CPLEX, replace variable with a constant - variables

Question

I already have a CPLEX model with variables x[0], x[1],...,x[n-1]. In order to create heuristics, I need to insert constants instead of all but one variable.
Say, I will keep x[0] and instead of x[i] I will insert constants a[i] (i=1,...,n-1).
One way to do this is to create a new model where variables will be replaced by constants manually. The model is big and I would need go through entire code to do this.
Is there another way?
What if I insert additional constraints: x[i]==a[i] (i=1,...,n-1) into the model? Will x[i] be converted to constants at the very beginning or the model with n variables (x[i], i=0,...,n-1) will be solved?

Answer 1

You can set Upper bound and lower bound on any decision variable to make it a constant.
For instance, with the zoo example , in OPL you can write
int nbKids=300;
float costBus40=500;
float costBus30=400;
float costBus50=700;
dvar int+ nbBus40;
dvar int+ nbBus30;
dvar int+ nbBus50;
minimize
costBus40*nbBus40 +nbBus30*costBus30+nbBus50*costBus50;
subject to
{
ctKids:40*nbBus40+nbBus30*30+nbBus50*50>=nbKids;
}
execute
{
writeln(nbBus30," ",nbBus40," ",nbBus50, " buses");
}
main
{
thisOplModel.generate();
cplex.solve();
thisOplModel.postProcess();
thisOplModel.nbBus50.UB=2;
thisOplModel.nbBus50.LB=2;
cplex.solve();
thisOplModel.postProcess();
}
that gives
2 6 0 buses
0 5 2 buses