Class ShiftedFitnessProportionalSelection

  • All Implemented Interfaces:
    Splittable<SelectionOperator>, SelectionOperator

    public final class ShiftedFitnessProportionalSelection
    extends FitnessProportionalSelection

    This class implements a variation of fitness proportional selection that uses shifted fitness values. Specifically, it shifts all fitness values by the minimum fitness minus one, such that the least fit population member's selection probability is based on a transformed fitness equal to 1. A member of the population is chosen randomly with probability proportional to this shifted fitness relative to the total shifted fitness of the population. For example, if the fitness of population member i is fi, and if the minimum fitness in the population is fmin, then the probability of selecting population member i is: (fi - fmin + 1) / ∑j (fj - fmin + 1), for j ∈ { 1, 2, ..., N }, where N is the population size. To select M members of the population, M independent random decisions are executed in this way, thus requiring generating M random numbers of type double.

    This selection operator is compatible with all fitness functions, even in the case of negative fitness values.

    The runtime to select M population members from a population of size N is O(N + M lg N).

    • Constructor Detail

      • ShiftedFitnessProportionalSelection

        public ShiftedFitnessProportionalSelection()
        Construct a shifted fitness proportional selection operator.
    • Method Detail

      • split

        public ShiftedFitnessProportionalSelection split()
        Description copied from interface: Splittable
        Generates a functionally identical copy of this object, for use in multithreaded implementations of search algorithms. The state of the object that is returned may or may not be identical to that of the original. Thus, this is a distinct concept from the functionality of the Copyable interface. Classes that implement this interface must ensure that the object returned performs the same functionality, and that it does not share any state data that would be either unsafe or inefficient for concurrent access by multiple threads. The split method is allowed to simply return the this reference, provided that it is both safe and efficient for multiple threads to share a single copy of the Splittable object. The intention is to provide a multithreaded search with the capability to provide spawned threads with their own distinct search operators. Such multithreaded algorithms can call the split method for each thread it spawns to generate a functionally identical copy of the operator, but with independent state.
        Specified by:
        split in interface Splittable<SelectionOperator>
        Overrides:
        split in class FitnessProportionalSelection
        Returns:
        A functionally identical copy of the object, or a reference to this if it is both safe and efficient for multiple threads to share a single instance of this Splittable object.
      • select

        public final void select​(PopulationFitnessVector.Integer fitnesses,
                                 int[] selected)
        Description copied from interface: SelectionOperator
        Selects a set of members of the population based on fitness. Implementations should ensure that the array of indexes of population members is in a random order. For some selection operators, this required behavior is met by definition (e.g., the common fitness proportionate selection will have this behavior as is). But other selection operators may require randomizing the array of indexes after selection. For example, the obvious implementation of stochastic universal sampling will likely have all copies of an individual population member ordered together, and thus will require a shuffling of the array before returning.
        Specified by:
        select in interface SelectionOperator
        Parameters:
        fitnesses - A vector of fitnesses of the members of the population.
        selected - An array for the result. The selection operator should select selected.length members of the population based on fitnesses, populating selected with the indexes of the chosen members. Note that selected.length may be different than the fitnesses.size().
      • select

        public final void select​(PopulationFitnessVector.Double fitnesses,
                                 int[] selected)
        Description copied from interface: SelectionOperator
        Selects a set of members of the population based on fitness. Implementations should ensure that the array of indexes of population members is in a random order. For some selection operators, this required behavior is met by definition (e.g., the common fitness proportionate selection will have this behavior as is). But other selection operators may require randomizing the array of indexes after selection. For example, the obvious implementation of stochastic universal sampling will likely have all copies of an individual population member ordered together, and thus will require a shuffling of the array before returning.
        Specified by:
        select in interface SelectionOperator
        Parameters:
        fitnesses - A vector of fitnesses of the members of the population.
        selected - An array for the result. The selection operator should select selected.length members of the population based on fitnesses, populating selected with the indexes of the chosen members. Note that selected.length may be different than the fitnesses.size().