Purpose
Reads a directives file to help direct the global search.
Synopsis
problem.readdirs(filename)
Argument
|
filename
|
A string of up to 200 characters containing the file name from which the directives are to be read. If omitted (or
None), the default
problem_name is used with a
.dir extension.
|
Example
The following example reads in directives from the file
dirfile.dir for use with the problem,
prob2:
p.read("prob2","")
p.readdirs("dirfile")
p.mipoptimize("")
Further information
1. Directives cannot be read in after a model has been presolved, so unless presolve has been disabled by setting
PRESOLVE to
0, this command must be issued before
problem.mipoptimize.
2. Directives can be given relating to priorities, forced branching directions, pseudo costs and model cuts. There is a priority value associated with each global entity. The
lower the number, the
more likely the entity is to be selected for branching; the
higher, the
less likely. By default, all global entities have a priority value of
500 which can be altered with a priority entry in the directives file. In general, it is advantageous for the entity's priority to reflect its relative importance in the model. Priority entries with values in excess of
1000 are illegal and are ignored. A full description of the directives file format may be found in the Xpress Optimizer reference manual.
3. By default,
problem.mipoptimize will explore the branch expected to yield the best integer solution from each node, irrespective of whether this forces the global entity up or down. This can be overridden with an
UP or
DN entry in the directives file, which forces
mipoptimize to branch up first or down first on the specified entity.
4. Pseudo-costs are estimates of the unit cost of forcing an entity up or down. By default
mipoptimize uses dual information to calculate estimates of the unit up and down costs and these are added to the default pseudo costs which are set to the
PSEUDOCOST control. The default pseudo costs can be overridden by a
PU or
PD entry in the directives file.
5. If model cuts are used, then the specified constraints are removed from the problem and added to the Optimizer cut pool, and only put back in the problem when they are violated by an LP solution at one of the nodes in the global search.
6. If creating a directives file by hand, wild cards can be used to specify several vectors at once, for example
PR x1* 2 will give all global entities whose names start with
x1 a priority of
2.
Related topics
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