[Scip] Upper bound on variables during pricing

[Gerald Gamrath]

Hi Hélène,

3. You can use both methods, SCIPfixVar() will internally use 
SCIPchgVarUb() in this case, anyway.
1. scip_prop() is called when a node is processed for the first time 
(and sometimes also later again, if a node was marked to be 
repropagated). You don't need to do it again if you enter the same node 
again during the tree search (when switching to a node in the 
corresponding subtree), because the bound changes are locally stored, 
such that they will be undone when you leave the node (or its subtree), 
and will be re-applied when you re-enter the node (or its subtree).
In contrast to that, scip_active() is called every time a node is 
entered, i.e., also when you switch to a new node in the corresponding 
subtree. It should be used to update your internal data, which SCIP 
cannot do automatically.
2. You are only allowed to change bounds either globally or at the 
current focus node, i.e., the node that is currently processed. However, 
when you switch from one node to another in the tree, all nodes from the 
common ancestor to the newly focused node are activated, so 
scip_active() is called. But these nodes are not the focusnode, so you 
are not allowed to change bounds.

Regarding your issue with CPLEX as LP solver, you can enable the LP 
solver output by setting the parameter "display/lpinfo" to TRUE, that 
should help you to check whether its a CPLEX error. Best you also set 
"display/verblevel" to 5 to see the full output. It might also be that 
SCIP does not accept the CPLEX LP solution when checking for its (primal 
and dual) feasibility; in this case, SCIP reoptimizes the LP after 
changing some LP settings several times until the LP solution passes the 
SCIP checks.

Best,
Gerald




Am 19.03.2013 14:32, schrieb Hélène Toussaint:
> Hi Gerald,
>
> Thank you for your answer. I enclose the statistics.
>
> Indeed I use propagation to propagate my modifications on the problem 
> data and variables. But I have some doubts concerning the scip_prop() 
> and scip_active() methods of the constraint handler for branching 
> decisions:
>
> 1. Is scip_prop() always called after scip_active() when SCIP enter a 
> node?
> 2. Why I can't fix variables in scip_active()? If I try then 
> SCIPnodeAddBoundinfer() displays an assertion about the nodetype 
> (which is equal to 8 in this case)
> 3. In scip_prop(), what method should I use to set a variable to 0: 
> SCIPchgVarUb() or SCIPfixVar()?
>
> Furthermore I have tried to link with cplex instead of soplex. All my 
> instances give good results except one which stops with the following 
> error message:
>
> "ERROR: (node 2502) unresolved numerical troubles in LP 3348 cannot be 
> dealt with"
>
> Is it an error from cplex? How can I see what happens? Debug mode 
> doesn't give more information.
>
> Best,
> Hélène
>
>
> Le 19/03/2013 12:48, Gerald Gamrath a écrit :
>> Hi Hélène,
>>
>> you are right, that should not happen. Can you send me the solving 
>> statistics, which you get by typing "display statistics" in the 
>> interactive shell or calling SCIPprintStatistics()? This should help 
>> to identify the plugin which changes the bound.
>>
>> Besides, you best disable the other propagators, too, if you don't 
>> want SCIP to change bounds itself. You can either disable all 
>> propagation by setting propagating/maxrounds and 
>> propagating/maxroundsroot to 0, but I guess your modifications to the 
>> problem data after branching are also done during propagation? In 
>> this case, you need to disable all default propagators individually 
>> by setting their freqency to -1.
>>
>> Best,
>> Gerald
>>
>> Am 18.03.2013 11:37, schrieb Hélène Toussaint:
>>>
>>> Dear all,
>>>
>>> I use SCIP to code a branch & price algorithm on positive continuous 
>>> variables. I branch on instance data (using a constraint handler to 
>>> store the branching decisions). When I modify the instance data some 
>>> variables become forbidden (I fix their upper bound to 0).
>>>
>>> I'd like SCIP to not set upper bound on the other variables. As 
>>> explained in the FAQ I have marked all constraints containing priced 
>>> variables as modifiable and added SCIPsetIntParam(scip, 
>>> "propagating/rootredcost/freq", -1). But occasionally there is a 
>>> variable with an upper bound equals to 0 although it isn't forbidden 
>>> in the current node. How is this possible? Could you please help me 
>>> with this issue?
>>>
>>> Best regards,
>>>
>>> Hélène
>>>
>>>
>>>
>>> _______________________________________________
>>> Scip mailing list
>>> Scip at zib.de
>>> http://listserv.zib.de/mailman/listinfo/scip
>>
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://listserv.zib.de/mailman/private/scip/attachments/20130319/7c3d92ed/attachment.html