Class: Selection::Sampling

Inherits:
Roulette show all
Defined in:
../lib/sampling.rb

Overview

Stochastic Universal Sampling selection method. The probability of the individual selection is proportional to some (usually fitness) non-negative value (as in Roulette selection). However, more individuals can be selected at once by the single run of a wheel, which brings a better spread of the results (in comparision with Roulette method).

See en.wikipedia.org/wiki/Stochastic_universal_sampling

Instance Attribute Summary

Attributes inherited from Roulette

#population, #proportional_by, #random

Attributes included from SelectMore

#unique_winners

Instance Method Summary (collapse)

Constructor Details

- (Sampling) initialize(proportional_by = nil, &block)

Set the proportional_by or the block for obtaining invividual’s proportion. See Roulette#proportional_by 



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# File '../lib/sampling.rb', line 17

def initialize( proportional_by=nil, &block )
  super
end

Instance Method Details

- (Object) select(how_much, population = self.population)

Select individuals from the population. It can be specified how_much individuals will be selected. 



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# File '../lib/sampling.rb', line 23

def select( how_much, population=self.population )
  raise "Sampling: cannot select from an empty population" if population.empty?
  raise "Sampling: cannot select more than population.size" if how_much > population.size
  return [] if how_much == 0

  @sum,@wheel = wheel_core population 
  @population = population

  step = @sum.to_f/how_much
  ballot = step * @random.rand 

  width = 0.0
  winners = []
  @wheel.each_with_index do |slot,index|
    width += slot.width
    next if ballot > width
    winners.push slot.original   
    ballot += step
  end

  winners
end

- (Object) select_one(population = self.population)

Select one individual from the population (same as Roulette#select_one, but a bit less effective). 



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# File '../lib/sampling.rb', line 47

def select_one population=self.population
  select( 1, population ).first
end