Originally posted by: Namal

Hello, I am using cplex for solving randomly generated SAT problems. Now I have encountered a strange error and I don't know exactly what the problem is. I am using ilolplex1.cpp as the template and populatebynonzeros because my clauses are stored in vectors where the last element is the compared to value.

Now, why does this problem (4 variables, 2 literals, 5 clauses) leads to the error:  Failed to optimize LP
Concert exception caught: IloExtractable 3 IloNumVarI has not been extracted by IloAlgorithm 0x205bda0

  0 -1  1  0  0
  0  0 -1  1  0
  0 -1  0  1  0
 -1 -1  0  0 -1
 -1  1  0  0  0

while this one does not?

  1 -1  0  0  0
  1  0  0 -1  0
 -1  0  1  0  0
  0  0  0 -1  0
  1  1  0  0  1
Solution status = Infeasible

I assume that I ask cplex to extract some variables that are not there but I don't see which those are.

Thank you for your answers!

This error usually occurs if you attempt to query the value for a variable that does not appear in any constraint or objective function. I suggest you double check the code that creates the model. Maybe also use IloCplex::exportModel() to export the model to an LP file and double-check that it looks as expected.

In order to avoid this problem, you can use IloModel::add() to explicitly add each variable you create. This way you can query the variable's value even if it does not occur in any constraint/objective.

Originally posted by: Namal

Thank you very much, Daniel.  The error was indeed when I used IloModel::add(). I added variables respectively to literals and not the variables. I have another question about the objective function though. I understand that when I am looking for any solution I don't include the objective function at all.  Still I get the solution status as optimal. Is this just the default status and there are better solutions or is this really an optimal solution.

If you don't add an objective function then the objective is always 0 for any feasible solution. That means that any feasible solution is optimal. That is why you get the "optimal" status.

Originally posted by: Namal

Alright, I have another question though. Sometimes I generate problems in which all elements in one column remain zero and for some of those problems I get an error

Concert exception caught: IloExtractable 4 IloNumVarI has not been extracted by IloAlgorithm 0x273dda0
 

sometimes I don't. For instance this problem in which there are no entries for x4 leads to the error:

\ENCODING=ISO-8859-1
\Problem name: IloCplex

Minimize
 obj:
Subject To
 c1: - x1 - x3 >= -1
 c2: - x3 + x2 >= 0
 c3: - x3 - x2 >= -1
 c4: x1 - x2 >= 0
 c5: - x1 + x2 >= 0
Bounds
 0 <= x1 <= 1
 0 <= x3 <= 1
 0 <= x2 <= 1
Binaries
 x1  x3  x2
End

and this one doesn't:

\ENCODING=ISO-8859-1
\Problem name: IloCplex

Minimize
 obj:
Subject To
 c1: - x1 + x2 >= 0
 c2: - x1 - x2 >= -1
 c3: x1 - x2 >= 0
 c4: x1 + x2 >= 1
 c5: - x2 - x3 >= -1
Bounds
 0 <= x1 <= 1
 0 <= x2 <= 1
 0 <= x3 <= 1
Binaries
 x1  x2  x3
End

Is there still something wrong with my IloModel::add() ?

This is indeed quite odd. Are you sure that you are looking at the correct LP files here? I would expect the second file to throw the same exception. What if you export the respective IloModel instance to XML, like so

#include <ilconcert/iloxmlcontext.h>

IloXmlContext ctx(env);
ctx.writeModel(model, "model.xml");
ctx.end();

Can you spot any interesting difference between the two models concerning variable x4? Maybe you can attach the two XML files here as well?