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   <title>Frequently Asked Questions</title>

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   <questions group="General">
      <question text="Why isn't serialization working?">
         answer goes here
      </question>

      <question text="How do I set the size of a matrix at runtime?">
         Long answer, read the <a href="matrix_ex.cpp.html">matrix example program</a>.
         <br/><br/>
         Short answer, here are some examples:
<code_box>
   matrix&lt;double&gt; mat;
   mat.set_size(4,5);

   matrix&lt;double,0,1&gt; column_vect;
   col_vect.set_size(6);

   matrix&lt;double,0,1&gt; column_vect2(6);

   matrix&lt;double,1&gt; row_vect;
   row_vect.set_size(5);
</code_box>

      </question>

      <question text="Where is the documentation for &lt;object/function&gt;?">
         If you can't find something then check the <a href="term_index.html">index</a>.
         <br/><br/>
         Also, the bulk of the documentation can be found by following the 
         <b><font style='font-size:1.3em' color='#007700'>Specification</font></b> links.   
      </question>

   </questions>

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   <questions group="Machine Learning">

      <question text="Why is RVM training is really slow?">
         answer
      </question>

      <question text="Why is cross_validate_trainer_threaded() crashing?">
      </question>

      <question text="How can I define a custom kernel?">
      </question>

      <question text="Can you give advice on feature generation/kernel selection?">
         <p>
            Picking the right kernel all comes down to understanding your data, and obviously this is 
            highly dependent on your problem. :)  
         </p>

         <p>
            One thing that's sometimes useful is to plot each feature against the target value.  You can get an idea of 
            what your overall feature space looks like and maybe tell if a linear kernel is the right solution.  But 
            this still hides important information from you.  For example, imagine you have two diagonal lines which 
            are very close together and are both the same length.  One line is of the +1 class and the other is the -1 
            class.  Each feature (the x or y coordinate values) by itself tells you almost nothing about which class 
            a point belongs to but together they tell you everything you need to know.  
         </p>

         <p>
            On the other hand, if you know something about the data you are working with then you can also try and 
            generate your own features.  So for example, if your data is a bunch of images and you know that one 
            of your classes contains a lot of lines then you can make a feature that attempts to measure the number 
            of lines in an image using a hough transform or sobel edge filter or whatever.  Generally, try and 
            think up features which should be highly correlated with your target value.  A good way to do this is 
            to try and actually hand code N solutions to the problem using whatever you know about your data or 
            domain.  If you do a good job then you will have N really great features and a linear or rbf kernel 
            will probably do very well when using them.
         </p>

         <p>
            Or you can just try a whole bunch of kernels, kernel parameters, and training algorithm options while 
            using cross validation.  I.e. when in doubt, use brute force :)   There is an example of that kind of 
            thing in the <a href="model_selection_ex.cpp.html">model selection</a> example program. 
         </p>
      </question>

      <question text="Why does my decision_function always give the same output?">
      </question>

      <question text="Which machine learning method should I use?">
      </question>
   </questions>

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</doc>
