Originally posted by: Jernej1

Hello,

I am building a ILP model that I am interested in solving and examining all its solutions. The underlying problem is pretty much a (integer) partition problem with some constraints on partition sizes. By following the ilopopulate.cpp example, I have come up with the following code

static void refine(const unsigned d[CONF_SIZE]) {

    unsigned i,j;
    int status;

    try {

        IloModel model(env);
        IloNumVarArray var(env);
        IloRangeArray con(env);
        IloCplex cplex(model);
   
        cplex.setParam(IloCplex::Param::MIP::Pool::Intensity, 4); 
        cplex.setParam(IloCplex::Param::MIP::Limits::Solutions, SOLSBOUND);
        cplex.setParam(IloCplex::Param::MIP::Limits::Populate, SOLSBOUND);

        for (i = 0; i < 2*CONF_SIZE; i++) {
            var.add( IloNumVar(env, 0, d[i], IloNumVar::Int));
            var.add( IloNumVar(env, 0, d[i], IloNumVar::Int));
        }   
    
        for (i = 0; i < CONF_SIZE; i++) {
            con.add(var[2*i] + var[2*i+1] == d[i]);
        }   

        for (i = 0; i < 2*CONF_SIZE; i++) {
            IloExpr expr = (N-K+1)*var[i];
            for (j = 0; j < K; j++) {
                expr += var[i ^ (1<<j)];
            }   
            con.add( expr >= 1<<(N-K) );
        }   
        model.add(con);
        cplex.populate();

        int numsol = cplex.getSolnPoolNsolns();

        for (i = 0; i < numsol; i++) {
            IloNumArray vals(env);
            cplex.getValues(vals, var, i);
        }
    } catch(IloException& e) {
        std::cerr << "Concert exception caught: " << e << std::endl;
    }
    catch(...) {
        std::cerr << "Unknown exception caught" << std::endl;
    }
}

Unfortunately this example raises an exception due to the call to getValues, namely 

Concert exception caught: IloExtractable 9 IloNumVarI has not been extracted by IloAlgorithm 0x3f0bf10

I have no idea why this error is being caused. Can someone help me understand how to use populate properly so that I can access all the feasible solutions of the modeled problem?

This exception is thrown if you attempt to query the value of a variable that does not appear in any constraint or the objective function.

As a workaround, you can just call model.add(var) to explicitly add all variables. However, I think in your case it is unexpected that variables do not appear in the model. To figure out what is the offending variable just use IloNumVar::setName() to assign a name to each variable, then the name of that variable will appear in the exception message.

Is it intended that your array 'var' contains 4*CONF_SIZE variables? From the rest of the code it seems only 2*CONF_SIZE variables are intended (I might be misreading)?