Re: [Bug-glpk] Bug in glpk 4.21?

[Andrew Makhorin]

> For the attached testcase, we obtain that

> $ glpsol p0033.mps --max -o output.gomory --gomory

> gives an optimum of 5201, while

> $ glpsol p0033.mps --max -o output.intopt --intopt

> gives an optimum of 5131.  The optimum 5201 is confirmed
> by a completely different MIP solver.
> Does this behavior indicate there is a bug somewhere?

The bug appears if the constant term of the objective is non-zero
either originally or due to fixing some columns at non-zero value by
the mip preprocessor.

In my previous post I just commented out the corresponding fragment
leading to wrong results.

To fix the bug in a correct way please replace lines 748 and 751
(the routine ios_preprocess_node, file src/glpios02.c):

748:              L[0] = -DBL_MAX, U[0] = mip->mip_obj;

751:              L[0] = mip->mip_obj, U[0] = +DBL_MAX;

by the following ones:

748:              L[0] = -DBL_MAX, U[0] = mip->mip_obj - mip->c0;

751:              L[0] = mip->mip_obj - mip->c0, U[0] = +DBL_MAX;

Please note that these changes will appear in the next version of
the package.


Andrew Makhorin