How can I get decision or w and b，i am a linear kernel ，with C++ Code about libsvm HOT 1 CLOSED

Please see the following faq by using the matlab interface.
Alternatively you can consider liblinear, which directly gives you w.

Q: How could I generate the primal variable w of linear SVM?

Let's start from the binary class and assume you have two labels -1 and
+1. After obtaining the model from calling svmtrain, do the following to
have w and b:

w = model.SVs' * model.sv_coef;
b = -model.rho;

if model.Label(1) == -1
w = -w;
b = -b;
end

If you do regression or one-class SVM, then the if statement is not
needed.

For multi-class SVM, we illustrate the setting in the following example
of running the iris data, which have 3 classes

[y, x] = libsvmread('../../htdocs/libsvmtools/datasets/multiclass/iris.scale');
m = svmtrain(y, x, '-t 0')

m =

Parameters: [5x1 double]
  nr_class: 3
   totalSV: 42
       rho: [3x1 double]
     Label: [3x1 double]
     ProbA: []
     ProbB: []
       nSV: [3x1 double]
   sv_coef: [42x2 double]
       SVs: [42x4 double]

sv_coef is like:

+-+-+--------------------+
|1|1| |
|v|v| SVs from class 1 |
|2|3| |
+-+-+--------------------+
|1|2| |
|v|v| SVs from class 2 |
|2|3| |
+-+-+--------------------+
|1|2| |
|v|v| SVs from class 3 |
|3|3| |
+-+-+--------------------+

so we need to see nSV of each classes.

m.nSV

ans =

 3
21
18

Suppose the goal is to find the vector w of classes 1 vs 3. Then y_i
alpha_i of training 1 vs 3 are

coef = [m.sv_coef(1:3,2); m.sv_coef(25:42,1)];

and SVs are:

SVs = [m.SVs(1:3,:); m.SVs(25:42,:)];

Hence, w is

w = SVs'*coef;

For rho,

m.rho

ans =

1.1465
0.3682

-1.9969

b = -m.rho(2);

because rho is arranged by 1vs2 1vs3 2vs3.

from libsvm.