Accessing Formulation Matrix before solve: Linear Program

GAMSPy
,
1

Description of the issue

I would like to access the A b and c matrix values, and the order of variables x before passing to solver. Is it possible to generate and write these out in gamspy?

Error message and stack trace

NA

GAMSPy version

NA- any gamspy

2

Hi @vpranjal ,

You could use getEquationListing and getVariableListing to get the generated variables and equations.

As an alternative, you could also use the convert “solver” and export the generated model instance to a gdx file

myModel.solve(solver="convert", solver_options={"dumpgdx": "path\\to\\myGDXFile.gdx"})

I hope this helps!

1 Like
3

Thanks for the reply. I am able to explore the GDXfile now (listVariables and listEquations are useful options to view).

I was trying to recover the following matrices from the formulation (A, b, Aeq and beq) similar to the following:
https://in.mathworks.com/help/optim/ug/mpsread.html

If it is possible. Thanks

4

You could extract the right hand side b and the sense of an equation by looking at the lo and up attributes of symbol e in the gdx file.

  • if they are equal, it is an equality constraint
  • if lo is -inf it is a =l= constraint
  • if up is +inf it is a =g= constraint
5

Thanks for the direction.

The following works:
(Let m be the container we read the .gdx file into)

from gamspy import Container, Set
m = Container()
m.loadRecordsFromGdx(“myGDXFile.gdx”)

m[“A”].records in GAMSPy - GAMS Studio has a way to do this in Table view
m[“A”].records.pivot(index=‘i’, columns=‘j’, values=‘value’) or something along these lines in GAMSPy for table view

and m.[“e”].records in GAMSPy - m.[“e”].records[‘level’] I believe for b matrix
lower and upper column for lo and up

m[“j”].records give the mapping between x variables and user given variable names