Re: [Help-glpk] forall and exists?

[Jay Hutfles]

I don't think you meant to address Andrew, but thanks for replying.

I realize if-then constraints aren't naturally supported in linear programming (or mixed integer programming).  But you can formulate constraints that are logically equivalent to if-then constraints.  See the whole part about "changing from the form 'if p then q' to 'not p or q.' "  The constraint is still true under the same conditions of the logical implication, so it constrains the binary variables in the same manner.  Yes, it may not be the most efficient, but it is a proper way to model such a constraint in a linear programming problem.

On Tue, May 7, 2013 at 5:54 AM, <address@hidden> wrote:

Hello Andrew,

GLPK is a linear programming solver. A constraint with an IF would not be linear.

In linear programming you might introduce binary variables for your purpose.

Or use a constraint programming solver.

Best regards

Heinrich Schuchardt

http://www.xypron.de

Am 07.05.13 um 07:48 schrieb Jay Hutfles

> Oh, I don't think there's anything wrong with them.  Well, except for how
>
> I'm trying to use them.
>
>
>
> I see that the examples only use them in computable parameters, though.  I
>
> was trying to use them in constraints, and was getting errors along the
>
> lines of "forall function does not exist" (sorry, I don't have the exact
>
> error with me).  The constraints were of the form:
>
>
>
>    for all a in A, if x[a] =1 then there exists a b in B such that
>
> (something or another based on a)
>
>
>
> I was having trouble directly implementing constraints of this form, so I
>
> changed the "if p then q" form to "not p or q" like this:
>
>
>
>    subject to C {a in A} :
>
>       (1 - x[a]) + (if exists {b in B} (...something or other based on a..)
>
> then 1 else 0) >= 1;
>
>
>
> I'll have to try again in the morning when I'm more awake.  Thanks for the
>
> guidance, Andrew.
>
>
>
>
>
>
>
> On Mon, May 6, 2013 at 11:36 PM, Andrew Makhorin <address@hidden> wrote:
>
>
>
> >
>
> > > Can you provide a link to an example of each?  Any help would be
>
> > > appreciated.
>
> > >
>
> >
>
> > Please see glpk/examples/color.mod (for 'exists') and
>
> > glpk/examples/egypt.mod (for 'forall').
>
> >
>
> > What is wrong with these operators?
>
> >
>
> >
>
> >
>

> _______________________________________________
>
> Help-glpk mailing list
>
> address@hidden
>
> https://lists.gnu.org/mailman/listinfo/help-glpk