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International Journal of Engineering and Technical Research (IJETR)
ISSN: 2321-0869 (O) 2454-4698 (P), Volume-7, Issue-5, May 2017
IV. DIFFERENTIAL EVOLUTION

offspring; otherwise,
directly becomes the offspring.
Setting the minimum problem as an example, the selection
method is shown in Equation (8), where f is the fitness
function such as a cost or forecasting error function. The
equation is given as follows:

DE [4] is a population based stochastic search which can be
efficiently used as a global optimizer in the continuous search
domain. DE has been successfully applied in diverse fields
such as large scale power dispatch problem [7], global
numerical optimization [8], power loss minimization [6] and
pattern reorganization. DE also has been extensively used in
different types of clustering like image pixel clustering [9],
text document clustering and dynamic clustering for any
unknown datasets [10]. Like any other evolutionary
algorithms, DE also starts with a population of NP
D-dimensional parameter vectors. Two other parameters used
in DE are scaling factor F and cross over rate CR.
The standard DE consists of four main operations:
initialization, mutation, crossover, and selection.
a)

(8)

V. GENETIC ALGORITHM
In GA, the evolution starts from a population of completely
random individuals and occur in generations. In each
generation, the fitness of the whole population is evaluated;
multiple individuals are stochastically selected from the
current population (based on their fitness), and modified
(mutated or recombined) to form a new population [12]. The
new population is then used in the next iteration of the
algorithm.

Initialization

Real number coding is used for the DE. In this operation,
several parameters, including population size N, length of
chromosome D, scaling or mutation factor F, crossover rate
CR, and the range of gene value [
], are initialized.
The population is randomly initialized as follows:

a)

Chromosomes are selected from the population to become
parents to crossover. The problem is how to select these
chromosomes. There are many methods to select the best
chromosomes, such as, roulette wheel selection, Boltzman
selection, tournament selection, rank selection, steady state
selection and many others. Every method has some merits as
well as some limitations. In this thesis, Roulette wheel
selection is used to select the chromosomes. Lastly, elitism is
used to copy the best chromosome (or a few best
chromosomes) to new population. Elitism helps in increasing
the performance of GA, because it prevents losing the best
found solution.

(5)
Where i = 1, 2... N, j = 1, 2... D and rand is a random number
with a uniform probability distribution.
b)

Mutation

For each objective individual , i = 1, 2. . . N, thestandard
DE algorithm generates a corresponding mutatedindividual,
which is expressed:
(6)
Where the individual serial numbers ,
, and
are
different and randomly generated. None of the numbers is
identical to the objective individual serial number i.
Therefore, the population size NP4. The scaling factor F,
which controls the mutation degree, is within the range of
[0,2], as mentioned [11].
c)

b)

Crossover

Crossover selects genes from parent chromosomes and
creates a new offspring. The simplest way to do this is to
choose randomly some crossover point and interchange the
value before and after that point.

Crossover
c)

The crossover operation method, which is shown in Equation
(7), generates an experimental individual as follows:

Mutation

Mutation takes place after crossover. Mutation changes
randomly the new offspring. For binary encoding, we can
switch a few randomly chosen bits from 1 to 0 or 0 to 1.
Mutation ensures genetic diversity within population. This
entire process is continued until the convergence criterion is
satisfied.

(7)
Where r(j) is a randomly generated number in the uniform
distribution [0, 1], and j denotes the j th gene of an individual.
The crossover rate CR is within the range of [0, 1], which has
to be determined by the user. The randomly generated number
rn(i) e [1, 2, . . .,D] is the gene index. This index is applied to
ensure that at least one dimension of the experimental
individual is from the mutated individual. Equation (7) shows
that the smaller the CR is, the better the global search effect.
d)

Selection

VI. ADAPTIVE DIFFERENTIAL EVOLUTION
The mutation factor F determines the scaling ratio of the
differential vector. If F is too big, then the efficiency of the
DE will be low; that is, the global optimal solution acquired
by the DE exhibits low accuracy. By contrast, if F is too small,
then the diversity of the population will not be ensured as the
algorithm will mature early. Consequently, we propose the
adaptive mutation factor shown in Equation (9). F changes as
the algorithm iterates. It is large during the initial stage, which
can guarantee the diversity of the population. During the later
stage of the algorithm, the smaller mutation factor can retain
the excellent individuals.

Selection

A greedy search strategy is adopted by the DE. Each objective
individual
has to compete with its corresponding
experimental individual
, which is generated after the
mutation and crossover operations. When the fitness value of
the experimental individual
is better than that of the
objective individual
,
will be chosen as the

(9)

21

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