Introduction to Genetic Algorithms Son Kuswadi Robotics and Automation Based on Biologically- Inspired Technology (RABBIT) Research Groups Electronic Engineering

Introduction toIntroduction to Genetic Genetic AlgorithmsAlgorithmsSon KuswadiSon KuswadiRobotics and Automation Based on Biologically-Robotics and Automation Based on Biologically-Inspired Technology (RABBIT) Research GroupsInspired Technology (RABBIT) Research GroupsElectronic Engineering Polytechnic Institute of Electronic Engineering Polytechnic Institute of Surabaya (EEPIS)Surabaya (EEPIS)Institut Teknologi Sepuluh Nopember (ITS)Institut Teknologi Sepuluh Nopember (ITS)

Road MapRoad Map• Basic Idea• Quick Review• Classes of Search Techniques• Simple GA• State of The Art


Basic IdeaBasic Idea

• Suppose you have a problem• You don’t know how to solve it• What can you do?• Can you use a computer to somehow

find a solution for you?• This would be nice! Can it be done?

Basic IdeaBasic Idea• A dumb solution

A “blind generate and test” algorithm:

Repeat Generate a random possible solution Test the solution and see how good it is

Until solution is good enough

Basic IdeaBasic IdeaCan we used this dumb idea?• Sometimes - yes:

– if there are only a few possible solutions– and you have enough time– then such a method could be used

• For most problems - no:– many possible solutions– with no time to try them all– so this method can not be used

Basic IdeaBasic Idea• Dumb Idea… but it was works….

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h

Basic IdeaBasic IdeaA “less-dumb” idea (GA)Generate a set of random solutions

RepeatTest each solution in the set (rank them)

Remove some bad solutions from set

Duplicate some good solutions

make small changes to some of them

Until best solution is good enough


• Obviously this depends on the problem!• GA’s often encode solutions as fixed

length “bitstrings” (e.g. 101110, 111111, 000101)

• Each bit represents some aspect of the proposed solution to the problem

• For GA’s to work, we need to be able to “test” any string and get a “score” indicating how “good” that solution is

How do you encode a solution?How do you encode a solution?


• Imagine you had to drill for oil somewhere along a single 1km desert road

• Problem: choose the best place on the road that produces the most oil per day

• We could represent each solution as a position on the road

• Say, a whole number between [0..1000]

Silly Example - Drilling for OilSilly Example - Drilling for Oil

Basic IdeaBasic IdeaWhere to drill for oil?Where to drill for oil?

0 500 1000

Road

Solution2 = 900Solution1 = 300


• The set of all possible solutions [0..1000] is called the search space or state space

• In this case it’s just one number but it could be many numbers or symbols

• Often GA’s code numbers in binary producing a bitstring representing a solution

• In our example we choose 10 bits which is enough to represent 0..1000

Digging for OilDigging for Oil

Basic IdeaBasic IdeaConvert to binary stringConvert to binary string

512 256 128 64 32 16 8 4 2 1

900 1 1 1 0 0 0 0 1 0 0

300 0 1 0 0 1 0 1 1 0 0

1023 1 1 1 1 1 1 1 1 1 1

In GA’s these encoded strings are sometimes called “genotypes” or “chromosomes” and the individual bits are

sometimes called “genes”

Basic IdeaBasic IdeaDrilling for OilDrilling for Oil

01000

Road

Solution2 = 900 (1110000100)

Solution1 = 300 (0100101100)

O I

L

Location

305


We have seen how to:• represent possible solutions as a

number• encoded a number into a binary string• generate a score for each number given

a function of “how good” each solution is - this is often called a fitness function

Basic IdeaBasic IdeaBack to the (GA) Algorithm

Generate a set of random solutions

Repeat

Test each solution in the set (rank them)

Remove some bad solutions from set

Duplicate some good solutions

make small changes to some of them

Until best solution is good enough


• Various method inspired by Darwinian evolution are used to update the set or population of solutions (or chromosomes)

• Two high scoring “parent” bit strings or chromosomes are selected and combined

• Producing two new offspring (bit strings)• Each offspring may then be changed randomly

(mutation)

Replication and MutationReplication and Mutation


• Many schemes are possible so long as better scoring chromosomes more likely selected

• Score is often termed the fitness• “Roulette Wheel” selection can be used:

– Add up the fitness's of all chromosomes– Generate a random number R in that range– Select the first chromosome in the population

that - when all previous fitness’s are added - gives you at least the value R

Selecting ParentsSelecting Parents


Quick ReviewQuick Review

• A class of probabilistic optimization algorithms• Inspired by the biological evolution process• Uses concepts of “Natural Selection” and

“Genetic Inheritance” (Darwin 1859)• Originally developed by John Holland (1975)• Particularly well suited for hard problems where

little is known about the underlying search space• Widely-used in business, science and engineering

Quick ReviewQuick Review• Typically applied to:

– discrete optimization

• Attributed features:– not too fast– good heuristic for combinatorial problems

• Special Features:– Traditionally emphasizes combining

information from good parents (crossover)– many variants, e.g., reproduction models,

operators

Quick ReviewQuick Review• Holland’s original GA is now known

as the simple genetic algorithm (SGA)

• Other GAs use different:– Representations– Mutations– Crossovers– Selection mechanisms


Classes of Search Classes of Search TechniquesTechniques

Search Techniqes

Calculus Base Techniqes

Guided random search techniqes

Enumerative Techniqes

BFSDFS Dynamic Programming

Tabu Search Hill Climbing

Simulated Anealing

Evolutionary Algorithms

Genetic Programming

Genetic Algorithms

Fibonacci Sort

ReferencesReferencesC. Darwin. On the Origin of Species by Means of Natural Selection; or, the Preservation of flavored Races in the Struggle for Life. John Murray, London, 1859.W. D. Hillis. Co-Evolving Parasites Improve Simulated Evolution as an Optimization Procedure. Artificial Life 2, vol 10, Addison-Wesley, 1991.J. H. Holland. Adaptation in Natural and Artificial Systems. The University of Michigan Press, Ann Arbor, Michigan, 1975. Z. Michalewicz. Genetic Algorithms + Data Structures = Evolution Programs. Springer-Verlag, Berlin, third edition, 1996. M. Sipper. Machine Nature: The Coming Age of Bio-Inspired Computing. McGraw-Hill, New-York, first edition, 2002.D. Goldberg, Genetic Algorithms in Search, Optimization and Machine Learning, Addison Wesley, 1989


Simple GASimple GA

produce an initial population of individuals

evaluate the fitness of all individuals

while termination condition not met do

select fitter individuals for reproduction

recombine between individuals

mutate individuals

evaluate the fitness of the modified individuals

generate a new population

End while

The Evolutionary CycleThe Evolutionary Cycle

selection

population evaluation

modification

discard

deleted members

parents

modifiedoffspring

evaluated offspring

initiate& evaluate

Simple Example Simple Example (Goldberg98)(Goldberg98)

• Simple problem: max x2 over {0,1,…,31}• GA approach:

– Representation: binary code, e.g. 01101 13– Population size: 4– 1-point xover, bitwise mutation – Roulette wheel selection– Random initialization

• We show one generational cycle done by hand

SelectionSelection

CrossoverCrossover

MutationMutation

Simple GASimple GA• Has been subject of many (early) studies

– still often used as benchmark for novel GAs

• Shows many shortcomings, e.g.– Representation is too restrictive– Mutation & crossovers only applicable for bit-

string & integer representations– Selection mechanism sensitive for converging

populations with close fitness values


State of The ArtState of The Art

• Crossover: Alternative Operators• Crossover vs Mutation• Other Representation: Gray coding, integer, floating point• Binary Representation: Required Precision Set

Crossover – Crossover – Alternative OperatorsAlternative Operators

• Performance with 1 Point Crossover depends on the order that variables occur in the representation– more likely to keep together genes that are

near each other– Can never keep together genes from opposite

ends of string– This is known as Positional Bias– Can be exploited if we know about the

structure of our problem, but this is not usually the case


• Choose n random crossover points• Split along those points• Glue parts, alternating between parents• Generalisation of 1 point (still some positional

bias)

N-point crossover


• Assign 'heads' to one parent, 'tails' to the other• Flip a coin for each gene of the first child• Make an inverse copy of the gene for the second

child• Inheritance is independent of position

Uniform crossover

Crossover vs MutationCrossover vs Mutation• Decade long debate: which one is better /

necessary / main-background

• Answer (at least, rather wide agreement):– it depends on the problem, but– in general, it is good to have both– both have another role– mutation-only-EA is possible, xover-only-EA

would not work

Crossover vs MutationCrossover vs MutationExploration: Discovering promising areas in the search space,

i.e. gaining information on the problem

Exploitation: Optimising within a promising area, i.e. using

information

There is co-operation AND competition between them

• Crossover is explorative, it makes a big jump to an area

somewhere “in between” two (parent) areas

• Mutation is exploitative, it creates random small diversions,

thereby staying near (in the area of ) the parent

Crossover vs MutationCrossover vs Mutation• Only crossover can combine information from two

parents

• Only mutation can introduce new information (alleles)

• To hit the optimum you often need a ‘lucky’ mutation

Other RepresentationOther Representation• Gray coding of integers (still binary

chromosomes)– Gray coding is a mapping that means that

small changes in the genotype cause small changes in the phenotype (unlike binary coding). “Smoother” genotype-phenotype mapping makes life easier for the GA

Nowadays it is generally accepted that it is better to encode numerical variables directly as:

• Integers• Floating point variables

Integer RepresentationInteger Representation• Some problems naturally have integer variables,

e.g. image processing parameters • Others take categorical values from a fixed set

e.g. {blue, green, yellow, pink}• N-point / uniform crossover operators work• Extend bit-flipping mutation to make

– “creep” i.e. more likely to move to similar value

– Random choice (esp. categorical variables)– For ordinal problems, it is hard to know correct

range for creep, so often use two mutation operators in tandem

Floating Point Floating Point RepresentationRepresentation

• Many problems occur as real valued problems, e.g. continuous parameter optimisation f : n

• Illustration: Ackley’s function (often used in EC)

Binary RepresentationBinary Representation• For example, search space x [-1..2],

with six places after decimal point• Therefore 3x1000000 equal size ranges• This means that 22 bit are required since:

– 2097152=221 <3000000<222=4194304

Binary RepresentationBinary Representation• Mapping from binary string <b21b20…b0> into a real number x

from range [-1..2]:

• convert the binary string <b21b20…b0> from the base 2 to base 10:

• find a corresponding real number x:

•

'2...10

21

0202021 xbbbbi

ii

12

3'.0.1

22 xx

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Introduction to Genetic Algorithms Son Kuswadi Robotics and Automation Based on Biologically- Inspired Technology (RABBIT) Research Groups Electronic Engineering