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ECJ, A Java-based Evolutionary Computation Research System. ECJ is a research EC system written in Java. It was designed to be highly flexible, with nearly all classes (and all of their settings) dynamically determined at runtime by a user-provided parameter file. All structures in the system are arranged to be easily modifiable. Even so, the system was designed with an eye toward efficiency. ECJ is developed at George Mason University's ECLab Evolutionary Computation Laboratory. The software has nothing to do with its initials' namesake, Evolutionary Computation Journal. ECJ's sister project is MASON, a multi-agent simulation system which dovetails with ECJ nicely.

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This package provides various common selection operators.  In ECJ, selection
operators (called SelectionMethods) appear as leaf nodes in breeding pipelines.
If you're having trouble selecting, we suggest you use TournamentSelection,
perhaps with a tournament size of 2.

Some of these methods require that all fitnesses be >= 0.

Other methods, when first requested to select an individual, buffer up a large
number of precomputed selections to provide later, or compute certain
statistics to be used later.  Such selection methods cannot be used efficiently
in steady state algorithms.

ec.select.MultiSelection 
	This isn't a selection method so much as a utility procedure.  The
	algorithm maintains some number of subsidiary selection methods
	(the 'selects').  When asked to produce a child, it picks a subsidiary
	according to its associated probability; it then asks for a child from
	that subsidiary.  The probability is defined in the 'probability'
	parameter in the subsidiary pipeline.  Compare to 
	ec.breed.MultiBreedingPipeline, which does more or less the same
	thing, but as a BreedingPipeline, always copies the children produced
	from its sources.  MultiSelection, like a good selection operator,
	makes no such copies (and so is a bit more efficient in this context).

ec.select.BestSelection
	Always selects uniformly from among the best N individuals in the
	population.  Can also optionally select from among the N worst.

ec.select.TournamentSelection		[acceptable for Steady State]
	When asked to select an individual, first picks N individuals at
	random with replacement, then returns the fittest individual among
	those N.  The size of N is known as the tournament size, and you
	specify this.  Can also optionally select the worst among the N. 

ec.select.FitProportionateSelection
	Selects from among the individuals in the population in proportion
	to their fitness values (which must all be >= 0).

ec.select.SUSSelection
	Performs Stochastic Universal Sampling selection, a variant of
	fitness proportionate selection which guarantees that sufficiently
	fit individuals will always be selected at least once.  When first
	asked to select, SUSSelection computes selection choices for an entire
	populations' worth of individuals beforehand.  It then begins handing
	these choices out in response to successive requests.  When the choices
	have been depleted, SUSSelection computes another populations's worth.
	Like fitness proportionate selection, SUSSelection requires that all
	fitnesses be >= 0.

ec.select.GreedyOverselection
	A version of FitProportionateSelection used in early forms of genetic
	programming.  The individuals are divided by fitness into two groups:
	the good group and the bad group.  To select an individual, the 
	algorithm first picks either the good group or the bad group, then
	performs fitness proportionate selection within that group.  You 
	specify the size of the good group and the probability that it will
	be selected.  Like fitness proportionate selection, SUSSelection
	requires that all fitnesses be >= 0.

ec.select.RandomSelection		[acceptable for Steady State]
	Always provides a random individual.  This is equivalent to a
	tournament selection with a tournament size of 1.

ec.select.FirstSelection		[acceptable for Steady State]
	Always selects the first individual in the population array.  Only
	used for debugging purposes.

ec.select.BoltzmannSelection		[NOTE: UNTESTED]
	Performs Boltzman Selection, variation of fitness proportionate
	selection which makes fitness differences insignificant initially
	and more prominent as time goes on (think Simulated Annealing).
	Each generatiowe compute a temperature, defined as the initial
	temperature (which you define) minus the current generation times
	the cooling rate (which you also define).  If the temperature is
	below 1.0, the fitnesses are not adjusted.  Otherwise each
	fitness is adjusted as E^(fitness / temperature).

ec.select.SigmaScalingSelection		[NOTE: UNTESTED]
	Performs Sigma Scaling, a variation of fitness propoprtionate
	selection which stretches fitnesses out if they have been
	all bunched up together, enabling fitness proportionate 
	selection methods to still distinguish between them.  We begin
	by computing the mean and standard deviation of the current
	fitnesses.  If the standard deviation is not zero, we then
	adjust each fitness as 1 + (fitness - mean)/(2 * standardDeviation).
	If the standard deviation is zero, all fitnesses are set to 1.
	If any fitness is less than a minimal acceptable fitness (the "floor"),
	it is set to the floor.  This prevents fitnesses from getting stretched
	so small that they cannot compete.  You specify the floor.