[SCIP] SCIP question: priced variables never show up in LP branching candidates

[myroslav]

Dear all,


I am a PhD student at the university of Würzburg, I work on Graph 
Drawing and am trying to implement a branch and price algorithm to solve 
ILPs arising from some graph drawing problems using SCIP. I am quite new 
to IP solvers and algorithms and wanted to ask two questions: 1) about 
what could possibly go wrong in my implementation and 2) about how 
branch and price algorithm works in  SCIP.

1)

For any node in the branching tree when I do branching in the callback 
BRANCHEXECL  it turns out that the priced variables never show up among 
the LP branching candidates (obtained using method 
SCIPgetLPBranchCands). It looks like they always have zero values in the 
solution of the LP.  That is weird, because if the pricer has added 
them, they should have nonzero values in the LP solution.  At the end of 
the solving process the solution eventually contains some of the priced 
variables even thought none of them was branched on. I wondered, what I 
possibly could be doing wrong.

2)

Also I wanted to ask specifically about the branch and price algorithm 
just to make sure if I understood it correctly. Does an LP at a 
branching node get the priced variables of the parent node or the 
pricing starts anew from a scratch(with no priced variables initially)?


Thanks in advance,

Myroslav