Modify Selector (NSGA2)

HolyOne
2010-05-17
2013-02-08
  • HolyOne

    HolyOne - 2010-05-17

    Hi,

    i want to extend the multiobjective evolutionary algorithm with special constraint handling. I'm in the beginning of the process, but if I get decent result, i can also share this with you.
    For this reason, I might modify the NSGA2-Selector and I have a question: Where exactly is the Selector (NSGA2) chosen? I believe it's done by the "@ImplementedBy"-Annotation in "org.opt4j.optimizer.ea.Selector.java", but if you could nugde me into the right direction where to say "use MySelector!", I would really appreciate it!

    Also could you have a look at "org.opt4j.optimizer.ea.Nsga2.java" line 104, I don't think the loop makes any sense (for (int t = 0; t < 0; t++){…}), but as the default tournament value is 0, I believe this doesn't influence the current algorithm.

    Thanks.

     
  • Felix Reimann

    Felix Reimann - 2010-05-19

    Hi,

    you're right: The "@ImplementedBy"-Annotation points to the default implementation of the Selector, if no explicit binding is made. If you now want to use your own "YourSelector implements Selector", you have to explicitly bind it. How this is done can be seen, e.g., in the org.opt4j.optimizer.ea.Spea2Module. I.e. additionally to YourSelector you must implement a "YourSelectorModule extends SelectorModule" and bind your selector there:
    @Override
    public void config() {
    bindSelector(YourSelector.class);
    }
    If you then start the configuration GUI, you find the new module at modules->optimizer->selector from where you can choose it.

    Thank you very much for the hint with the for loop in Nsga2. This was a bug. I fixed it in revision 508.

    Cheers
    Felix

     
  • HolyOne

    HolyOne - 2010-05-26

    Thank you for your answer! As I won't use the Gui, I will still have to find out some stuff, but this was actually helpful!

    Thx

     
  • HolyOne

    HolyOne - 2010-05-27

    Thanks again!

    FYI according to current plans, I'm going to integrate constraints using the concept of constraint-domination as described in , p.192.

    But there is another thing that I forgot to post earlier:
    If you could have a look at "org.opt4j.optimizer.ea.Nsga2.java" line 106 et seqq., the second part of the if-clause seems strange to me considering the body:
    if (rank < rank || opponent == winner) {
    winner = opponent;
    }
    I believe the niched comparison operator that also considers the crowding distance should be applied here…

    Kalyanmoy D. Deb, Amrit Pratap, Sameer Agarwal, and T. Meyarivan. A fast and elitist multiobjective
    genetic algorithm : Nsga-ii. Evolutionary Computation, IEEE Transactions on, 6(2):182-197,
    August 2002.

     
  • HolyOne

    HolyOne - 2010-05-31

    1)
    Well, this actually proves my point. In the PISA implementation, the fitness value incorporates both, the pareto rank and the crowding distance, whereas in Opt4J only the pareto rank is considered.
    PISAs fitness is a double value where a fitness=3,14 means, that the solution has a rank of 3 and a represented crowding distance of 0,14 (it is the inverse of the calculated C.D. to be exact). In PISAs evironmentalSelection(), the two aspects (P.R. & C.D.) of the NSGA2 are combined:
    for (i = 0; i < size; i++)    {
      pp_all->ind_array_->fitness += 1.0 / dist;
    }
    I still don't know the sense of the second part of the above mentioned if-clause, maybe in C there are special optimizations by the compiler, but I still believe the crowding distance should be considered at this point (at least additionally).

    2)
    I found out today that the Objectives class of Opt4J is not meant to be changed. As I want to change the dominance criterion, I would like to know if you are even interested in such kind of constraint handling, because this could also affect the API (I'll maybe make a module for objectives and for domination strategies, as soon as I find out how :) meaning that existing implementations would require adjustments.
    Otherwise I would just do it according to our needs._

     
  • Martin Lukasiewycz

    1) This confuses me somehow, but I am sure youre right that this is not the 1-to-1 PISA implementation. You can send me an SVN patch and then I can "fix" the NSGA2 in the repository. Thanks.
    2) Yeah, I needed this once and then I copied all the objectives to a new Objectives instances and called individual.setObjectives(..). However, adapting the code of the Objectives slightly might also allow changes, e.g., an invalidate() method that resets the double array to null. If you would share the constraint handling with us, we would appreciate it. Changing the API is always acceptable if things dont become more complicated.

     
  • HolyOne

    HolyOne - 2010-06-08

    It could take a while till I get to the coding since I'm still designing the architecture, but I'll try my best…

     
  • Martin Lukasiewycz

    Great! Take your time.

     

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