« Return to Thread: [AMPL 2536] Condicions in AMPL function

[AMPL 2542] Re: Condicions in AMPL function

by Paul A. Rubin :: Rate this Message:

The value of x and y changes from what? Did you previously have a way
of solving the problem?

If you have a known optimal solution, check whether this approach
gives the same objective value but different x and y. If the
objective value is the same, you have multiple optimal solutions.

Another possibility, if M is very large, is that rounding error may be
causing issues. You generally want the smallest value of M for which
you can be confident that |x-y| < M.

On May 27, 6:53 am, plozmar <lozano_pal...@...> wrote:

> Thanks so much, but now I have problems with the parameter M because
> it makes that the result optimization of the variables x and y
> changes, I don´t know why. I take a big number (sufficiently large
> that |x-y|) the values of x and y changes (and the program doesn´t
> work ok) Why is it?
>
> Thanks for all
>
> On 26 mayo, 22:37, Paul <ru...@...> wrote:
>
> > I assume x and y are variables? You'll need to introduce a binary
> > variable.
>
> > var z; # objective value
> > var b binary; # decides which of x and y is smaller
> > param M > 0; # sufficiently large parameter that |x-y| < M for all
> > feasible x, y
>
> > minimize totalcost: z;
> > s.t. IfXSmaller: z >= x - M*b;
> > s.t. IfYSmaller: z >= y - M*(1-b);
>
> > On May 26, 5:12 am, plozmar <lozano_pal...@...> wrote:
>
> > > Hi,
> > > now I want to make this, using CPLEX
>
> > > minimize totalcost:
>
> > > min(x,y)
>
> > > How can I implement this statement? Can I with this solver?
>
> > > Thanks to all,- Ocultar texto de la cita -
>
> > - Mostrar texto de la cita -
>
>

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