Genetic algorithms full lecture

Genetic AlgorithmsGenetic Algorithms

IntroductionIntroduction
 After scientists became disillusioned withAfter scientists became disillusioned with
classical and neo-classical attempts atclassical and neo-classical attempts at
modeling intelligence, they looked in othermodeling intelligence, they looked in other
directions.directions.
 Two prominent fields arose, connectionismTwo prominent fields arose, connectionism
(neural networking, parallel processing) and(neural networking, parallel processing) and
evolutionary computing.evolutionary computing.
 It is the latter that this essay deals with -It is the latter that this essay deals with -
genetic algorithms and genetic programming.genetic algorithms and genetic programming.

What is GAWhat is GA
 AA genetic algorithmgenetic algorithm (or(or GAGA) is a search technique) is a search technique
used in computing to find true or approximateused in computing to find true or approximate
solutions to optimization and search problems.solutions to optimization and search problems.
 Genetic algorithms are categorized as global searchGenetic algorithms are categorized as global search
heuristics.heuristics.
 Genetic algorithms are a particular class ofGenetic algorithms are a particular class of
evolutionary algorithms that use techniques inspiredevolutionary algorithms that use techniques inspired
by evolutionary biology such as inheritance,by evolutionary biology such as inheritance,
mutation, selection, and crossover (also calledmutation, selection, and crossover (also called
recombination).recombination).

 Genetic algorithms are implemented as a computerGenetic algorithms are implemented as a computer
simulation in which a population of abstractsimulation in which a population of abstract
representations (called chromosomes or the genotyperepresentations (called chromosomes or the genotype
or the genome) of candidate solutions (calledor the genome) of candidate solutions (called
individuals, creatures, or phenotypes) to anindividuals, creatures, or phenotypes) to an
optimization problem evolves toward better solutions.optimization problem evolves toward better solutions.
 Traditionally, solutions are represented in binary asTraditionally, solutions are represented in binary as
strings of 0s and 1s, but other encodings are alsostrings of 0s and 1s, but other encodings are also
possible.possible.

 The evolution usually starts from a population ofThe evolution usually starts from a population of
randomly generated individuals and happens inrandomly generated individuals and happens in
generations.generations.
 In each generation, the fitness of every individual inIn each generation, the fitness of every individual in
the population is evaluated, multiple individuals arethe population is evaluated, multiple individuals are
selected from the current population (based on theirselected from the current population (based on their
fitness), and modified (recombined and possiblyfitness), and modified (recombined and possibly
mutated) to form a new population.mutated) to form a new population.

 The new population is then used in the nextThe new population is then used in the next
iteration of the algorithm.iteration of the algorithm.
 Commonly, the algorithm terminates whenCommonly, the algorithm terminates when
either a maximum number of generations haseither a maximum number of generations has
been produced, or a satisfactory fitness levelbeen produced, or a satisfactory fitness level
has been reached for the population.has been reached for the population.
 If the algorithm has terminated due to aIf the algorithm has terminated due to a
maximum number of generations, amaximum number of generations, a
satisfactory solution may or may not havesatisfactory solution may or may not have
been reached.been reached.

Key termsKey terms
 IndividualIndividual - Any possible solution- Any possible solution
 PopulationPopulation - Group of all- Group of all individualsindividuals
 Search SpaceSearch Space - All possible solutions to the problem- All possible solutions to the problem
 ChromosomeChromosome - Blueprint for an- Blueprint for an individualindividual
 TraitTrait - Possible aspect (- Possible aspect (features)features) of anof an individualindividual
 Allele -- Possible settings of trait (black, blond, etc.)
 LocusLocus - The position of a- The position of a genegene on theon the chromosomechromosome
 GenomeGenome - Collection of all- Collection of all chromosomeschromosomes for anfor an
individualindividual

Genotype and Phenotype
 Genotype:
– Particular set of genes in a genome
 Phenotype:
– Physical characteristic of the genotype
(smart, beautiful, healthy, etc.)

GA RequirementsGA Requirements
 A typical genetic algorithm requires two things to be defined:A typical genetic algorithm requires two things to be defined:
 a genetic representation of the solution domain, anda genetic representation of the solution domain, and
 a fitness function to evaluate the solution domain.a fitness function to evaluate the solution domain.
 A standard representation of the solution is as an array of bits.A standard representation of the solution is as an array of bits.
Arrays of other types and structures can be used in essentiallyArrays of other types and structures can be used in essentially
the same way.the same way.
 The main property that makes these genetic representationsThe main property that makes these genetic representations
convenient is that their parts are easily aligned due to theirconvenient is that their parts are easily aligned due to their
fixed size, that facilitates simple crossover operation.fixed size, that facilitates simple crossover operation.
 Variable length representations may also be used, butVariable length representations may also be used, but
crossover implementation is more complex in this case.crossover implementation is more complex in this case.
 Tree-like representations are explored in GeneticTree-like representations are explored in Genetic
programming.programming.

RepresentationRepresentation
Chromosomes could be:Chromosomes could be:
 Bit strings (0101 ... 1100)Bit strings (0101 ... 1100)
 Real numbers (43.2 -33.1 ... 0.0 89.2)Real numbers (43.2 -33.1 ... 0.0 89.2)
 Permutations of element (E11 E3 E7 ... E1 E15)Permutations of element (E11 E3 E7 ... E1 E15)
 Lists of rules (R1 R2 R3 ... R22 R23)Lists of rules (R1 R2 R3 ... R22 R23)
 Program elements (genetic programming)Program elements (genetic programming)
 ... any data structure ...... any data structure ...

GA RequirementsGA Requirements
 The fitness function is defined over the genetic representationThe fitness function is defined over the genetic representation
and measures theand measures the qualityquality of the represented solution.of the represented solution.
 The fitness function is always problem dependent.The fitness function is always problem dependent.
 For instance, in theFor instance, in the knapsack problemknapsack problem we want to maximizewe want to maximize
the total value of objects that we can put in a knapsack ofthe total value of objects that we can put in a knapsack of
some fixed capacity.some fixed capacity.
 A representation of a solution might be an array of bits, whereA representation of a solution might be an array of bits, where
each bit represents a different object, and the value of the biteach bit represents a different object, and the value of the bit
(0 or 1) represents whether or not the object is in the knapsack.(0 or 1) represents whether or not the object is in the knapsack.
 Not every such representation is valid, as the size of objectsNot every such representation is valid, as the size of objects
may exceed the capacity of the knapsack.may exceed the capacity of the knapsack.
 TheThe fitnessfitness of the solution is the sum of values of all objects inof the solution is the sum of values of all objects in
the knapsack if the representation is valid, or 0 otherwise. Inthe knapsack if the representation is valid, or 0 otherwise. In
some problems, it is hard or even impossible to define thesome problems, it is hard or even impossible to define the
fitness expression; in these cases, interactive geneticfitness expression; in these cases, interactive genetic
algorithms are used.algorithms are used.

A fitness functionA fitness function

Basics of GABasics of GA
 The most common type of genetic algorithm works like this:The most common type of genetic algorithm works like this:
 a population is created with a group of individuals createda population is created with a group of individuals created
randomly.randomly.
 The individuals in the population are then evaluated.The individuals in the population are then evaluated.
 The evaluation function is provided by the programmer andThe evaluation function is provided by the programmer and
gives the individuals a score based on how well they performgives the individuals a score based on how well they perform
at the given task.at the given task.
 Two individuals are then selected based on their fitness, theTwo individuals are then selected based on their fitness, the
higher the fitness, the higher the chance of being selected.higher the fitness, the higher the chance of being selected.
 These individuals then "reproduce" to create one or moreThese individuals then "reproduce" to create one or more
offspring, after which the offspring are mutated randomly.offspring, after which the offspring are mutated randomly.
 This continues until a suitable solution has been found or aThis continues until a suitable solution has been found or a
certain number of generations have passed, depending on thecertain number of generations have passed, depending on the
needs of the programmer.needs of the programmer.

General Algorithm for GAGeneral Algorithm for GA
 InitializationInitialization
 Initially many individual solutions are randomlyInitially many individual solutions are randomly
generated to form an initial population. Thegenerated to form an initial population. The
population size depends on the nature of the problem,population size depends on the nature of the problem,
but typically contains several hundreds or thousandsbut typically contains several hundreds or thousands
of possible solutions.of possible solutions.
 Traditionally, the population is generated randomly,Traditionally, the population is generated randomly,
covering the entire range of possible solutions (thecovering the entire range of possible solutions (the
search spacesearch space).).
 Occasionally, the solutions may be "seeded" in areasOccasionally, the solutions may be "seeded" in areas
where optimal solutions are likely to be found.where optimal solutions are likely to be found.

 SelectionSelection
 During each successive generation, a proportion of theDuring each successive generation, a proportion of the
existing population is selected to breed a new generation.existing population is selected to breed a new generation.
 Individual solutions are selected through aIndividual solutions are selected through a fitness-basedfitness-based
process, where fitter solutions (as measured by a fitnessprocess, where fitter solutions (as measured by a fitness
function) are typically more likely to be selected.function) are typically more likely to be selected.
 Certain selection methods rate the fitness of each solution andCertain selection methods rate the fitness of each solution and
preferentially select the best solutions. Other methods ratepreferentially select the best solutions. Other methods rate
only a random sample of the population, as this process mayonly a random sample of the population, as this process may
be very time-consuming.be very time-consuming.
 Most functions are stochastic and designed so that a smallMost functions are stochastic and designed so that a small
proportion of less fit solutions are selected. This helps keepproportion of less fit solutions are selected. This helps keep
the diversity of the population large, preventing prematurethe diversity of the population large, preventing premature
convergence on poor solutions. Popular and well-studiedconvergence on poor solutions. Popular and well-studied
selection methods include roulette wheel selection andselection methods include roulette wheel selection and
tournament selection.tournament selection.

 In roulette wheel selection, individuals areIn roulette wheel selection, individuals are
given a probability of being selected that isgiven a probability of being selected that is
directly proportionate to their fitness.directly proportionate to their fitness.
 Two individuals are then chosen randomlyTwo individuals are then chosen randomly
based on these probabilities and producebased on these probabilities and produce
offspring.offspring.

Roulette Wheel’s Selection Pseudo Code:Roulette Wheel’s Selection Pseudo Code:
for all members of populationfor all members of population
sum += fitness of this individualsum += fitness of this individual
end forend for
probability = sum of probabilities + (fitness / sum)probability = sum of probabilities + (fitness / sum)
sum of probabilities += probabilitysum of probabilities += probability
end forend for
loop until new population is fullloop until new population is full
do this twicedo this twice
number = Random between 0 and 1number = Random between 0 and 1
if number > probability but less than next probability thenif number > probability but less than next probability then
you have been selectedyou have been selected
end forend for
endend
create offspringcreate offspring
end loopend loop

 ReproductionReproduction
 The next step is to generate a second generation population ofThe next step is to generate a second generation population of
solutions from those selected through genetic operators:solutions from those selected through genetic operators:
crossover (also called recombination), and/or mutation.crossover (also called recombination), and/or mutation.
 For each new solution to be produced, a pair of "parent"For each new solution to be produced, a pair of "parent"
solutions is selected for breeding from the pool selectedsolutions is selected for breeding from the pool selected
previously.previously.
 By producing a "child" solution using the above methods ofBy producing a "child" solution using the above methods of
crossover and mutation, a new solution is created whichcrossover and mutation, a new solution is created which
typically shares many of the characteristics of its "parents".typically shares many of the characteristics of its "parents".
New parents are selected for each child, and the processNew parents are selected for each child, and the process
continues until a new population of solutions of appropriatecontinues until a new population of solutions of appropriate
size is generated.size is generated.

 These processes ultimately result in the nextThese processes ultimately result in the next
generation population of chromosomes that isgeneration population of chromosomes that is
different from the initial generation.different from the initial generation.
 Generally the average fitness will haveGenerally the average fitness will have
increased by this procedure for the population,increased by this procedure for the population,
since only the best organisms from the firstsince only the best organisms from the first
generation are selected for breeding, alonggeneration are selected for breeding, along
with a small proportion of less fit solutions, forwith a small proportion of less fit solutions, for
reasons already mentioned above.reasons already mentioned above.

CrossoverCrossover
 the most common type is single point crossover. In singlethe most common type is single point crossover. In single
point crossover, you choose a locus at which you swap thepoint crossover, you choose a locus at which you swap the
remaining alleles from on parent to the other. This is complexremaining alleles from on parent to the other. This is complex
and is best understood visually.and is best understood visually.
 As you can see, the children take one section of theAs you can see, the children take one section of the
chromosome from each parent.chromosome from each parent.
 The point at which the chromosome is broken depends on theThe point at which the chromosome is broken depends on the
randomly selected crossover point.randomly selected crossover point.
 This particular method is called single point crossover becauseThis particular method is called single point crossover because
only one crossover point exists. Sometimes only child 1 oronly one crossover point exists. Sometimes only child 1 or
child 2 is created, but oftentimes both offspring are createdchild 2 is created, but oftentimes both offspring are created
and put into the new population.and put into the new population.
 Crossover does not always occur, however. Sometimes, basedCrossover does not always occur, however. Sometimes, based
on a set probability, no crossover occurs and the parents areon a set probability, no crossover occurs and the parents are
copied directly to the new population. The probability ofcopied directly to the new population. The probability of
crossover occurring is usually 60% to 70%.crossover occurring is usually 60% to 70%.

MutationMutation
 After selection and crossover, you now have a new populationAfter selection and crossover, you now have a new population
full of individuals.full of individuals.
 Some are directly copied, and others are produced bySome are directly copied, and others are produced by
crossover.crossover.
 In order to ensure that the individuals are not all exactly theIn order to ensure that the individuals are not all exactly the
same, you allow for a small chance of mutation.same, you allow for a small chance of mutation.
 You loop through all the alleles of all the individuals, and ifYou loop through all the alleles of all the individuals, and if
that allele is selected for mutation, you can either change it bythat allele is selected for mutation, you can either change it by
a small amount or replace it with a new value. The probabilitya small amount or replace it with a new value. The probability
of mutation is usually between 1 and 2 tenths of a percent.of mutation is usually between 1 and 2 tenths of a percent.
 Mutation is fairly simple. You just change the selected allelesMutation is fairly simple. You just change the selected alleles
based on what you feel is necessary and move on. Mutation is,based on what you feel is necessary and move on. Mutation is,
however, vital to ensuring genetic diversity within thehowever, vital to ensuring genetic diversity within the
population.population.

 TerminationTermination
 This generational process is repeated until aThis generational process is repeated until a
termination condition has been reached.termination condition has been reached.
 Common terminating conditions are:Common terminating conditions are:
 A solution is found that satisfies minimum criteriaA solution is found that satisfies minimum criteria
 Fixed number of generations reachedFixed number of generations reached
 Allocated budget (computation time/money) reachedAllocated budget (computation time/money) reached
 The highest ranking solution's fitness is reaching or hasThe highest ranking solution's fitness is reaching or has
reached a plateau such that successive iterations no longerreached a plateau such that successive iterations no longer
produce better resultsproduce better results
 Manual inspectionManual inspection
 Any Combinations of the aboveAny Combinations of the above

Genetic ProgrammingGenetic Programming
 In programming languages such as LISP, the mathematicalIn programming languages such as LISP, the mathematical
notation is not written in standard notation, but in prefixnotation is not written in standard notation, but in prefix
notation. Some examples of this:notation. Some examples of this:
 + 2 1+ 2 1 :: 2 + 12 + 1
 * + 2 1 2* + 2 1 2 :: 2 * (2+1)2 * (2+1)
 * + - 2 1 4 9 :* + - 2 1 4 9 : 9 * ((2 - 1) + 4)9 * ((2 - 1) + 4)
 Notice the difference between the left-hand side to the right?Notice the difference between the left-hand side to the right?
Apart from the order being different, no parenthesis! TheApart from the order being different, no parenthesis! The
prefix method makes it a lot easier for programmers andprefix method makes it a lot easier for programmers and
compilers alike, because order precedence is not an issue.compilers alike, because order precedence is not an issue.
 You can build expression trees out of these strings that thenYou can build expression trees out of these strings that then
can be easily evaluated, for example, here are the trees for thecan be easily evaluated, for example, here are the trees for the
above three expressions.above three expressions.

 You can see how expression evaluation is thus a lotYou can see how expression evaluation is thus a lot
easier.easier.
 What this have to do with GAs? If for example youWhat this have to do with GAs? If for example you
have numerical data and 'answers', but no expressionhave numerical data and 'answers', but no expression
to conjoin the data with the answers.to conjoin the data with the answers.
 A genetic algorithm can be used to 'evolve' anA genetic algorithm can be used to 'evolve' an
expression tree to create a very close fit to the data.expression tree to create a very close fit to the data.
 By 'splicing' and 'grafting' the trees and evaluatingBy 'splicing' and 'grafting' the trees and evaluating
the resulting expression with the data and testing it tothe resulting expression with the data and testing it to
the answers, the fitness function can return how closethe answers, the fitness function can return how close
the expression is.the expression is.

Applications of Genetic AlgorithmsApplications of Genetic Algorithms
 Uses of genetic programming can lie in:Uses of genetic programming can lie in:
 stock market predictionstock market prediction
 advanced mathematicsadvanced mathematics
 and military applicationsand military applications
 Composing MusicComposing Music
 GamingGaming
 Market StrategiesMarket Strategies
 RoboticsRobotics

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