to C guru.

i dont understand why this code fail
because of implicit declaration.
undefined reference to `__finite'

#define isfinite(x) \
     (sizeof (x) == sizeof (float)                              \
      ? __finitef (x)                                                 \
      : sizeof (x) == sizeof (double)                         \
      ? __finite (x) : __finitel (x))



  for (class_num = 0; class_num < classifier->settings->class_num;
       class_num++)
    {
      // maximum belief
      double x =
        classifier->belief[class_num] / ((double) classifier->sample_num);
      argmax = isfinite (x) && (x > max) ? class_num : argmax;
      max = isfinite (x) && (x > max) ? x : max;
      
      // mean beliefs
      classifier->mean_belief[class_num] +=
        (classifier->belief[class_num] / ((double) classifier->sample_num) -
         classifier->mean_belief[class_num]) /
        ((double) classifier->window_num);
    }

Is that something you copied out of a header file which came with your compiler?

Or did you find that macro on the net somewhere, and just copy/pasted it into your code?

Well that code is going to be specific to one particular OS/Compiler.

Which means if you're using something else, you need to do some work. You need to say what you're using to compile with to get any further.