HW2 5.3 plot decision boundary

[fred king]

Hi all,

Does anyone can tell how to draw decision boundary? it seems the meshgrid and contourf can only deal with regularly mesh . Many thanks.

[Krikamol Muandet]

Hi,

I assume you are using Matlab. In that case, you can try the ezplot function. Here is an example.

svm_fun = @(x,y) w'*[x; y]+b;

ezplot(svm_fun);

where w and b is the parameter of SVM.

Krik

On 19 February 2013 18:39, fred king <jinyi...@gmail.com> wrote:

Hi all,

Does anyone can tell how to draw decision boundary? it seems the meshgrid and contourf can only deal with regularly mesh . Many thanks.

--
http://alex.smola.org/teaching/cmu2013-10-701 (course website)
http://www.youtube.com/playlist?list=PLZSO_6-bSqHQmMKwWVvYwKreGu4b4kMU9 (YouTube playlist)
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--

Krikamol Muandet
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[fred king]

Hi Krik,

It seems work, but sine w and b is not output, do you mean we should plot them inside the svm_primal_classify? Another question is that in dual problem, i.e. RBF, I can not obtain w in this case, I use \sum \alpha_i y_i k(x_i,x) + b  , then how do I plot using ezplot?

在 2013年2月19日星期二UTC-5下午12时46分36秒，Krik写道：

Hi,

I assume you are using Matlab. In that case, you can try the ezplot function. Here is an example.

svm_fun = @(x,y) w'*[x; y]+b;

ezplot(svm_fun);

where w and b is the parameter of SVM.

Krik

On 19 February 2013 18:39, fred king <jinyi...@gmail.com> wrote:

Hi all,

Does anyone can tell how to draw decision boundary? it seems the meshgrid and contourf can only deal with regularly mesh . Many thanks.

--
http://alex.smola.org/teaching/cmu2013-10-701 (course website)
http://www.youtube.com/playlist?list=PLZSO_6-bSqHQmMKwWVvYwKreGu4b4kMU9 (YouTube playlist)
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[Krikamol Muandet]

Hi Fred,

You can use the property that w can be written as a linear combination of feature maps. That is,

svm_fun = @(x,y) \sum \alpha_i y_i k(x_i,x) + b;

if you don't know w and b, but be able to compute the classification, i.e., either +1 or -1, then you may use this value directly.

Krik

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