[SCIP] Restart after root node

[Jakob Witzig]

Hi Emily,

you can use the method SCIPvarChgType to convert the variables into 
integers, but the method can only be called if the variables are not 
part of the problem, e.g., before you call SCIPtransformProb. Do you use 
sub-SCIPs to solve your subproblems? If yes, than you can call the 
method mentioned above before you start solving the subproblem.

I don't get the connection between your two ways. Why is changing the 
variables and changing the modifiability flag of the constraint equivalent?

If you restart your problem SCIP tries to transform cuts into globally 
valid constraints and will fix all variables that have equal global 
upper and lower bounds, i.e., the variables can removed from the 
problem. Hence, if you could not tighten the bounds of your variables 
enough or you didn't found global valid cuts a restart directly after 
the root will give you the same problem as before.

I think changing the flags of a constraint should be done before you 
start solving your subproblem.

Best,
Jakob


On 08/31/15 04:38, Aao Du wrote:
> Dear all,
>
> I have a decomposed model, a master problem and subproblems. I am trying
> to perform pricing only at the root node, and then change the variables
> to integer and solve it again. I have tried two ways. One is to print
> the lp format of the problem after root node, with variables as integer,
> and give it to SCIP binaries. In this way, the second part of the
> solving process ( solving the final master problem with binaries) is
> performed fast.
> The other way, is to define the variables as integer from begining, and
> stop pricing at the root node, and change the constarints to
> unmodifiable. In this way, after the lp of master problem is solved, it
> takes way more than the first method to solve it. Following a previous
> email, I tried to restart the master problem after root node, but it did
> not make any change in the time. I thought maybe i am doing sth wrong.
> What is the proper place in the pricerobj to change the modifiable flag
> of constraints? I think it should be somewhere before redcost, or maybe
> in it.
>
> Sorry for the long email.
>
> Thanks,
> Emily
>
>
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-- 
Jakob Witzig

Konrad-Zuse-Zentrum für
Informationstechnik Berlin (ZIB)

Division Mathematical Optimization and Scientific Information
Research Group Mathematical Optimization Methods

Takustrasse 7
14195 Berlin

Tel. : +49 (0)30 84185-416
Fax  : +49 (0)30 84185-269
email: witzig at zib.de