How to call Highs.writeLocalModel in C++

[rbp-bowmaker]

Hi, I'm stuck trying to get Highs.writeLocalModel to work. Has anyone got this working in C++, and have a code snippet to share?

The definition is:
HighsStatus Highs::writeLocalModel(HighsModel& model, const std::string& filename)
... so you need to pass it a reference to a model object.
The problem is that if I pass it a HighsModel object that has not yet been passed to Highs using Highs.passModel, it throws an exception. After a lot of debugging, it seems the problem is that HighsModel.lp_.a_matrix.num_col_ and num_row_ are not yet initialized, leading to an index out of bounds exception.
These two values are initialized when the model is passed to Highs, so it would work if I could pass Highs.model_ to writeLocalModel. However, the only way to access this model_ object in my code is to call the Highs.getModel(), but this returns a const reference to the model, whereas the writeLocalModel takes a non-const reference, so trying this results in a compilation error.

Anyone know how to do this?

[jajhall]

Sorry, Highs::writeLocalModel was introduced to save code duplication in Highs::writeModel and Highs::writePresolvedModel, where a previously "validated" internal HighsModel is passed. and not intended to be called by users. However, I can understand why it's useful if you want an MPS file of your model. Unfortunately, as well as the values of HighsModel.lp_.a_matrix.num_col_ and HighsModel.lp_.a_matrix.num_row_, there are other internal properties of HighsModel that must be correct for the model file to be written reliably.

That said, does it not work if you set HighsModel.lp_.a_matrix.num_col_ and HighsModel.lp_.a_matrix.num_row_ yourself?

If you've gone as far as passing your model to HiGHS, just call Highs::writeModel("foo.mps") to generate your MPS file.

[jajhall]

Updated Highs::writeLocalModel to set HighsModel.lp_.a_matrix.num_col_ and HighsModel.lp_.a_matrix.num_row_, and trap errors that would lead to segfaults.