SCIPincludeBranchruleMybranchingrule() called many times

Question

I'm including my own branching rule on SCIP and I'm using the SCIPincludeBranchruleMybranchingrule() function to initialize some branching rule data. One of the things I do is to call the SCIPgetNVars() function. When I run the code, I see that the function is called many times (not once, as I thought, before the B&B algorithm starts) and I get the following error triggered by the SCIPgetNVars() function:

[src/scip/scip.c:10048] ERROR: invalid SCIP stage <0>

I'm confused about the use of SCIPincludeBranchruleMybranchingrule(), since the documentation states that this function can be use to initialize branching rule data. I would like to initialize some data that can be used at every B&B node, maybe the branching rule data is not the right way of doing it.

I'll appreciate any help!

Answer 1

The important thing to note here is that there is no problem available yet for which you want to access the variables.

Branching rules of SCIP provide several callbacks for data initialization. The include- callback is only called once when SCIP starts, aka in the SCIP_STAGE_INIT stage of SCIP. At this stage, you want the branching rule to inform SCIP that it exists, and optionally introduce some user parameters that are problem-independent.

There are two more callback-functions that allow for storing data which are better suited for what you intend to do; SCIPbranchruleInitsolMybranchingrule which is called just before the (presolved) problem is about to be solved via branch-and-bound, and SCIPbranchruleInitMybranchingrule, which is called after a newly read problem was transformed.

Since the execution of a branching-rule is restricted to within the branch-and-bound-process, your callback is SCIPbranchruleInitSolMybranchingrule which you should implement by moving all problem specific data initialization there. Don't forget to also implement SCIPbranchruleExitsolMybranchrule to free the stored data every time the branch-and-bound search is terminated, either if search was terminated, or if a time limit was hit, or SCIP decided that it wants another restart.

FYI: Data that is allocated during the include-callback can be freed with the SCIPbranchruleFreeMybranchingrule-callback, which is executed once when SCIP is about to exit and free all left system-memory.