[SCIP] write customized branching codes

[Gregor Hendel]

Dear Ziye,

what you describe can be much easier (where easily depends on your C 
skills, of course) accomplished by using the branching priorities for 
variables. By default, all default branching plugins will only consider 
fractional candidates with the highest priority for branching. That 
means, if you partition your variables into two classes A and B, to 
which you assign the priorities 2 and 1, SCIP will branch on variables 
in A as long as a fractional variable of A remains.

A soft version of prioritization can be achieved by using so-called 
branching factors of variables. These factors affect the branching score 
that SCIP calculates, as you would expect. If you have two variables A 
and B, A has a factor of 1.0 and B has a factor of 10.0, then A is only 
preferred over B if A's branching score (without the factor) is ten 
times better than B's. Of course, factors and priorities can also be 
combined.

Since you create the problem yourself, you probably know the variable 
types you prefer at creation time. Then you can use the methods

SCIPvarChgBranchPriority 
<http://scip.zib.de/doc/html/var_8c.php#aba9d5313231dd58bab3f55dbc44dc646>

or

SCIPchgVarBranchFactor 
<http://scip.zib.de/doc/html/group__PublicVariableMethods.php#gaf616016202a9ea9eb28f6d3d8da555ef>

and its friends SCIPaddVarBranchFactor or SCIPscaleVarBranchFactor and 
let SCIP do the rest. You save the writing and reading part. Note that 
branching priorities or factors are not part of any problem file format 
and thus cannot be read in by SCIP. After reading in a problem from an 
mps-file, all variables have both their priority and their factor set to 
1, as one would expect.

Hope this helps,
Gregor

Am 28.05.2017 um 23:11 schrieb Ziye Tang:
> Hi there,
>
> My current task is the following: I'd like to implement a customized 
> branching rules for my MIP problem. I have some prior knowledge to 
> partition variables into a few sets with `priorities' used for 
> branching, and the branching rules is (roughly) branching according to 
> decreasing `priorities'. Since I seem to be the first guy among my 
> workmates to code in SCIP, I'd really appreciate it if you could give 
> me some guidance here. I've decomposed this task into the following steps.
>
> 1. I can output my problem formulation in commonly used format like 
> .mps, .lp etc.. So I need a plugin to read my problem and partition 
> variables in the format into a few sets/blocks. Can I do that in the 
> already implemented reader plugin, or i need to follow the Binpacking 
> example and write my own struct?
>
> 2. Once I have created blocks of variables, should I follow the 
> branching code in Binpacking example?
>
> I am reading your example while posting this question. Since I am not 
> a fluent C programmer, it might take me several days to understand 
> what is going on, I hope getting your reply while reading the code 
> could accelerate the whole process.
>
> Thanks very much in advance,
> Ziye
>
>
>
>
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