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Evolutionary Algorithms

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Evolutionary Algorithms in a NutshellORHow I copied the comp.ai.genetic FAQ onto a bunch of slides at 5am this morningAniruddh Nath24 Sep 2006What are Evolutionary Algorithms?●“A subset of evolutionary computation, a generic population-based metaheuristic optimization algorithm.” - Wikipedia●Wuh?●A set of biologically inspired algorithms that work on the 'survival of the fittest' principle.Key Features●Population●Genetic operators (recombination, mutation)●Reproduction●Fitness function●ExplorationTypes of Evolutionary Algorithms●Genetic algorithms●Evolutionary programming●Evolution strategies●Genetic programming●Learning classifier systemsGenetic Algorithms●Search technique, often used for optimization problems.●Population: usually a set of binary strings (“chromosomes”).●Looking for a binary string that is exactly or approximately optimal, given a fitness function.Pseudocode (from c.a.genetic)t := 0;initpopulation P (t);evaluate P (t);while not done do t := t + 1; P' := selectparents P (t); recombine P' (t); mutate P' (t); evaluate P' (t); P := survive P,P' (t);odToy ExampleGive me a number as close to '42' as possible.Representation: unsigned binary strings.Fitness function: f(x) = 42 - |42 – x|Completion condition: f(x) > 40.Toy Example (cont'd)Init: P = {010010, 110011, 100001}Evaluate:f(010010) = 18f(110011) = 33f(100001) = 33Generation 1Select parents: 110011, 100001Recombination: we're using one point crossover. Other methods are possible.Mutation: 111001, 100010P = {110011, 100001, 111001, 100010}Condition not met.110 011100 001110001100011Generation 2Select parents: 110011, 100010Recombination: Mutation: 010010, 101011P = {110011, 100010, 010010, 101011}f(101011) = 41 > 40. Done.110 011100 010110010100011Evolutionary Programming●Similar idea, but not restricted to 'chromosome' structure.●Solutions can have any structure; various mutations are possible based on how you define your solutions.●Recombination tends not to play a role.Evolutionary Strategy●Uses vector of reals rather than bit string.●Mutation: add a random value to each element of the vector.●Recombination: take the mean of each element of the parent vector:[1.0, 2.0, 3.0], [2.0, 1.0, 4.0] [1.5, 1.5, 3.5]Genetic Programming●Solutions are actual programs, usually represented as parse trees.●Recombination consists of programs exchanging subtrees.●Mutation is generally not used.●This is awesome.Learning Classifier Systems●Population is a set of 'binary classifiers' (if-then rules about an environment).●Uses reinforcement learning: the environment gives the agent a reward for choosing a set of rules; agent aims to maximize award.Problems with EAs●Local maxima.●Highly dependent on problem formulation.●Not guaranteed to ever find a good solution (if you're really unlucky with your operators).Why bother?●Yields surprisingly good results on optimization problems.●Example: find a pretty good way to schedule jobs on a processor (minimize waiting time).●Scheduling problems tend to be NP-hard; cannot calculate exact solution.●EAs can also be used to find numbers close to 42.A Brief History of EAs●Nils Aall Barricelli used EAs in 1954 to play a card game.●The father of modern GAs is Prof. John Holland, at the University of Michigan.●Prof. David Goldberg in the GE department worked with Prof. Holland, and now runs the Illinois Genetic Algorithms Laboratory (IlliGAL).References/Further Reading●Goldberg, David. Genetic Algorithms in Search, Optimization, and Machine Learning.●comp.ai.genetic newsgroup.●Wikipedia.●Some website I found off


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