paradiseo/mo/tutorial/Lesson5

How to use Iterated Local Search

In this lesson, an Iterated Local Search is presented. The Tabu Search of the Lesson 4 is used with an order neighborhood based on a shift operator, to solve the Queen problem.

1. Iterated Tabu Search on the Queen problem.
2. Exercise

1. Iterated Tabu Search (example on the Queen problem)

As in Lesson 4, you have to define a Solution, the method to initialize and evaluate it. Then you have to define a Tabu Search.

Declaration of the Tabu Search:

moTS<shiftNeighbor> ts(orderShiftNH, fullEval, shiftEval, 1, 7);

To use a simple Iterated Local Search, a mutation operator is needed. So the swap mutation defined in EO is used:

eoSwapMutation<Queen> mut;

Now, a simple Iterated Tabu Search can be declared as follow:

moILS<shiftNeighbor> localSearch1(ts, fullEval, mut, 3);

This constructor has got 4 parameters:

1. a local search (ts)
2. a full evaluation function (fullEval)
3. a mutation operator (mut)
4. a number of iterations (3)

localSearch1 performs the Tabu Search 3 times. The first solution of each iteration(except the first one) is obtained by applying the mutation operator on the last visited solution.

A constructor allows to specify the continuator. Be carefull, the continuator must be templatized by a "moDummyNeighbor":

moIterContinuator<moDummyNeighbor<Queen> > cont(4, false);

The explorer of the Iterated local search don't use its own neighborhood. Here, the neighborhood of the Tabu Search is used. But to respect the conception, we create a "moDummyNeighbor" using as template for Iterated Local Search.

An Iterated Tabu Search with this continuator can be declared as:

moILS<shiftNeighbor> localSearch2(ts, fullEval, mut, cont);

A general constructor is available allowing to specify the perturbation operator and the acceptance criteria. First, you have to declare a perturbation operator:

moMonOpPerturb<shiftNeighbor> perturb(mut, fullEval);

And, the acceptance criteria:

moSolComparator<Queen> solComp;
moBetterAcceptCrit<shiftNeighbor> accept(solComp);

Finally, the Iterated Local Search can be declared as:

moILS<shiftNeighbor> localSearch3(ts, fullEval, cont, perturb, accept);

You can test these three algorithms by changing problem sizes(use parameter file or the option --vecSize=X on command line to execute "testILS"). It prints the initial and the final solutions.

2. Exercise

• Try to implement an Iterated Hill Climbing on the Queen problem with these caracteristics:
  1. Hill Climbing with a "moShiftNeighborhood" and a "moTrueContinuator"
  2. Iterated Local Search using a "moIterContinuator" and a "moNeighborhoodPerturb" with a "moSwapNeighborhood".