# ARDE-SPX: Adaptive Repository with Fitness Based Selection

In document THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR (Page 118-125)

## CHAPTER 4: DIFFERENTIAL EVOLUTION WITH ADAPTIVE

### 4.3.3 ARDE-SPX: Adaptive Repository with Fitness Based Selection

Algorithm 4.4: Update the value of πΆππ scheme

01: Input: πΆππ (mean value of previous successful πΆπ), ππΆπ(set of successful crossover rates), π = 1.5 02: Output: πΆπΜπ (updated mean value of πΆπ)

03: BEGIN

04: Step 1: πππππππ€ = β πππ€ππ((πππ€ππ(πΆπ|π π,π)

πΆπ| ),1/π)

|ππΆπ|

π=1 where πΆππβ ππΆπ

05: Step 2: π€πΆπ= 0.9 + 0.1 Γ ππππ(0,1)

06: Step 3: πΆπΜ = π€π πΆπΓ πΆππ+ (1 β π€πΆπ) Γ πππππππ€ 07: END

Algorithm 4.5: Generate πΉ value scheme

01: Input: π  (the cellβs index in the repository), πΉπ (mean value of πΉ) 02: Output: πΉ (new πΉ value)

03: BEGIN

04: IF (βπΉπβ in πΆππππ ) THEN πΉ = πππππ(πΉπ, 0.1) ELSE πΉ = πππππ(πΉπ, 0.1) 05: END

Algorithm 4.6: Generate πΆπ value scheme

01: Input: π  (the cellβs index in the repository), πΆππ (mean value of πΆπ) 02: Output: πΆπ (new πΆπ value)

03: BEGIN

04: IF (βπΆππβ in πΆππππ ) THEN πΆπ = πππππ(πΆππ, 0.1) ELSE πΆπ = πππππ(πΆππ, 0.1) 05: END

restriction on negative fitness function. Finally, it is a fast technique because it does not require any further calculations for the average of fitness or any other population statistics, all the solutions are sent to the same processor using only their relative fitness values (Elsayed, Sarker, & Essam, 2014).

The basic idea of FTS is to stochastically select individuals from the current generation to create the basis of the next generation. The selection process has replicated nature in that the individuals with better fitness values will tend to have better chance to survive to the next generation; whereas the individuals with weaker fitness values will have less probability to survive. The general idea of the FTS can be encapsulated as follows:

ο· Evaluate each individual using the fitness function to get the fitness values ππ.

ο· Set a value to the selection probability ππ β [0,1]. In practice, it has been found that it is always better to set the value of ππ to be greater than 0.5 in order to favor the fittest individuals and increase the selection pressure. The tournament selection with ππ = 1.0 is called deterministic tournament and the tournament selection with ππ < 1.0 is called stochastic tournament.

ο· Determine the tournament size ππ  β [2, ππ], because at least two individuals should be involved in the competition. In practice, the larger the tournament size, the higher probability that the new population will contain individuals with fitness values above average. The lower tournament size will produce population consists of individuals with low fitness values.

ο· Generate randomly uniformly distributed number π β [0,1] . Then if π β€ ππ, the fitter candidate individual among the ππ  individuals is selected; otherwise the weaker individual is chosen.

The last two steps are repeated until the desired number of survival individuals is obtained. Table 4.1 illustrates step-by-step on how to get the number of copies for 5

individuals using the simple fitness function, f(x)= π₯2. The problem is minimization of the fitness function. The values of ππ and ππ  are set to 0.7 and 2, respectively.

Table 4.1: The π₯2 example of fitness tournament selection detailed steps

No. Individual x f(x)= ππ Compete individuals r Winners

1 11001 25 625 2 1 0.46 2

2 01100 12 144 4 2 0.39 2

3 00111 7 49 1 3 0.78 1

4 11100 28 784 3 5 0.50 3

5 10001 17 289 5 4 0.14 5

From Table 4.1, it can be noted that individual 2 has two copies to be included in the next generation because of its low fitness value compared to other individuals; whereas individual 4 fails to have even one copy. However, individual 3, even with its low fitness value, could obtain only one copy to be included in the next generation. For this reason, it is preferable to use either deterministic tournament or at the very least set the selection pressure to high probability values. The standard procedure of the FTS algorithm is shown in Algorithm 4.7. In this algorithm, the function bestind returns the best individual from a set of solutions, and worstind returns the worst individual from a set of solutions.

Algorithm 4.7: The standard FTS algorithm

01: Input: Np (population size), π (solution vectors), ππ (selection probability), and ππ  (tournament size) 02: Output: M (mating pool)

03: Begin

04: Step 1: Evaluate each individual using fitness function, ππ= π(ππ), (π = 1,2 β¦ , ππ) 05: Step 2: WHILE (π < ππ) DO

06: Step 2.1: Select ππ  individuals randomly from the current population 07: Step 2.2: Generate uniformly random number,

π = ππππ(0,1) 08: Step 2.3: Selection of the survival individual

if ( π β€ ππ) then ππ= πππ π‘πππ (π1, π2, β¦, πππ )

else ππ= π€πππ π‘πππ (π1, π2, β¦, πππ ) 09: Step 2.4: π = π + 1

10: END

The FTS method has been used as the base technique for the new adaptive procedure of the ARDE algorithm. This new adaptive procedure is implemented in conjunction with

the selection step of DE, as follows:

Step 1 (Repository Construction): In the same selection step, a repository of empty cells is created. This repository is a result of uniting the two initial repositories of the DE strategies and the parameter control schemes into one repository called adaptive repository (π΄π). The address of each cell in this repository is one combination of the DE strategies and the parameter control schemes (DE strategy, πΉ adaptation scheme and/or πΆπ adaptation scheme) that are involved in the ARDE algorithm. So, the total number of the cells is 20 because there are six DE strategies, two πΉ adaptation schemes, and two πΆπ adaptation schemes to be involved in constructing the π΄π; so the total number is (6 Γ 2 Γ 2) β 4 = 20 cells. This number is subtracted by 4 because the DE/current-to-pbest/1 with archive and DE/rand-to-pbest/1 with archive strategies do not require crossover strategy. The content of the cell is the fitness value(s) that has been obtained from applying its relative combination to the corresponding individual(s);

otherwise, it is an empty cell if its relative combination has not been attempted in the search process. Figure 4.2 depicts the strategies and schemes combinations repository in the ARDE.

Step 2 (Individuals Classification): In the selection step of ARDE, the target vectors π(π‘)βs and the trial vectors π(π‘)βs are first evaluated. Then a one-by-one competition is started to determine the vectors of the next generation π(π‘ + 1)βs. These new vectors are either successful individuals (i.e. the trial vectors that their fitness values are better than their corresponding target vectors after applying the DE strategy and parameter control schemes) or failure individuals (i.e. the target vectors that their fitness values are still better than their corresponding trial vectors even after applying the DE strategy and the parameter control schemes).

Step 3 (Averaging Cells): In this step, the cell that contains more than one fitness value is averaged using the equation,

πππππ = βππππ=1πππ,π

ππππ (4.5)

where ππ,π is the fitness values and ππππ is the total number of fitness values in cell π. So, the result is a repository with cells that are either empty from a value (non-attempted in the search process) or occupied with a value (attempted in the search process).

Step 4 (Cells Selection): In this step of ARDE, the π(π‘ + 1) individuals are generated by assigning DE strategies and parameter control schemes to the π(π‘ + 1)sβ successful and failure individuals using an adaptive technique as,

Step 4.1: For the successful individuals, the FTS mechanism is used to assign the DE strategies and parameter control schemes to them from the π΄π created in Step 1.

In this adaptive method, the same table of FTS is created with only one difference is that the column of individuals is replaced with a column that includes the attempted cells of the repository with their average fitness values, as illustrated in Table 4.2.

Then, for each successful individual, the FTS procedure determines which combination is to be assigned to it.

Table 4.2: The FTS method used to assign the DE strategies and parameter control schemes to the successful individuals

Successful Individuals

Repository of Strategies (AR)

Compete

Cells r Winner Cell

Cells Cellsβ

Avg.

Fitness Cell

No.

DE Strategies F Scheme

CR Scheme Individual

11

1 DE/current-to-pbest/1/bin

Fc CRc 9.703E+04 2 1 0.46 1

Individual 23

2 DE/current-to-pbest/1

Fc - 1.090E+05 4 2 0.39 4

Individual 15

3 DE/rand-to-pbest/1/bin

Fn CRn 8.386E+04 1 3 0.78 1

Individual 30

4 DE/current-to-pbest/1/exp

Fn CRc 6.236E+04 3 5 0.50 3

Individual 45

5 DE/rand-to-pbest/1/exp

Fc CRn 1.129E+05 5 4 0.14 4

In the same table, the problem is minimization and the values of ππ and ππ  are set to 0.7 and 2, respectively.

In the same step, two counters are created. The first counter is called πΆππ’ππ‘π  which records the number of times its corresponding cell has been selected during one generation. The second counter is called πΆππ’ππ‘π€ which records the number of times in which this cell has won the competition in FTS.

Step 4.2: For the failure individuals, a re-initialization mechanism is used to assign a combination of the strategies and schemes from the attempted and non-attempted cells of the π΄π with equal probability. In case there is not non-attempted cell, then the selection of combinations from the π΄π is applied among the attempted cells with equal probability.

Step 5 (Repository Cleaning, Rewarding, and Penalizing Processes):

Step 5.1 (Cleaning Process): In this step a cleaning mechanism is applied to discriminate the individuals on the basis of the age. In this mechanism the concept of

"history" or βfirst-in-first-out (FIFO)β technique is used to eliminate the oldest fitness values in the cells. For each fitness value stored in a cell, its generation number (i.e., its history of birth) is stored too. Then cleaning is applied to those old fitness values existing in the π΄πβs cells. A fitness value in a cell of the π΄π can be tagged as "old" if its history is earlier than the current generation number by 10 generations. For example, if the current number of generations is 11, then the clean operation is applied to all those fitness values with history equals to 1. And at generation 12, the clean operation removes all fitness values with history 2, and so on. Accordingly, the cell that contains one fitness value with βoldβ tag is turned to be empty cell (non-attempted).

Figure 4.2: The complete structure of the adaptive repository, π΄π in the ARDE algorithm

Repository of Parameters Control Schemes and DE Strategies

Cell 1

Cell 2

Cell 3

Cell 4

Cell 5

Cell 6

Cell 7

Cell 8

Cell 9

Cell 10

Cell 11

Cell 12

Cell 13

Cell 14

Cell 15

Cell 16

Cell 17

Cell 18

Cell 19

Cell 20 DE/rand-to-pBest/1/bin

Fn; CRc

DE/rand-to-pBest/1/bin Fn; CRn

DE/current-to-pBest/1/exp Fc; CRc

DE/current-to-pBest/1/exp Fc; CRn

DE/current-to-pBest/1/exp Fn; CRc

DE/current-to-pBest/1/exp Fn; CRn

DE/current-to-pBest/1/bin Fc; CRc

DE/current-to-pBest/1/bin Fc; CRn

DE/current-to-pBest/1/bin Fn; CRc

DE/current-to-pBest/1/bin Fn; CRn

DE/current-to-pBest/1 Fc

DE/rand-to-pBest/1 Fn

DE/current-to-pBest/1 Fc

DE/current-to-pBest/1 Fn

DE/rand-to-pBest/1/exp Fc; CRc

DE/rand-to-pBest/1/exp Fc; CRn

DE/rand-to-pBest/1/exp Fn; CRc

DE/rand-to-pBest/1/exp Fn; CRn

DE/rand-to-pBest/1/bin Fc; CRc

DE/rand-to-pBest/1/bin Fc; CRn

Empty Cell

(Non-attempted Cell) Occupied Cell

(attempted Cell)

Cell Content

Ft=1

Ft=1 Ft=2 Ft=3

Ft=4 Fitness

Value Fitness Age

Step 5.2 (Rewarding and Penalizing Processes): At each generation, the superior cell (cell with the highest π) is rewarded by eliminating its worst fitness value. The inferior cell (cell with the lowest π) is penalized by eliminating its best fitness value;

where π is the cellβs winning probability for each attempted-cell selected in the FTS mechanism and calculated as,

πΆππππ . π = πΆππππ  . πΆππ’ππ‘π€ πΆππππ  . πΆππ’ππ‘π

(4.6)

where π  is the index of the attempted-cell that participated in the FTS mechanism during the current generation.

These steps of ARDE evolution are repeated till the stopping criteria are met. In Figure 4.2 and Table 4.2, the letters π and π attached with πΉ and πΆπ refer to the parameters adaptation schemes of Normal distribution and Cauchy distribution, respectively.

In document THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR (Page 118-125)