[Coin-discuss] GLPK AND SYMPHONY not getting the same solutions

[Tobias Achterberg]

The problem seems to be in the MPS standard.

As you stated the MPS file, you defined ALL variables to be binary, because the MPS standard says
that variables that are included in a

    MARK0000  'MARKER'                 'INTORG'
    ...
    MARK0001  'MARKER'                 'INTEND'

section are binary variables by default. If you want them to be general integer variables, you have
to specify bounds for the variables in the BOUNDS section. That means, your

BOUNDS
    BV    BND1   Y1

is redundant, but if you want to declare variables X1, X2, and X3 to be integer, you would have to
include them in the BOUNDS section.

I attached a modified MPS file, in which the X variables are general integers without bounds (i.e.
-inf <= Xi <= +inf). The solution to this problem is X1 = X2 = -299, X3 = Y = 0, obj = -5980000.

If the variables are defined to be general integer with a lower bound of 0, the solution is the zero
vector.

Probably, there is a difference in the MPS file readers of Symphony and GLPK...

Regards, Tobi


ZT wrote:
> Hello,
> 
> im having problems getting the same solutions using
> both programs. I attached the MPS file that containts
> the problem, consisting of 3 integer variables and 1
> binary. GLPK calculates the problem correctly,
> SYMPHONY does not. Anyone knows how to get the same
> solution out of SYMPHONY?
> 
> thanks! bye
> 
> 
> 		
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-- 
Tobias Achterberg          Konrad-Zuse-Zentrum fuer
                           Informationstechnik Berlin
Tel: +49 (0)30 84185-301   Takustr. 7
Email: achterberg at zib.de   D-14195 Berlin, Germany
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