Turning off all presolving operations in SCIPpresolve() + retrieving symmetry information

[FZixuan]

Hello everyone,

I want to retrieve the constraint classification information for MIP problems and I need to completely turn off the presolving and all possible operations that could be triggered by calling SCIPpresolve() (e.g. symmetry detection). For now, I'm using the following settings and I wonder whether there are any other parameters I need to change.

presolving/maxrounds = 0
misc/usesymmetry = 0

Additionally, I wonder whether there is any way to retrieve symmetry information after I turn it on (misc/usesymmetry = 7), such as the number of symmetries of each type and their sizes.

Thank you in advance for your help!

Best,
Zixuan

[christopherhojny]

Dear Zixuan,

Your current setup should be fine in principle. By setting the two parameters, presolving and symmetry detection are deactivated and the provided problem should not be manipulated. However, since SCIP's upgrade methods for linear constraints are called during presolve, all constraints of a MILP will be classified as cons_linear when presolving is disabled. If you are interested in classifications like cons_knapsack, SCIP natively supports this only during presolving.

A possible work around could be to write your own C-code. After disabling presolving and symmetry detection, you need to iterate over all constraints and call the function cons_linear.c:SCIPupgradeConsLinear() manually to trigger an upgrade of constraints. This way, SCIP will not perform any presolving/symmetry handling and the constraint classification will be visible. Note, however, that SCIPupgradeConsLinear() possibly manipulates the constraints because it calls functions like normalizeCons() and tightenSides(). That is, the upgraded constraints do not necessarily correspond to the constraints of the original problem even when presolving is disabled.

To retrieve symmetry information, you can indeed set misc/usesymmetry = 7 to enable all symmetry handling methods. Via the SCIP shell, you can then get the symmetry information by calling presolve (even when presolving is disabled, this will then only trigger symmetry detection) and afterwards display symmetry. If you are writing your own C-code, then the function prop_symmetry.c:SCIPgetSymmetry() allows you to retrieve symmetry information.

Please note, however, that SCIP does not provide you all symmetries of the problem, but only a set of permutations that generate the entire (formulation) symmetry group. Moreover, some symmetry handling methods can compute additional redundant generators. For example, for miplib2010/cov1075.mps.gz, SCIP will report symmetry computation finished: 9 generators found, but calling display symmetry will show 37 permutations. The first 9 permutations will then correspond to the initially computed generators, whereas the remaining 28 permutations are additional redundant permutations.

Best regards,
Christopher