[Scip] Fractional solution, pruned node

[Gerald Gamrath]

Dear Christina,

it should not be a problem if you do not set a lowerbound. Just to be 
sure, you could set it to -SCIPinfinity().

So we will need to investigate your problem further. Could you send me a 
log file (including statistics)?

About the integer LP solutions: This should automatically be done by the 
simplerounding heuristic. Did you perhaps disable the heuristic by accident?

Best,
Gerald

On 06.03.2015 17:08, Cristina Núñez del Toro wrote:
> Dear Gerald,
>
> thank you for you response.
>
>
> About 2) yes, I am sure that pricing is performed at this node, and 3) 
> I start the B&P with a with a set of initial variables that gives a 
> feasible primal solution to the integer problem. Both, initial and 
> priced variables are marked as removable and all constraints are 
> marked as modifiable.
>
> About 1), I think this could be actually the problem. I do not compute 
> the lower bound at any point. I just followed the binpacking example 
> to create my own implementation but I missed this issue. In fact, I 
> also noticed that whenever an integer LP solution gets into the 
> pricing callback, scip do not update the best upper bound in case of 
> promising one. I read a previous email about this issue and 
> recommended to use SCIPupdateCutoofbound() and/or SCIPsetObjlimit(). 
> However, what I am more concerned about why this integral and feasible 
> solution is not stored as a Primal bound of the original Integer 
> Master Problem thant setting a new cuttoffbound. If you can help me 
> explaining me a little bit more about this because I'm find myself 
> quite lost with that.
> Best regards,
>
>
>
> 2015-03-04 19:35 GMT+01:00 Gerald Gamrath <gamrath at zib.de 
> <mailto:gamrath at zib.de>>:
>
>     Dear Christina,
>
>     sorry for the late reply, but we were quite busy in the last weeks.
>
>     There might be different reasons for this behavior.
>
>     1) Does your pricing callback compute a lower bound and sets the
>     lowerbound pointer accordingly? If this is higher than the cutoff
>     bound, the node will be cut off.
>
>     2) Perhaps the propagation already detected infeasibility? Are you
>     sure that you perform pricing at this node?
>
>     3) Are all your variables created by pricing and all constraints
>     marked to be modifiable? Otherwise, the enforcement might also
>     detect infeasibility of an unmodifiable constraint.
>
>     Best,
>     Gerald
>
>     Am 19.02.2015 um 15:27 schrieb Cristina Núñez del Toro:
>>     Dear all,
>>
>>     I am currently implemented a Branch&Price algorithm. For my
>>     problem, I have 3 types of variables, say "x","y" and "z". I have
>>     just finished my on branching rule that implies to only branch on
>>     the "z" variables. Apparently, everything goes ok; I mean,
>>     everytime SCIP enters to the branchexeclp routine, it looks for
>>     the most fractional "z" variable and do branch on it. However, I
>>     have noticed that a certain point of the algorithm, after
>>     finishing the pricing loop, SCIP "skips" (sorry for the joke) the
>>     branching phase (the node is cutted off/pruned), I mean, it does
>>     not enter to any branching callback method and goes directly to
>>     the handler constraint to propagate another node. As far I
>>     understand, this would be of course a normal behaviour if, after
>>     finishing the pricing stage :
>>
>>     a) the objective value of the current LP is greater or equal than
>>     the incumbent,
>>     b) the current LP solution is an integer solution,
>>     c) the current LP solution is an integer solution and it is optimal.
>>
>>     However, I found a pruned node with a fractional LP solution
>>     (inluding some "z" variables with fractional value) but with the
>>     objective value strictly lower than the incumbent.
>>
>>     Is there any reason for expecting this?
>>
>>     Thanks in advances,
>>
>>     Best regards,
>>
>>
>>     -- 
>>     ---
>>     Cristina Nuñez
>>
>>
>>     _______________________________________________
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>>     Scip at zib.de  <mailto:Scip at zib.de>
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>
>
>
>
> -- 
> ---
> Cristina Nuñez

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