[PPL-devel] [GIT] ppl/ppl(pip): Added a standalone PIP solver demo program.

[Sven Verdoolaege]

On Wed, Oct 28, 2009 at 04:47:39PM +0100, François Galea wrote:
> However, some results I found are different from those which are in the  
> corresponding '.ll' solution files. For instance, in 'boulet.pip', I get  
> the solution minimum {0 ; 0 ; 0}, since all constraints have a  
> nonnegative constant term. PIPlib finds a solution with negative  
> elements : "if (p>=-5) then {-3*p-15 ; -p-5 ; p+5}."
>
> This may be valid if variables and parameters are not supposed  
> nonnegative, but my current implementation of the solver does not  
> support negative variables and parameters.

That's what the options "Urs_parms" and "Urs_unknowns" in boulet.pip
mean.  That is, both the parameters and the unknowns have unrestricted
sign.  So, piplib's answer is correct (see the piplib manual,
where this example originates from).

A bit of history.  The original version of pip would assume that
all unknowns and parameters are non-negative, but allowed the
user to specify a "big parameter" that is treated in a special
way by piplib.  By reformulating the problem a little bit
and by applying yet another trick to deal with urs parameters,
the user could trick piplib into handling urs unknowns and
parameters.  A couple of years ago, I made some changes to piplib
that allow the user to simply _indicate_ that the unknowns and
parameters have urs and then piplib will perform all these tricks
internally.  The default was not changed for reasons of backward
compatibility.

Now, I actually think it is wrong to expose this implementation
detail to the user, so, in isl, I make no assumption about the
sign of the unknowns or the parameters.  In the demo application
isl_pip, I then add the appropriate ">= 0" constraints if the
user did _not_ specify the Urs_parms or Urs_unknowns options.

skimo