How to do multi class classification using Support Vector Machines (SVM)

Question

In every book and example always they show only binary classification (two classes) and new vector can belong to any one class.

Here the problem is I have 4 classes(c1,

Answer 1

LibSVM uses the one-against-one approach for multi-class learning problems. From the FAQ:

Q: What method does libsvm use for multi-class SVM ? Why don't you use the "1-against-the rest" method ?

It is one-against-one. We chose it after doing the following comparison: C.-W. Hsu and C.-J. Lin. A comparison of methods for multi-class support vector machines, IEEE Transactions on Neural Networks, 13(2002), 415-425.

"1-against-the rest" is a good method whose performance is comparable to "1-against-1." We do the latter simply because its training time is shorter.