PipeOC = 147071.35583463698
  Solution 4: PipeOC = 147071.35583463698
  Solution 5: PipeOC = 147071.35583463698
  Solution 6: PipeOC = 147071.35583463698
  Solution 7: PipeOC = 147071.35583463698
  Solution 8: PipeOC = 147071.35583463698
  Solution 9: PipeOC = 147071.35583463698
  Solution 10: PipeOC = 147071.35583463698

   

  Thank you for your help.

  Mohammad

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