Originally posted by: StephanBeyer

Hi,

the IloCplex documentation says that IloCplex::getNnodes() returns 0 if the model is not a MIP. I attach a simple example (but stumbled over the bug in much bigger models) where it returns 0 although it is a MIP. I guess it still returns 0 because the model can be solved using the root node only. However, the correct answer should be 1 then. (Or the documentation must be fixed.)

Note that the "if it is not a MIP"  sentence occurs also for other methods of IloCplex (possibly the same bug).

Best
Stephan
 

I think you misread the documentation. What the documentation says is that "if the problem is not a MIP, the function will return 0".

The reverse is however not true. In other words, if the function returns 0 then you cannot conclude that the problem is not a MIP.

It may be debatable whether CPLEX should return 0 or 1 if the problem solves very early (in your case in presolve). We chose to return 0 in that case.

In order to test the problem type you can use functions like IloCplex::isMIP(), IloCplex::isQC(), IloCplex::isQO().

Originally posted by: StephanBeyer

You are right, it says "if" and not "if and only if". So it is perhaps no bug ;-)

However, if 0 means "it is solved in presolve" and not "it is solved in the root node", then it is weird that it shows a time for the root relaxation > 0 (I think it does, I have to recheck with some bigger problem).

Also please note that I used IloCplex::isMIP() in the example code. ;-)

It might be that there is some fuzziness in the definition of "root node" among the several functions. If you find anything then we will be happy to fix that up.