I need to solve a hard scheduling problem. For that purpose, I get an initial feasible solution using a greedy heuristic. This heuristic gives me the starting time of every job in every machine.
How can I initialize CP Optimizer with this initial solution? ("warmstart")
Thanks in advance
With opl Cplex CPo you May See an example in
https://github.com/AlexFleischerParis/zooopl/blob/master/zoowarmstartapicpo.mod
//we saw how to warm start in CPLEX. Let s see how we can do the same with CPO in CPLEX:
using CP;
int nbKids=300;
// a tuple is like a struct in C, a class in C++ or a record in Pascal
tuple bus
{
key int nbSeats;
float cost;
}
// This is a tuple set
{bus} pricebuses=...;
// asserts help make sure data is fine
assert forall(b in pricebuses) b.nbSeats>0;assert forall(b in pricebuses) b.cost>0;
// To compute the average cost per kid of each bus
// you may use OPL modeling language
float averageCost[b in pricebuses]=b.cost/b.nbSeats;
// Let us try first with a naïve computation, use the cheapest bus
float cheapestCostPerKid=min(b in pricebuses) averageCost[b];
int cheapestBusSize=first({b.nbSeats | b in pricebuses : averageCost[b]==cheapestCostPerKid});
int nbBusNeeded=ftoi(ceil(nbKids/cheapestBusSize));
float cost0=item(pricebuses,<cheapestBusSize>).cost*nbBusNeeded;
execute DISPLAY_Before_SOLVE
{
writeln("The naïve cost is ",cost0);
writeln(nbBusNeeded," buses ",cheapestBusSize, " seats");
writeln();
}
int naiveSolution[b in pricebuses]=
(b.nbSeats==cheapestBusSize)?nbBusNeeded:0;
// decision variable array
dvar int+ nbBus[pricebuses];
// objective
minimize
sum(b in pricebuses) b.cost*nbBus[b];
// constraints
subject to
{
sum(b in pricebuses) b.nbSeats*nbBus[b]>=nbKids;
}
float cost=sum(b in pricebuses) b.cost*nbBus[b];
execute DISPLAY_After_SOLVE
{
writeln("The minimum cost is ",cost);
for(var b in pricebuses) writeln(nbBus[b]," buses ",b.nbSeats, " seats");
}
main
{
thisOplModel.generate();
// Warm start the naïve solution
var sol=new IloOplCPSolution();
for(var b in thisOplModel.pricebuses)
sol.setValue(thisOplModel.nbBus[b],thisOplModel.naiveSolution[b]);
cp.setStartingPoint(sol);
cp.solve();
thisOplModel.postProcess();
}
/*
.dat
pricebuses={<40,500>,<30,400>};
which gives
The naïve cost is 4000
8 buses 40 seats
The minimum cost is 3800
6 buses 40 seats
2 buses 30 seats
and in the log we see
! ----------------------------------------------------------------------------
! Minimization problem - 2 variables, 1 constraint
! Using starting point solution
*/
and with scheduling
using CP;
range r = 1..10;
dvar interval x[r] size 1;
dvar interval y[r] size 1;
// The following array of values (defined as data) will be used as
// a starting solution to warm-start the CP Optimizer search.
int values[i in r] = (i==5)? 10 : 0;
minimize sum( i in r ) startOf(x[i]) + sum( j in r ) startOf(y[j]);
subject to
{
ctSum: sum( i in r ) startOf(x[i]) >= 10;
forall( j in r ) ctEqual: startOf(y[j]) == j;
}
execute
{
for(i in r) write(Opl.startOf(x[i])," ");
writeln();
}
main
{
thisOplModel.generate();
var sol=new IloOplCPSolution();
for(var i=1;i<=10;i++) sol.setStart(thisOplModel.x[i],thisOplModel.values[i]);
cp.solve();
thisOplModel.postProcess();
cp.setStartingPoint(sol);
cp.solve();
thisOplModel.postProcess();
}