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AI and Robotics

Oct 23, 2013 (4 years and 8 months ago)

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Genetic Algorithms

2

Introduction To Genetic
Algorithms (GAs)

3

What Are Genetic Algorithms
(GAs)?

Genetic Algorithms are
search

and
optimization

techniques based on
Darwin’s Principle of
Natural Selection
.

Characteristics of GA

Parallel
-
search procedures that can be
implemented on parallel processing
machines for speeding operations

Applies to both continuous and discrete
optimization problems

Stochastic in nature and less likely to get
caught in local minima

Facilitates both structure and parameter
identification

5

Darwin’s Principle Of Natural
Selection

I

IF there are organisms that reproduce, and

IF offsprings inherit traits from their parents, and

IF there is variability of traits, and

IF the environment cannot support all members of a
growing population,

THEN those members of the population with less
-
adaptive traits (determined by the environment) will die
out, and

THEN those members with more
-
(determined by the environment) will thrive

The result is the evolution of
species
.

Working of GA

GA encodes each point in a parameter space
into a binary bit called chromosome

Each point is associated with a fitness
function

Gene pool is a population of all such points

In each generation GA constructs a new
population using genetic operators

Crossover

Mutation

Components of GA

Encoding schemes

Crossover operators

Mutation operators

Encoding schemes

Transforms points in parameter space into string
representations

Eg (11,6,9) is represented as
1011
0110
1011

Encoding schemes provide a way of translating
problem
-
specific knowledge directly into GA
framework

After this the fitness function is evaluated

Next selection is based on the fittest survivor

Fitness evaluation

How is it different from other
optimization and search procedures?

1.
Works with a coding of the parameter
set, not the parameters themselves

2.
Search for a population of point and not a
single point

3.
Use objective function information and
not derivatives or other auxiliary
knowledge

How GA is used and different from
other optimization techniques?

The first step in GA is to code the parameter x as
a finite length string

Example 1 can be code as string of 5 bits with an
output f=f(s) , where s=string of bits

Successive populations are generated using the
GA

For effective check GA requires only objective
functions associated with individual strings

Simple genetic algorithm

Reproduction:
-

individual strings are copied
according to their objective fn: values f(FITNESS
FUNCTION)

Crossover:
-

Members of the newly reproduced
strings are mated at random. Each pair of strings
undergoes crossing overs.

Mutation:
-
supplements reproduction and
crossover and acts as an insurance policy against
premature loss of important notions

13

Basic Idea Of Principle Of Natural
Selection

“Select The Best, Discard The Rest”

Example 1

Maximize f(x) =x
2

on the integer
scale from 0
-
31

0

31

x

f(x)

1000

Example 1

No:

String

Fitness

% of total

1

01101

169

14.2

2

11000

576

49.2

3

01000

64

5.5

4

10011

361

30.9

Total

1170

100

Roulette wheel with slots sized
according to fitness

Crossover

A1=0 1 1 0 1

A2=1 1 0 0 0

A1’=0 1 1 0 0

A2’=1 1 0 0 1

Simple GA by
Hand(Reproduction)

No:

String

x

f(x)

x
2

pselect

fi/
Ʃf

Expected count

n.pselect

Actual count

(Roulette Wheel0

1

01101

13

169

.14

.58

1

2

11000

24

576

.49

1.97

2

3

01000

08

64

.06

0.24

0

4

10011

19

361

0.31

1.24

1

Sum

1170

1.00

4

4.0

Average

293

0.25

1.00

1.0

Maximum

576

.49

1.97

2

Crossover

Mating Pool after
Reproduction(Cross Site
shown)

Mate

Crossov
er

New population

x

F(x)

0110|1

2

4

0 1 1 0 0

12

144

1100|0

1

4

1 1 0 0 1

25

625

11|000

4

2

1 1 0 1 1

27

729

10|011

3

2

1 0 0 0 0

16

256

Sum

1754

Average

439

Maximum

729

Probability of mutation in this test is 0.001. With 20 transferred bit
positions we should expect 20*0.001=0.02

Grist for the search mill

How does the directed search guide
help improvement?

Seeking similarities among strings in
population

Causal relationships between similarities
and high fitness

SCHEMATA

21

Evolution in the real world

Each cell of a living thing contains
chromosomes

-

strings
of
DNA

Each chromosome contains a set of
genes

-

blocks of DNA

Each gene determines some aspect of the organism (like eye
colour)

A collection of genes is sometimes called a
genotype

A collection of aspects (like eye colour) is sometimes called a
phenotype

Reproduction involves recombination of genes from parents
and then small amounts of
mutation

(errors) in copying

The
fitness

of an organism is how much it can reproduce
before it dies

Evolution based on “survival of the fittest”

Basic Idea Of Principle Of Natural Selection

“Select The
Best
Rest

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Population

Chromosomes could be:

Bit strings (0101 ...
1100)

Real numbers (43.2
-
33.1 ... 0.0
89.2)

Permutations of element (E11 E3 E7 ... E1 E15)

Lists of rules (R1 R2 R3 ... R22
R23)

... any data structure ...

population

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Fitness Function

A fitness function quantifies the optimality of a
solution so that that particular solution may be
ranked against all the other solutions.

A fitness value is assigned to each solution depending
on how close it actually is to solving the problem.

Ideal fitness function correlates closely to goal + quickly
computable.

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Crossover

Mimics biological recombination

Some portion of genetic material is
swapped between chromosomes

Typically the swapping produces an
offspring

CROSSOVER

(1 2 9 3 0 7 )

(1 2 9 7 9 5)

(4 6 1 7 9 5 )

(1 2 9 3 0 7 )

( 4 6 1 7 9 5)

(4 6 1 7 9 5 )

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Algorithm

Generate
Initial Population

do

Calculate the
F
itness

of each member

do

{

Select
Parents

from current population

Perform
Crossover

population

Merge

new population into the current

population

Mutate

current population till result

is obtained

}

Mutation

Selects a random locus

gene location

with some probability and alters the
allele at that locus

The intuitive mechanism for the
preservation of variety in the
population

Mutation: Local Modification

Before:

(1 0 1 1 0 1 1 0)

After:

(0 1 1 0 0 1 1 0)

Before:

(1.38
-
69.4 326.44 0.1)

After:

(1.38
-
67.5 326.44 0.1)

The Problem

The
Traveling Salesman Problem

is defined as:

G
iven:

1) A set of cities

2) Symmetric distance matrix that indicates the cost of
travel from each city to every other city.

Goal:

1) Find
the shortest circular tour
,
visiting every city
exactly once.

2)
M
inimize the total travel cost
, which includes the cost
of traveling from the last city back to the first city’.

Traveling Salesperson Problem

38

Encoding

Represent every city with an integer .

Consider 6 Indian cities

Mumbai, Nagpur , Calcutta, Delhi, Bangalore and
Pune assign a number to each.

Mumbai 1

Nagpur 2

Calcutta 3

Delhi 4

Bangalore 5

Pune

6

39

Encoding

Thus a path would be represented as a
sequence

of
integers from 1 to 6.

The path
[1 2 3 4 5 6]

represents a path from

Mumbai to Nagpur
-

Nagpur to Calcutta
-

Calcutta to Delhi
-

Delhi
to Bangalore
-

Bangalore to Pune and pune to Mumbai.

Fitness Function

The fitness function will be the
total cost of the tour

represented by each chromosome.

This can be calculated as the
sum of the distances

traversed in each travel segment.

The
Lesser The Sum, The Fitter The Solution

Represented By That Chromosome.

1

2

3

4

5

6

1

0

863

1987

1407

998

163

2

863

0

1124

1012

1049

620

3

1987

1124

0

1461

1881

1844

4

1407

1012

1461

0

2061

1437

5

998

1049

1881

2061

0

841

6

163

620

1844

1437

841

0

Distance/Cost Matrix For TSP

Fitness Function

(contd.)

So, for a chromosome [4 1 3 2 5 6 ], the total cost
of travel or fitness will be calculated as shown below

Fitness

= 1407 + 1987 + 1124 + 1049 + 841

=

6408

kms.

Since our objective is to
Minimize

the distance, the
lesser the total distance, the fitter the solution.

Initial Population for TSP

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(4,3,6,2,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

Select Parents

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(4,3,6,2,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

Try to pick the better ones.

Create Off
-
Spring

1 point

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(4,3,6,2,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

(
3
,
4
,
5
,
6
,
2
)

(
3
,
4
,
5
,
6
,
2
)

Create More Offspring

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(4,3,6,2,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

(
5
,
4
,
2
,
6
,
3
)

(
3
,
4
,
5
,
6
,
2
)

(
5
,
4
,
2
,
6
,
3
)

Mutate

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(4,3,6,2,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

Mutate

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(
2
,3,6,
4
,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

(
3
,
4
,
5
,
6
,
2
)

(
5
,
4
,
2
,
6
,
3
)

Eliminate

(5,3,4,6,2)

(2,4,6,3,5)

(4,3,6,5,2)

(2,3,4,6,5)

(
2
,3,6,
4
,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

Tend to kill off the worst ones.

(
3
,
4
,
5
,
6
,
2
)

(
5
,
4
,
2
,
6
,
3
)

Integrate

(5,3,4,6,2)

(2,4,6,3,5)

(
2
,3,6,
4
,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

(
3
,
4
,
5
,
6
,
2
)

(
5
,
4
,
2
,
6
,
3
)

Restart

(5,3,4,6,2)

(2,4,6,3,5)

(2,3,6,4,5)

(3,4,5,2,6)

(3,5,4,6,2)

(4,5,3,6,2)

(5,4,2,3,6)

(4,6,3,2,5)

(3,4,2,6,5)

(3,6,5,1,4)

(3,4,5,6,2)

(5,4,2,6,3)

When to Use a GA

Alternate solutions are too slow or overly
complicated

Need an exploratory tool to examine new
approaches

Problem is similar to one that has already been
successfully solved by using a GA

Want to hybridize with an existing solution

Benefits of the GA technology meet key problem
requirements

53

SUMMARY

Genetic Algorithms

(GAs) implement optimization strategies
based on simulation of the natural law of evolution of a
species by
natural selection

The basic GA Operators are:

Encoding

Recombination

Crossover

Mutation

GAs have been applied to a variety of function optimization
problems, and have been shown to be highly effective in
searching a
large, poorly defined search space

THANK YOU