faq.html

支撑向量机库文件

If that still doesn't work, you may want to contact us. We can show you some
tricks on improving the training time.

<p><li> How do I get the decision value(s) ?
<p>
We print out decision values for regression. For classification,
we solve several binary SVMs for multi-class cases, so 
you obtain values by easily calling the subroutine
svm_predict_values. Their corresponding labels
can be obtained from svm_get_labels. 
Details are in 
README of libsvm package. 

<p>
We do not recommend the following. But if you would
like to get values for 
TWO-class classification with labels +1 and -1
(note: +1 and -1 but not things like 5 and 10)
in the easiest way, simply add 
<pre>
		printf("%f\n", dec_values[0]*model->label[0]);
</pre>
after the line
<pre>
		svm_predict_values(model, x, dec_values);
</pre>
of the file svm.cpp.
Positive (negative)
decision values correspond to data predicted as +1 (-1).

<!--
<p><li>For some problem sets if I use a large cache 
(i.e. large -m) on a linux machine, why sometimes
I get "segmentation fault ?"

<p>
This may happen only  when the cache is large, but each cached row is
not large enough. <b>Note:</b> This problem is specific to 
gnu C library which is used in linux.
The solution is as follows:

<p>
In our program we have malloc() which uses two methods 
to allocate memory from kernel. One is
sbrk() and another is mmap(). sbrk is faster, but mmap 
has a larger address
space. So malloc uses mmap only if the wanted memory size is larger
than some threshold (default 128k).
In the case where each row is not large enough (#elements < 128k/sizeof(float)) but we need a large cache ,
the address space for sbrk can be exhausted. The solution is to
lower the threshold to force malloc to use mmap
and increase the maximum number of chunks to allocate
with mmap.

<p>
Therefore, in the main program (i.e. svm-train.c) you want
to have
<pre>
      #include &lt;malloc.h&gt;
</pre>
and then in main():
<pre>
      mallopt(M_MMAP_THRESHOLD, 32768);
      mallopt(M_MMAP_MAX,1000000);
</pre>
You can also set the environment variables instead
of writing them in the program:
<pre>
$ M_MMAP_MAX=1000000 M_MMAP_THRESHOLD=32768 ./svm-train .....
</pre>
More information can be found by 
<pre>
$ info libc "Malloc Tunable Parameters"
</pre>
-->

<p><li>How do I disable screen output of svm-train and svm-predict ?
<p>
Simply update svm.cpp:
<pre>
#if 1
void info(char *fmt,...)
</pre>
to
<pre>
#if 0
void info(char *fmt,...)
</pre>

<p><li>I would like to use my own kernel but find out that
there are two subroutines for kernel 
evaluations: k_function() and kernel_function(). Which one
should I modify ?
<p>
The reason why we have two functions is as follows:
For the RBF kernel exp(-g |xi - xj|^2), if we calculate
xi - xj first and then the norm square, there are 3n operations.
Thus we consider exp(-g (|xi|^2 - 2dot(xi,xj) +|xj|^2))
and by calculating all |xi|^2 in the beginning, 
the number of operations is reduced to 2n.
This is for the training.  For prediction we cannot
do this so a regular subroutine using that 3n operations is
needed.

The easiest way to have your own kernel is
to  put the same code in these two
subroutines by replacing any kernel.

<p><li>What method does libsvm use for multi-class SVM ?
<p>
It is one-against-one. We chose it after doing the following
comparison:
C.-W. Hsu and C.-J. Lin.
<A HREF="http://www.csie.ntu.edu.tw/~cjlin/papers/multisvm.ps.gz">
A comparison of methods 
for multi-class support vector machines
</A>, 
<I>IEEE Transactions on Neural Networks</A></I>, 13(2002), 415-425.


<p><li> After doing cross validation, why there is no
model file outputted ?
<p>
Cross validation is used for selecting good parameters.
After finding them, you want to re-train the whole
data without the -v option.

<p><li> I would like to try different random partition
for cross validation, how could I do it ?
<p>
Right now we use the default seed so each time when you
run svm-train -v, folds of validation data are the same.
To have different seeds, you can add the following code
in svm-train.c:
<pre>
#include &lt;time.h&gt;
</pre>
and in the beginning of the subroutine do_cross_validation(),
<pre>
srand(time(0));
</pre>

<p><li> I would like to solve L2-SVM (i.e., error term is 
quadratic). How should I modify the code ?
<p>
It is extremely easy. Taking c-svc for example, only two 
places of svm.cpp have to be changed. 
First, modify the following line of 
solve_c_svc from 
<pre>
	s.Solve(l, SVC_Q(*prob,*param,y), minus_ones, y,
		alpha, Cp, Cn, param->eps, si, param->shrinking, param->cal_partial, param->gamma);
</pre>
to
<pre>
	s.Solve(l, SVC_Q(*prob,*param,y), minus_ones, y,
		alpha, INF, INF, param->eps, si, param->shrinking, param->cal_partial, param->gamma);
</pre>
Second, in  the class  of SVC_Q, declare C as 
a private variable:
<pre>
	double C;
</pre> 
In the constructor we assign it to param.C:
<pre>
        this->C = param.C;		
</pre>
Than in the the subroutine get_Q, after the for loop, add
<pre>
        if(i >= start && i < len) 
		data[i] += 1/C;
</pre>

<p><li> 
There seems to be a zero division ?
<p>
We intend
to have this zero division. 
Under the IEEE floating point standard, zero
division will returns infinity. Then with the 
operations later to bound it, things go back to normal numbers without any problem.

In general no warning messages happen. On some computers, you may
need to add an option (e.g. -mieee on alpha).

Reasons of doing so are described in libsvm document.

<p><li> 
How do I choose parameters for one-class svm as training data are in only one class ?
<p>
You have pre-specified true positive rate in mind and then search for
parameters which achieve similar cross-validation accuracy.

</ul>
<hr>
<h3>Graphic Interface</h3>
<ul>
<p><li>How can I save images drawn by svm-toy? <p>
For Microsoft windows, first press the "print screen" key on the keyboard.
Open "Microsoft Paint" 
(included in Windows) 
and press "ctrl-v." Then you can clip
the part of picture which you want.
For X windows, you can 
use the program "xv" to grab the picture of the svm-toy window.

<p><li>I press the "load" button to load data points but why
svm-toy does not draw them ?
<p>
The program svm-toy assumes both attributes (i.e. x-axis and y-axis
values) are in (0,1). Hence you want to scale your 
data to between a small positive number and 
a number less than but very close to 1.
<p><li>
I would like svm-toy to handle more than three classes of
data, what should I do ?
<p>
Taking windows/svm-toy.cpp as an example, you need to
modify it and  the difference
from the original file is as the following: (for five classes of
data)
<pre>
30,32c30
< 	RGB(200,0,200),
< 	RGB(0,160,0),
< 	RGB(160,0,0)
---
> 	RGB(200,0,200)
39c37
< HBRUSH brush1, brush2, brush3, brush4, brush5;
---
> HBRUSH brush1, brush2, brush3;
113,114d110
< 	brush4 = CreateSolidBrush(colors[7]);
< 	brush5 = CreateSolidBrush(colors[8]);
155,157c151
< 	else if(v==3) return brush3;
< 	else if(v==4) return brush4;
< 	else return brush5;
---
> 	else return brush3;
325d318
< 	  int colornum = 5;
327c320
< 		svm_node *x_space = new svm_node[colornum * prob.l];
---
> 		svm_node *x_space = new svm_node[3 * prob.l];
333,338c326,331
< 			x_space[colornum * i].index = 1;
< 			x_space[colornum * i].value = q->x;
< 			x_space[colornum * i + 1].index = 2;
< 			x_space[colornum * i + 1].value = q->y;
< 			x_space[colornum * i + 2].index = -1;
< 			prob.x[i] = &x_space[colornum * i];
---
> 			x_space[3 * i].index = 1;
> 			x_space[3 * i].value = q->x;
> 			x_space[3 * i + 1].index = 2;
> 			x_space[3 * i + 1].value = q->y;
> 			x_space[3 * i + 2].index = -1;
> 			prob.x[i] = &x_space[3 * i];
397c390
< 				if(current_value > 5) current_value = 1;
---
> 				if(current_value > 3) current_value = 1;
</pre>
</ul>
<hr>
<h3>Java version of libsvm</h3>
<ul>
<p><li>What is the difference between Java version and C++
version of libsvm?
<p>
They are the same thing. We just rewrote the C++ code
in Java.
<p><li>Is the Java version significantly slower than the
C++ version?
<p>
This depends on the VM you used. We have seen good
VM which leads the Java version to be quite competitive with
the C++ code. (though still slower)
<p><li> While training I get the following error 
message: java.lang.OutOfMemoryError. What is wrong?
<p>
You should try to increase the maximum Java heap size.
For example,
<pre>
java -Xmx256m svm_train.java ...
</pre>
sets the maximum heap size to 256M.
<p><li> Why you have the main source file svm.m4 
and then transform it to svm.java?
<p>
Unlike C, Java does not have a preprocessor built-in.
However,  we need some macros (see first 3 lines of svm.m4).

</ul>
<hr>
<h3>Python Interface</h3>
<ul>
<!--
<p><li> I include libsvm-python interface in my scripts but
get "segmentation fault" ?
<p>
There are two bugs in svm.py (version 2.35). 
Please manually correct them before we release the new version.
First, please add 
<pre>
            self.prob = prob
</pre>
after the line
<pre>
            prob,param = arg1,arg2
</pre>
in svm.py

<p>
Second, please 
move the first line 
of svm_parameter.__del__ to the last, so
the function becomes:
<pre>	
	def __del__(self):
                _free_int_array(self.weight_label)
                _free_double_array(self.weight)
                svmc.delete_svm_parameter(self.param)
</pre>
-->
<p><li>Using python on MS windows, it fails to load the dll file.
<p>
It seems the dll file is version dependent. So far we haven't
found out a good solution. Please email us if you have any
good suggestions.

<p><li> How to modify the python interface on MS windows
and rebuild the dll file ?
<p>

To modify the interface, follow the instructions given in
<a href=http://www.swig.org/Doc1.1/HTML/Python.html#n2>
http://www.swig.org/Doc1.1/HTML/Python.html#n2</a>

<p>

If you just want to build DLL for a different python version,
after libsvm 2.5, you can easily use the file Makefile.win.
See details in README.

Alternatively, you can use Visual C++:
<ol>
<li> Create a Win32 DLL project and set (Build->Active Configuration) to "Release."
<li> Add svm.cpp, svmc_wrap.c, python2x.lib to your project.
<li> Add the directories containing Python.h and svm.h to
the Additional include directories. (in Project
Settings->C/C++->Preprocessor)
<li> add __WIN32__ to Preprocessor definitions
<li> Make sure that in the "General" category of
Project->Settings->Settings For "Win32 Release", Output
directories should be "Release"
<li> Build the DLL.
</ol>


<!--
There do exist work arounds, however. In
http://aspn.activestate.com/ASPN/Mail/Message/python-list/983252
they presented a solution: 1) find the version of python in the
registry 2) perform LoadLibrary("pythonxx.dll") and 3) resolve the
reference to functions through GetProcAddress. It is said that SWIG
may help on this.
http://mailman.cs.uchicago.edu/pipermail/swig/2001-July/002744.html
presented a similar example.
-->
<p><li>Except the python-C++ interface provided, could I use Jython to call libsvm ?
<p> Yes, an example is <a href=http://www.csie.ntu.edu.tw/~cjlin/libsvm/jython>here</a>
</ul>
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