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Dynamic artificial bee colony algorithm for multiparameters optimization of support vector machinebased softmargin classifier
EURASIP Journal on Advances in Signal Processing volume 2012, Article number: 160 (2012)
Abstract
This article proposes a ‘dynamic’ artificial bee colony (DABC) algorithm for solving optimizing problems. It overcomes the poor performance of artificial bee colony (ABC) algorithm, when applied to multiparameters optimization. A dynamic ‘activity’ factor is introduced to DABC algorithm to speed up convergence and improve the quality of solution. This DABC algorithm is employed for multiparameters optimization of support vector machine (SVM)based softmargin classifier. Parameter optimization is significant to improve classification performance of SVMbased classifier. Classification accuracy is defined as the objection function, and the many parameters, including ‘kernel parameter’, ‘cost factor’, etc., form a solution vector to be optimized. Experiments demonstrate that DABC algorithm has better performance than traditional methods for this optimizing problem, and better parameters of SVM are obtained which lead to higher classification accuracy.
Introduction
Artificial bee colony (ABC) algorithm was first proposed by Karaboga in 2005 [1]. It has many advantages than earlier swarm intelligence algorithms, especially for constrained optimization problem.
A constrained optimization problem (1) is defined as finding solution \overrightarrow{\text{x}} that minimizes an objective function f\left(\overrightarrow{\text{x}}\right) subject to inequality and/or equality constraints [2]:
when D is larger and each element of \overrightarrow{\text{x}} represents a specific parameter, it is a multiparameters optimization problem.
Simulating the foraging behavior of honey bee swarm, ABC algorithm assumes solution \overrightarrow{\text{x}} as coordinate of nectar source in Ddimensional space, and defines objective function f\left(\overrightarrow{\text{x}}\right) which reflects quality of the nectar source. Small value of objective function indicates better nectar source. As bee swarm continually searching better nectar source, the algorithm could find the best solution \overrightarrow{\text{x}}.
However, ABC algorithm is criticized owing to its poor convergence rate and local optimization problems [3–6]. Many modified methods have been proposed. As the earlier idea of many researchers, poor performance is attributed to ‘roulette wheel’ selection mechanism, which is introduced in the onlooker phase of the original ABC algorithm. Boltzmann selection mechanism was employed instead of roulette wheel selection by Haijun and Qingxian [7]. Interactive ABC, proposed by Tsai et al. [8], introduced the Newtonian law of universal gravitation, which was also for modifying the original selection mechanism. Akbari et al. [9] proposed a modified formula for different phases of ABC algorithm. Actually, according to testing by abundant experiments, these modified methods could improve the original algorithm only when D is not too large. Nevertheless, our findings provide evidence that it is the ‘randomly single element modification (RSEM) process’, which principally leads ABC algorithm to poor performance. In traditional ABC algorithm, for each memorized solution \overrightarrow{\text{x}}, modifying operation is on single element x_{ k } (k∈[1,D) of \overrightarrow{\text{x}} in each cycle, and solution \overrightarrow{\text{x}} changes little after each modification. Moreover, element x_{ k } is randomly selected. It is uncertain whether the modification of x_{ k } could improve the solution \overrightarrow{\text{x}}, particularly when D is large. Consequently, more cycles are needed for searching best solution, and the algorithm performs poor efficiency relatively. Although Karaboga and Akay [2] introduced modification rate (MR) factor to randomly modify more elements of the solution vector in each cycle, robustness of the algorithm is not quite well. Furthermore, in ABC algorithm, optimization is hierarchical (from global to local), which is implemented mainly by operations of employed bees and onlooker bees, respectively. However, RSEM process is simultaneously utilized in these two phases, which could not effectively guarantee hierarchical optimization. Therefore, RSEM process is abandoned in our DABC algorithm. A dynamic ‘activity’ factor is introduced to modify appropriate number of elements of solution \overrightarrow{\text{x}} and achieve hierarchical optimization. In different optimizing stages, active degree of bees is properly set. More active bees modify more elements of \overrightarrow{\text{x}} . For bees with different division of labor, ‘activity’ factors are different set. Thus, hierarchical optimization is able to implement.
Based on structural risk minimization principle, support vector machine (SVM) was first proposed by Cortes and Vapnik [10] in the 1990s. It has many advantages on classification, but multiple parameters have to be properly selected. Many research studies have been carried out on this topic. For a specific set of training samples, once classification accuracy is employed as objective function \overrightarrow{\text{x}}, solution vector \overrightarrow{\text{x}} is formed by parameters of SVM, training of SVM classifier could be transformed into a multiparameters optimization problem. Traditionally, most methods for SVM parameter optimization are based on grid search algorithm and genetic algorithm (GA) [11]. The recent focus is swarm intelligence algorithmbased methods, such as ant colony algorithm, particle swarm optimization (PSO) algorithm [12]. ABC algorithm is introduced for SVM parameter optimizing by Hsieh and Yeh [13]. Since multiple parameters of SVMbased softmargin classifier need to be optimized, our DABC algorithm is highly suited for this purpose. Especially for multiclass classification problems, the length D of \overrightarrow{\text{x}} is larger, and parameters including ‘cost factor’ of each class and kernel parameter are need to be optimized. Performance of classifier is evaluated by average classification accuracy after kfold crossvalidation. Experiments demonstrate that comparing with earlier ABC algorithms, our method have great improvement on convergence rate, and better parameters are obtained which lead to higher classification accuracy.
The main contributions of this article are (1) a modified ABC algorithm is proposed, named DABC algorithm; (2) DABC algorithm is applied to multiparameters optimization of SVM softmargin classifier. The article is organized as follows. In the following section, we introduce traditional ABC algorithm and several modified process along with their drawbacks. Moreover, description of DABC algorithm is presented. Multiparameters optimization of SVM by DABC algorithm is illustrated in Section “Multiparameters optimization of SVMbased softmargin classifier”, and accordingly experimental settings and analysis are stated. Finally, the last section concludes this study.
Methodology
Traditional ABC algorithm
ABC algorithm is inspired by the foraging behavior of real bee colony. The objective of a bee colony is to maximize the nectar amount stored in the hive. The mission is implemented by all the members of the colony, by efficient division of labor and role transforming. Each bee performs one of following three kinds of roles: employed bees (EB), onlooker bees (OB), and scout bees (SB). They could transform from one role to another in different phases of foraging. The flow of nectar collection is as follow:

1.
In initial phase, there are only some SB and OB in the colony. SB are sent out to search for potential nectar source, and OB wait near the hive for being recruited. If any SB finds a nectar source, it will transform into EB.

2.
EB collect some nectar and go back to the hive, and then dance with different forms to share information of the source with OB. Diverse forms of dance represent different quality of nectar source.

3.
Each OB estimates quality of the nectar sources found by all EB, then follows one of EB to the corresponding source. All OB choose EB according to some probability. Better sources (more nectar) are more attractive (with larger probability to be selected) to OB.

4.
Once any sources are exhausted, the corresponding EB will abandon them, transform into SB and search for new source.
In this way, the bee colony assigns more members to collect the better source and few members to collect the ordinary ones. Thus, the nectar collection is more effective.
Analogously, in ABC algorithm, position of nectar source is presented by the coordinate in Ddimensional space. It is the solution vector \overrightarrow{\text{x}} of some special problem, and the quality of nectar source is presented by the objective function f\left(\overrightarrow{\text{x}}\right) of this problem. Accordingly, optimization of this problem is implemented by simulating behaviors of the three kinds of bees. The flowchart of original ABC algorithm is shown in Figure 1. The main steps are as follow.

1.
Parameters initialization of ABC algorithm. Population number (PN) and scout bee triggering threshold (Limit) are the key parameters of ABC algorithm. Maximum cycle number (MCN) or ideal fitness threshold (IFT) could be set for terminating algorithm. As stated in formula (1), all variables to be optimized form a Ddimensional vector \overrightarrow{\text{x}}. Restrict both upper bound (UB) and lower bound (LB) of each variable.

2.
Bee colony initialization. In ABC algorithm, since SB transform into EB, they are not reckoned in PN. Generally, the initial nectar sources are found by PN/2 SB, and then they all transform into EB. The other PN/2 bees are OB. The initial PN/2 solutions are generated by formula (2) in principle. Specified initial value could be used only if needed. All further modifications are based on these PN/2 solutions, which is corresponding to the PN/2 EB.
\begin{array}{l}{x}_{i}^{\left(j\right)}=L{B}^{\left(j\right)}+{\varphi}_{i}^{\left(j\right)}(U{B}^{\left(j\right)}L{B}^{\left(j\right)})\\ \text{where}\begin{array}{c}\hfill \begin{array}{cc}\hfill i=1,2,\dots ,PN/2\hfill & \hfill j=1,2,\dots ,D\hfill \end{array}\hfill \end{array}\end{array}(2)where {x}_{i}^{\left(j\right)} is the j th elements of the i th solution. {\varphi}_{i}^{\left(j\right)} is uniformly distributed random real number in the range of [0, 1]. Objective function f\left(\overrightarrow{\text{x}}\right) is introduced to estimate the fitness of each solution \overrightarrow{\text{x}}. For parameter optimization of SVM classifier, f\left(\overrightarrow{\text{x}}\right) could be minimum classification error or maximum classification accuracy. Vector Failure is a counter, length PN/2, and is set to zero for counting optimizing failure of each EB.

3.
Each cycle includes following phases:

1)
Employed bee: Each EB randomly modifies single element {x}_{i}^{\left(j\right)} of source i by formula (3). Then fitness of the two solutions (before and after modification) is estimated. Greedy selection criterion is introduced to choose the one with better fitness, and the reserved one becomes new solution of this EB. If fitness of EB is not improved after modification, corresponding Failure counters will increase by 1.
{\overline{x}}_{i}^{\left(j\right)}={x}_{i}^{\left(j\right)}+{\lambda}_{i}^{\left(j\right)}({x}_{i}^{\left(j\right)}{x}_{k}^{\left(j\right)})(3)where {x}_{i}^{\left(j\right)} is defined as in formula (2), and {\overline{x}}_{i}^{\left(j\right)} is the corresponding new element of the solution after modification. {\lambda}_{i}^{\left(j\right)} is uniformly distributed random real number in the range of [−1, 1], and {x}_{k}^{\left(j\right)} is the j th elements of {\overrightarrow{x}}_{k}. Note that k ≠ i.

2)
Estimate recruiting probability. By formula (4), fitness and recruiting probability of each EB are calculated.
\{\begin{array}{c}\hfill \text{Fitness(i)}=\frac{1}{1+f\left({\overrightarrow{\text{x}}}_{\text{i}}\right)}\hfill \\ \hfill \text{prob(i)}=\frac{\text{Fitness(i)}}{\sum _{i=1}^{\text{PN/}2}\text{Fitness(i)}}\hfill \end{array}(4) 
3)
Onlooker bee: ‘roulette wheel’ selection mechanism is introduced. It forces each OB following one of EB according recruiting probability. Owing to better solutions corresponding to larger recruiting probability, they obtain more chance to be optimized. Then each solution will be modified again by its followers (OB), using same steps as employed bee phase, from steps 1 to 6.

4)
Record best solution. All PN/2 solutions after modification are ranked according to their fitness, and best solution of current cycle is reserved. The termination conditions are then checked. When cycle counter reach the MCN or an ideal solution is found (reach IFT), the algorithm is over.

5)
Scout bee. If Failure counters of any solutions exceed Limit, the corresponding solution is abandoned, and scout bee is triggered. For example, if the l th solution is abandoned, a new solution is generated to replace the original one using formula (2), where set i = l.

1)
By above operations, ABC algorithm performs optimization. Nevertheless, in both EB and OB phases, the algorithm merely modify single element of the solution in each cycle. If the length of the solution vector D is large, it makes inefficiency improvement in each cycle. In [2], MR is proposed, which is a real number factor in [0, 1]. For element {x}_{i}^{\left(j\right)} of solution i, a uniformly distributed random real number (0\le {R}_{i}^{\left(j\right)}\le 1) is produced. If {R}_{i}^{\left(j\right)}\le MR, element {x}_{i}^{\left(j\right)} will be modified and others not. Moreover, if all {R}_{i}^{\left(j\right)} are larger than MR, ensure at least one parameter being modified by original algorithm. Although this MRABC algorithm improves the convergence rate of basic algorithm to some extent, its robustness is not ideal according to testing by abundant experiments.
DABC algorithm
The original idea of ABC algorithm is to perform hierarchical optimization. Overall, global searching is performed by EB and local searching is implemented by OB. However, this idea is not prominent in traditional ABC algorithms, because the modifying extent of EB and OB is similar and relatively fixed. Dynamic modifying extent is more reasonable. To achieve more effective optimization, the activity of bees must be dynamic in different stages of the algorithm. Our idea is that global searching should be dominant in early cycles and local searching should be primary in the posterior cycles. This could be more consistent with actions of real bees: EB become main force in the initial, then more and more OB follow, they play the major role afterwards. Specifically, in early stages of optimization, audaciously modify more elements of \overrightarrow{\text{x}} in EB phase. That makes the bees approaching better solution by a greater probability. Furthermore, OB become active in posterior stages, and they modify more elements of \overrightarrow{\text{x}}. That provides more opportunities to jump out of local optimal solution.
Consequently, we propose a dynamic ‘activity’ factor, and introduce it into modification operation of EB and OB phases. Adjust number of elements of the solution vector in each cycle. The ‘activity’ factor δ could be defined as following two forms by formulas (5) and (6), alternatively:
where C_{ c } is current cycle number, , D is length of solution, F_{ c } is current best fitness. The alternation of the two definitions depends on the termination condition of the algorithm. If using MCN to terminate the optimization, δ is defined as formula (5). And if IFT is employed, δ is defined as formula (6). δ_{EB} and δ_{OB} are ‘activity’ factor of EB and OB, respectively. Employ τ as the progress rate of the optimization, δ_{EB} and δ_{OB} subject to: (1) δ_{EB} grows with τ, when τ is not beyond half of total progress. {\delta}_{\mathit{EB}}\in \left[0,1\right]; (2) δ_{OB} grows with τ, when τ is beyond half of total progress.{\delta}_{\mathit{OB}}\in \left[0,1\right] . Explicit formulas could be determined according to specific problems. In this article, following scheme is suggested when utilize MCN as termination condition of the algorithm.

(1)
In early stages, for EB phase, δ_{EB} is defined as formula (7). It reduces with C_{ c } increasing, and N_{EB} elements are randomly picked to be modified; For OB phase, MR method is recommended. Audacious global modification and conservative local modification are implemented.
\begin{array}{c}\hfill EB\to {N}_{\mathit{EB}}=\left[{\delta}_{\mathit{EB}}\xb7D\right]=\left[(1\frac{{C}_{c}}{MCN})\xb7D\right]\hfill \\ \hfill OB\to \text{MR}\begin{array}{}\end{array}\hfill \end{array}\}if{C}_{c}\le \frac{MCN}{2}(7) 
(2)
In posterior stages, for EB phase, MR method is reused; For OB phase, δ_{OB} is defined as formula (8). It increases with C_{ c } growing, and N_{OB} elements are randomly picked to be modified. Conservative global modification and audacious local modification are implemented.
\begin{array}{c}\hfill EB\to \text{MR}\begin{array}{}\end{array}\hfill \\ \hfill OB\to {N}_{\mathit{OB}}=\left[{\delta}_{\mathit{OB}}\xb7D\right]=\left[\frac{{C}_{c}}{MCN}\xb7D\right]\hfill \end{array}\}\begin{array}{c}\hfill if\hfill \end{array}{C}_{c}>\frac{MCN}{2}(8)
Furthermore, DABC algorithm is closely to the length D of solution vector. When D is small, there is practically little difference between original ABC algorithm and DABC algorithm. And for larger D, the advantages of DABC algorithm are prominent on convergence rate and improving the quality of solutions.
Multiparameters optimization of SVMbased softmargin classifier
Introduction of SVM parameters optimization
As we all know, training softmargin classifier is a constrained optimization problem as formula (9). l is number of samples, x_{ i } is i th sample, and y_{ i } is the label of sample i.
It is a quadratic programming problem, which maximum the margin (2/\Vert w\Vert) when restricting the least classification error rate.
To solve unbalanced problem of training samples, ‘slack variable’ (ζ) and ‘cost’ factor (C) are introduced to process outlier samples and compromise the position of optimal separating hyperplane. Large C indicates attaching importance to the loss of outliers of different classes. SVM needs to assign different C for each class. If these cost factors are not properly set, poor classification result will be obtained. However, experiencebased setting is not robust. As a result, the multiparameters optimizing problem needs to be solved, and parameters to be optimized will increase with number of class.
Additionally, parameters of kernel function of SVM need to be optimized. Solving (9) with Lagrange multiplier method, the separating classification function could be obtained as formulas (10) and (11), where α_{ i } is Lagrange factor.
When samples are linearly inseparable, SVM processes nonlinear problem as linear classification in highdimensional, which is performed by kernel function as formula (12). Both number and type of parameters to be optimized are determined by the kernel function.
All above parameters to be optimized compose a vector \overrightarrow{\text{x}}, and the multiparameters optimization problem is defined as (13):
where C is the cost factor, γ is the kernel parameter, and n is number of labels {q}_{j}(j=1,2,\dots ,n), are weight parameters of each class, which set the cost factor C of class j to q, C. Moreover, to obtain creditable classification accuracy, ‘kfold’ crossvalidation is utilized for testing performance of SVM classifier. In our experiments, k is set to 10. Define the objective function f\left(\overrightarrow{x}\right) as the average classification accuracy of ‘10fold’ crossvalidation as formula (14):
Consequently, this problem could be solved by optimization algorithm. Owing to multiple parameters need to be optimized, our DABC algorithm is more suitable than traditional ABC algorithm. The flowchart of DABC algorithm based multiparameters optimization is shown in Figure 2.
For a set of training samples, DABC algorithm modifies parameters vector \overrightarrow{x}=\{C,\gamma ,{q}_{1},{q}_{2,}\dots ,{q}_{n}\} cyclebycycle, and search best \overrightarrow{x} for maximizing the classification accuracy.
Experiments
In this article, multiclass SVMbased softmargin classifier is performed by Csupport vector classification (CSVC) toolbox. It is from LIBSVM toolbox supplied by Cheng and Lin [14]. The toolbox supply several typical kernel functions. Radial basis function is employed as kernel function in our experiments, and kernel parameter γ need to be optimized. For nclass classification, all parameters to be optimized and their range are presented in Table 1. Obviously, the length of vector \overrightarrow{x} is D = n + 2. The dataset utilized for SVM training is as Table 2 shows. ‘Wine’ and ‘Image Segment’ are two typical testing dataset, which are widely used for testing SVMbased classifier. The two ‘building’ datasets are collected by us especially for multiparameters optimization problem.
Performances of PSO algorithm, original ABC algorithm, MRABC algorithm, and DABC algorithm are compared for this optimization problem. All algorithms are coded under MATLAB 2011b. Main hardware configuration of our computer: Intel®Core(TM)2 Duo CPU P8400@2.26 GHz 2.27 GHz, 2.00 GB RAM.
According to principle of fair comparison: (1) corresponding initialization parameters are same set in these algorithms as Table 3 shows,the settings are according to [2]; (2) using same starting searching points to initialize the colony, as shown in Table 4 and Appendix. Mean value of 20 times running by different algorithms are collected as the final results for the four datasets, which are shown in Figures 3, 4, 5, and 6 and Table 4. Particularly, to verify the robustness of different algorithm, standard deviations of the 20 times run are given by Figures 7 and 8, for the two highdimensional datasets.
Note that we choose measuring the convergence rate in cycle for following reasons: Generally, computational time of calling objection function is much larger than other parts of ABC algorithm, particularly when our objection function includes multiple times SVM training. The SVM training takes more than much time, and in each cycle, the code of objection function will be called many times (same times in each cycle for different algorithm, and times is determined by parameter PN). Objection function calling occupies more than 90% computational time of both original ABC algorithm and modified ones (for instance, for dataset 4, about 22.3 s are cost by running DABC algorithm for each cycle, 21.7 s for MRABC algorithm, and 20.5 s for original ABC algorithm. It is obviously that objection function cost most time in each cycle). Moreover, every time the objection function is called, the solution is modified. Therefore, it is more reasonable measuring the convergence rate in cycle than in time. On the contrary, if measuring the convergence rate in specified computational time, each solution might be modified different times, which could be unfair.
As is shown in Figure 3, for dataset 1, DABC algorithm rapidly find a solution \overrightarrow{\text{x}}, with which SVM could best training the data and obtain a classification accuracy of 98.88 %, while original ABC algorithm and MRABC obtain that solution slowly. Though PSO algorithm has a good convergence rate, it could not get an ideal solution. As is shown in Figure 4, for dataset 2, similarly, compared with original ABC algorithm and MRABC, DABC algorithm performs better convergence rate and obtains higher classification accuracy, and the improvement is more obvious than dataset 1. PSO algorithm still converges fast but an unsatisfactory solution. By contrast, DABC algorithm obtains a classification accuracy of 92.38 %.
The results above have demonstrated the advantages of DABC algorithm for lower dimension of parametervector like datasets 1 and 2. Furthermore, datasets 3 and 4 are collected for testing optimization of higher dimensional \overrightarrow{\text{x}}. As is shown in Figures 5, 6, and Table 5, similar conclusions could be obtained that DABC algorithm has certain advantages over other algorithms, which lead to greater improvement on convergence rate and quality of solution, especially when D is larger. Moreover, standard deviations (of 20 times run) for the two groups of datasets are shown in Figures 7 and 8, respectively. The curves illustrate the standard deviations of objection function in different optimizing cycles, and the relatively lower standard deviations have been obtained by DABC algorithm for both datasets 3 and 4, which indicates that DABC algorithm has good robustness.
Conclusion and discussion
In this article, two parts of work have been studied. First, DABC algorithm is introduced to improve the disadvantages of traditional ABC algorithms: poor convergence rate and local optimizing. Second, DABC algorithm is utilized for multiparameters optimization of SVM classifier. Experiments results demonstrate that DABC algorithm is in many ways superior to traditional ABC algorithms. It effectively ameliorates the convergence rate and local optimum. Typically, for multiparameters optimization, when length D of vector (to be optimized) is larger, our study has provided substantial evidence for the advantages of DABC algorithm on quality of solution and convergence rate. When DABC algorithm is employed for optimizing multiparameters of SVMbased softmargin classifier, great improvement is obtained on performance of the classifier. Moreover, the robustness of DABC algorithm is proofed. Furthermore, the idea of DABC algorithm could be associated with other modified ABC algorithms, whose modification is on other phase of original ABC algorithm, and it might further improve traditional ABC algorithm in future work.
Abbreviations
 ABC:

Artificial bee colony
 DABC:

Dynamic artificial bee colony
 EB:

Employed bees
 PSO:

Particle swarm optimization
 RSEM:

Randomly single element modification
 IFT:

Ideal fitness threshold
 LB:

Lower bound
 MCN:

Maximum cycle number
 MR:

Modification rate
 MRABC:

Modification rate artificial bee colony
 OB:

Onlooker bees
 PN:

Population number
 SB:

Scout bees
 SVM:

Support vector machine
 UB:

Upper bound.
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Yan, Y., Zhang, Y. & Gao, F. Dynamic artificial bee colony algorithm for multiparameters optimization of support vector machinebased softmargin classifier. EURASIP J. Adv. Signal Process. 2012, 160 (2012). https://doi.org/10.1186/168761802012160
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DOI: https://doi.org/10.1186/168761802012160