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How to customize SVM kernel parameters in Matlab

Asked by Ikra89

Ikra89 (view profile)

on 9 Jun 2013


I want to ask about how to customize SVM kernel parameters in Matlab.

For ex: I have linear kernel, polynomial kernel, and RBF kernel with:

C = 0.2;
gamma = 0.8;
r = 0.05;
d = 3;

My question is how do I customize it using svmstruct.

1. linear: ??

2. polynomial: ??

3. RBF:

svmstruct = svmtrain(data, groups, 'Kernel_Function', 'rbf', 'RBF_Sigma', 0.2, 'BoxConstraint', 0.8);

CMIIW about how to customize RBF kernel.

I'd really appreciate if anybody could help my research. Thanks.





Ikra89 (view profile)


2 Answers

Answer by Ikra89

Ikra89 (view profile)

on 10 Jun 2013

anybody can help? :)



Ikra89 (view profile)

Answer by Ahmed

Ahmed (view profile)

on 10 Jun 2013

Documentation given by

   help svmtrain

should give you all information you need. What is the meaning of your variable r?

1. the linear kernel has only parameter C

   svmstruct = svmtrain(data, groups, 'Kernel_Function', 'linear', 'BoxConstraint', 0.2);

2. polynomial has parameters C and polyorder

   svmstruct = svmtrain(data, groups, 'Kernel_Function', 'polynomial', 'polyorder',3,'BoxConstraint', 0.2);

3. RBF

you might have mixed up the constants C and gamma in your code



Ahmed (view profile)

on 10 Jun 2013

It looks like Matlab's svmtrain function only supports homogeneous polynomial kernels. You can however, define an arbitrary kernel function and pass a handle of it to svmtrain. See section kernel_function -> @kfun in the documentation for description and example.


Ikra89 (view profile)

on 12 Jun 2013

can you give me an example how to use @kfun

i have tried many times but still error


Ahmed (view profile)

on 6 Jul 2013

Using an anonymous function of a linear kernel would be:

svmstruct = svmtrain(data, groups, ...
'Kernel_Function',@(x1,x2) x1*x2','BoxConstraint', 0.2);

Note, that your kernel function must lead to a valid kernel.


Ahmed (view profile)

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