I have been using GLPK since 2004, all my PhD study. So, firstly, thanks to the GLPK creators :-)
And now is the first time I got a problem. Doing a branch&cut
exploration, I need to remove a row of the problem whenever I go up a
level in my exploration tree. And this seems not work. I do the branch
on variables called zk. If start with z3 = 1 for instance, I have to
add a row when testing z3 = 1 childs, as can be z3 = 1 and z4 = 1. It
keeps working. z3 = 1 and z4 = 0. It continues ok. And after these
tests, I remove the row with z4 = 0 and change the previous to z3 = 0.
And GLPK gives me back a "solution is undefined" code. If I start with
z3 = 0, the solution exists.
I have extracted the problem in any case, with the lpx_print_prob. The
files are exactly the same when I try z3 = 0 at the begining than when
I try firstly z3 = 1, then z3 = 1 and z4 = 1, then z3 = 1 and z4 = 0,
and finally z3 = 0. The files written by GLPK for the case z3 = 0 are
exactly the same, I insist, even thought the value of the solution in
the first case is 7.00 (as it is supposed to be), ans in the second
"solution is undefined". So, I think the instruction that is not
working is lpx_del_rows. Furthermore, after three years having been using
GLPK, is the first time I need it.
Thank you very much for your attention, as well
Carles Franquesa