[Scip] Calling a separator

[Gerald Gamrath]

Dear Hélène,

> I have implemented a separator using the C++ class. I have redefined 
> only the scip_execlp method. I see in the doc that this method is 
> called in the LP solving loop. However it is never called when I run 
> my program. I have called SCIPincludeObjSepa before solving and tried 
> several priority flags. I put freq = maxbounddist = 1 and delay = false.
> I'd like scip_execlp be called before branching (since it should help 
> me to get rid of some fractional solutions). Do I forget something ?
well, that is strange. Did you perhaps disable separation in your 
settings to turn off the default separators? If the 
separating/maxcuts(root) and separating/maxrounds(root) parameters are 
set to 0, no separators are called, whatever their priorities are.
How do you know that your separator is never called? Did you add output 
messages or did you just look at the statistics of SCIP? In the latter 
case, it might be that you just forgot to set the result-pointer 
correctly, if it is set to SCIP_DIDNOTRUN, the call is not counted, even 
if you did something within the method.
> Morever I have also  a pricer so the constraints generated  by the 
> separator must be updated when I get new variables. I would like to 
> know if it may cause potential errors in SCIP ?
Your pricer just has to add newly created variables not only to the 
constraints, but also to the cuts that were already generated and you 
have to take into account the dual values of the cuts in your pricing 
problem. If you do this, there should be no problems doing 
branch-cut-and-price with SCIP.

> If somebody have an example implementing a separator I would be happy 
> to have a look to it :).
I just realized that we do not have an example in the SCIP release with 
a separator, but you could have a look at the TSP example. The 
constraint handler in there also does separation and the separation 
callback for constraint handlers is rather similar to the one of separators.

Best,
Gerald
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