Gurobi 9.1.1: GRBaddsos failed: Problem adding SOS constraints.

Answered

Anna Thünen

August 31, 2021 13:29

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Hi there!

I am using Gurobi with AMPL.
I am solving a MINLP with SOS1 constraints.
Gurobi does the job great on small instances.
In order to be able to scale the problem and solve it with many more variables and constraints, I intend to investigate and to develop heuristics. (..e.g. to hot start Gurobi)
Therefore I want to use Gurobi to solve the NLP relaxation of the problem, i.e. with ignored binary condition.

Ok, now the issue:
For the NLP relaxation I get the following error code:
Gurobi 9.1.1: GRBaddsos failed:
Problem adding SOS constraints.
(which is weird because it also happens if I initialize Gurobi with the MINLP solution)
Also if I start the Gurobi in the optimal NLP solution, it returns the very same error code.

My code for the SOS1 is:
SOS1{n in Grid_t}: 1 = sum{j in In} w[n,j];

I would be glad to get some hints and tips how to resolve that issue.
Best,
Anna

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  June 23, 2025 02:48 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 06, 2021 12:30

  Hi Anna,

  SOS variables are set via the sosno suffix as described in How can I use the solver's "special ordered sets" feature?

  There is also a different way described by Bob Fourer in the conversation about AMPL SOS constraints on Google Groups.

  Best regards,
  Jaromił

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