support for bias different from 1

[daslu]

The liblinear bias parameter is defined as follows (README.md at commit 7b6c6a56c1 of liblinear java):
https://github.com/bwaldvogel/liblinear-java/blob/master/README.md
"if bias >= 0, instance x becomes [x; bias]; if < 0, no bias term added (default -1)".

clj-liblinear does support the use of bias (thanks to #4 by @akhudek).

Currently in clj-liblinear, the only supported bias values are 0 and 1:
https://github.com/lynaghk/clj-liblinear/blob/master/src/clj_liblinear/core.clj#L54
Also, the bias=0 case results in not adding the extra featue:
https://github.com/lynaghk/clj-liblinear/blob/master/src/clj_liblinear/core.clj#L64

I suggest to the following changes:
(1) Support all bias values.
(2) Let the bias=0 case behave like documented in liblinear-java: adding an extra feature of value 0.

Reasons for the change:

• It seems reasonable for the behaviour of training to match the documented behaviour of liblinear-java.
• At least (1) is actually useful: tuning the value of the constant feature makes it possible to control the importance of regularizing the intercept vs. regularizing the other coefficients.
• Maybe (2) is useful too?

If desired, I can suggest a pull request with these changes.
What do you think?

[lynaghk]

Agreed, this seems like a very reasonable addition.
A pull request would be awesome.
My only request is that you handle the case of true/false inputs by converting to 1 or 0, respectively to maintain backwards compatibility for existing callers.

[daslu]

Hi,

I have seen the deprecation notice, so I guess the pull request is not relevant here anymore.
I am fixing this at my fork.

Let us keep in touch in case you are interested in it again, one day.

Again, many thanks for this very nice project.

[lynaghk]

Will do, I'll contact you about a redesign if that is ever in the cards.

[akhudek]

Hey daslu, (2) would be the same thing as simply leaving the bias feature out since w*0=0.

[daslu]

Thanks,
yes, this seems to be true for all the various objective functions supported by liblinear-java.
I have no idea why liblinear-java supports adding a zero column.