Strange weka instance results

Question

Hi,

strange results come up while using a J48 tree. I need to classify a vector of 48 features, which works very well, but when i tried to "optimize", I run into strange results.

I have a method classify:

    public boolean classify(double feature1, double feature2, double[] featureVec ) {
        Instance toBeClassified = new Instance(2+featureVec.length);
        toBeClassified.setValue(0, feature1);
        toBeClassified.setValue(1, feature2);
        for (int i = 2; i < f.length + 2; ++i) {
            toBeClassified.setValue(i, featureVec [i - 2]);
        }
        toBeClassified.setDataset(dataset);

        try {
            double _class = tree.classifyInstance(toBeClassified);
            return _class > 0;
        } catch (Exception e1) {
            if(Logging.active) { 
                logger.error(e1.getMessage(), e1.getCause());}
            }
        return false;
    }
}

It works quite well, and i hope i'm doing things right. But I wanted to remove the instance creation which is done at every method call, so I moved the Instance toBeClassified = new Instance(48); line into the class body - so it is created only once. That works well too, despite of the fact, that I get slightly different results compared with the other. lets say, from 400 classifications, one is different (not to say, incorrect). But I don't see a reason for this...I hope here are some guys using weka, so that I understand whats going on/wrong. (Yes, 2+featureVec.length is 48).

Thanks and regards.

Answer 1

It's very unlikely that anything is wrong with J48. Both classifier creation and classification itself is deterministic. I'd recommend to post bigger part of Your code, because this one looks great (unbuggy).

As for Your 400 loop test: this one definitely should produce the same results every time, no exceptions. Two thoughts:

• Put assert that checks if the values of instance are same as the model one. That would rule out any bug in Instance.

• Does classification run in multi-threading manner? Are there any shared data objects?