Originally posted by: sdjena

Hello,

I just installed cplex v12.6.3 after having worked with v12.6.0 before. I have some strange behavior that the x solution of the network optimizer returns a 0 vector. Using the cplex console I have the same behavior:

- I run the file netex1.net (see attachment) via netopt on cplex v12.6.3 on one computer and with v12.6.0 on another computer; both report an optimal OF of 1029380.00

- "disp sol var *" on the v12.6.0 shows plenty of solution values for the arc variables, while on v12.6.3 I get "All arc variables matching '*' are 0."

A bug? Any chance to go back to 12.6.0 for me?

Thanks in advance and best regards,

Sanjay

Curious. I confirmed that the network optimizer (again on 12.6.3.0) behaves as described (correct objective value but all variables allegedly zero). Staying on 12.6.3.0, if I change the problem type to LP and run either primal or dual simplex, I get the same objective value and a slew of non-zero variable values.

Turning off presolve did not affect the behavior of the network optimizer. (Does the network optimizer do any presolving?) Changing the netfind parameter also had no effect.

Paul

Originally posted by: sdjena

Thanks for taking the time to check. Looks like a bug to me. The positive demand nodes should result in a few non-zero arc variables, no matter whether pre-solving is involved or not, isn't it ?

I need the netopt to solve thousands of transportation problems, so solving an LP instead won't do the job as this is too slow. Shortest fix for me right now would be to find cplex v12.6.0 :-)

Well, I just ran it in CPLEX 12.6.2.0 and got the variable values (using netopt), so you don't have to go quite that far back in history. :-)
 

Originally posted by: sdjena

Good point. Can I actually download an earlier version? I have not found them in the software catalog.

I'm not sure if IBM has the installers for past versions available anywhere. Someone from IBM will have to answer that. Since you only need to roll back one version, chances are good they have an installer lying around somewhere.

Good luck,

Paul

Unfortunately, you have indeed hit a bug in CPLEX. I have filed an appropriate bug report.

The workaround is the following: convert the problem to LP then solve the LP with the network optimizer (not simplex). For example, in the interactive do

CPLEX> read netex1.net
CPLEX> change prob LP
CPLEX> netopt

or

CPLEX> read netex1.net
CPLEX> set lpmethod 3
CPLEX> opt

Can you try whether this gives acceptable performance?

Originally posted by: sdjena

Hi,

I will try whether this gives me acceptable performance the next days. Can I use similar commands with the C callable library for the workaround? I.e., can I continue to use CPXNETprimopt to run the network optimizer and CPXNETsolution to access the solution?

In the meanwhile, is there any way I can download cplex v12.6.2 ?

Thanks and best regards.

Originally posted by: RWunderling

You can create the LP from the network using CPXcopynettolp() which will create an LP version and install the basis from the network optimizer if available.  Then with CPXprimopt() you solve the model which should yield 0 iterations (check the advance indicator!) and install the solution information that can then be queried.  Sorry for the inconvenience.

Originally posted by: sdjena

By chance, do you have any news for a bugfix? I have send requests on how to access previous cplex versions to the customer service but never received an answer since. Thanks!

I apologize for the delay and the lack of answer from customer service. A fix is not yet available, but it should be possible to provide an earlier CPLEX version.

Let's take this offline. Would you please send me an email?  I need to know which platform and which version would work best for you.  If you can also include the mail you sent to customer service, that may be helpful.

Thanks.

Xavier Nodet
xavier.nodet@fr.ibm.com