Very simple OPL : relax integrity constraints and dual value

Hi,

let ‘s start from https://www.ibm.com/developerworks/community/forums/html/topic?id=9748cb9c-fa58-471a-8f9b-fba546df609b&ps=25

Now let’s see how easy it is to relax integrity constraints and turn a MIP into LP, solve and get dual value (shadow price)

int nbKids=300;
float costBus40=500;
float costBus30=400;
 
dvar int+ nbBus40;
dvar int+ nbBus30;
 
minimize
 costBus40*nbBus40  +nbBus30*costBus30;
 
subject to
{
 ctKids:40*nbBus40+nbBus30*30>=nbKids;

}

main {
  var status = 0;
  thisOplModel.generate();
  if (cplex.solve()) {
    writeln(“Integer Model”);   
    writeln(“OBJECTIVE: “,cplex.getObjValue());   
    
  }

  // relax integrity constraint
  thisOplModel.convertAllIntVars();
 
  if (cplex.solve()) {
    writeln(“Relaxed Model”);   
    writeln(“OBJECTIVE: “,cplex.getObjValue());  
    
    writeln(“dual of the kids constraint = “,thisOplModel.ctKids.dual);
  }
   
 
}

gives

Integer Model
OBJECTIVE: 3800
Relaxed Model
OBJECTIVE: 3750
dual of the kids constraint = 12.5

And we can confirm that if we use only 40 seats buses the marginal cost of a seat within a 40 seats bus is costbus40/40=12.5

regards

PS:

Many other very simple OPL examples at https://www.linkedin.com/pulse/making-decision-optimization-simple-alex-fleischer/