CPLEX OPL, Iterative solves problem: the variable has already been set

Question

I'm trying to solve some quadratic integer programming problem with IBM ILOG CPLEX Optimization studio. So I would like to do solve the following quadratic integer programming problem, all the parameters has been defined.

float q = 0;

maximize
    sum(i in RR) sum(j in RR) (Num[i][j]*x[i]*x[j]  - q*Den[i][j]*x[i]*x[j]);

subject to {
    forall (i in R) sum(j in R) x[I*i + j] == 1;
    forall (i in R) sum(j in R) x[I*j + i] == 1;
}

The program manage to find the solution. But I also would like to solve for several values of q. So I setup the following main script loop, updating q for every iteration and solve.

main {
  var count = 1;
  thisOplModel.generate();
  var mdl = thisOplModel;
  while (1) {
    writeln("Running with q = " + mdl.q);

    cplex.solve();
    count = count + 1;
    if (count > 20) break;

    // prepare next iteration
    var def = mdl.modelDefinition;
    var data = mdl.dataElements;
  
    if ( mdl!=thisOplModel ) {
       mdl.end();
    }

    mdl = new IloOplModel(def,cplex);
    data.q = count;
    mdl.addDataSource(data);
    mdl.generate();
  }
}

But when I tried to run this, I got the error: I think the idea of what I'm trying to do should be clear, could anyone advice me how to do it properly? Thank you!

Alex Fleischer · Accepted Answer · 2022-02-04 17:01:43Z

in Easy Optimization see Flow control and changes with regenerate

main
{
  var source = new IloOplModelSource("sub.mod");
  var cplex = new IloCplex();
  var def = new IloOplModelDefinition(source);
 
  var opl1 = new IloOplModel(def,cplex);
  var data1=new IloOplDataElements();
  data1.costBus40=500;
  data1.costBus30=400;
  data1.maxBus40=100;
  data1.maxBus30=100;
  data1.nbKids=300;
 
 
  opl1.addDataSource(data1);
    
  opl1.generate();
  cplex.solve();
  opl1.postProcess();
 
//    //now 350 kids instead of 300
  writeln("now 350 kids instead of 300");
 
 
  data1.nbKids=350;
  var opl2 = new IloOplModel(def,cplex);
  opl2.addDataSource(data1);
    
  opl2.generate();
  cplex.solve();
  opl2.postProcess();

// no more than 4 buses 40 seats
  writeln("no more than 4 buses 40 seats");
  data1.maxBus40=4;
 
  var opl3 = new IloOplModel(def,cplex);
  opl3.addDataSource(data1);
    
  opl3.generate();
  cplex.solve();
  opl3.postProcess();
 
// change the objective so that cost for 40 seats is 450
// and remove the limit on the number of buses 40 seats

  writeln("change the objective so that cost for 40 seats is 450");
  writeln("and remove the limit on the number of buses 40 seats");    
 
  data1.maxBus40=100;
  data1.costBus40=450;
 
  var opl4 = new IloOplModel(def,cplex);
  opl4.addDataSource(data1);
    
  opl4.generate();
  cplex.solve();
  opl4.postProcess();


    }

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