Question

Im trying to build a text classifier in JAVA with Weka.
I have read some tutorials, and I´m trying to build my own classifier.

I have the following categories:

    computer,sport,unknown 

and the following already trained data

 cs belongs to computer
 java -> computer
 soccer -> sport
 snowboard -> sport

So for example, if a user wants to classify the word java, it should return the category computer (no doubt, java only exists in that category!).

It does compile, but generates strange output.

The output is:

      ====== RESULT ======  CLASSIFIED AS:  [0.5769230769230769, 0.2884615384615385, 0.1346153846153846]
      ====== RESULT ======  CLASSIFIED AS:  [0.42857142857142855, 0.42857142857142855, 0.14285714285714285]

But the first text to classify is java and it occures only in the category computer, therefore it should be

      [1.0 0.0 0.0] 

and for the other it shouldnt be found at all, so it should be classified as unknown

      [0.0 0.0 1.0].

Here is the code:

    import java.io.FileNotFoundException;
    import java.io.Serializable;
    import java.util.Arrays;

    import weka.classifiers.Classifier;
    import weka.classifiers.bayes.NaiveBayesMultinomialUpdateable;
    import weka.core.Attribute;
    import weka.core.FastVector;
    import weka.core.Instance;
    import weka.core.Instances;
    import weka.filters.Filter;
    import weka.filters.unsupervised.attribute.StringToWordVector;

    public class TextClassifier implements Serializable {

        private static final long serialVersionUID = -1397598966481635120L;
        public static void main(String[] args) {
            try {
                TextClassifier cl = new TextClassifier(new NaiveBayesMultinomialUpdateable());
                cl.addCategory("computer");
                cl.addCategory("sport");
                cl.addCategory("unknown");
                cl.setupAfterCategorysAdded();

                //
                cl.addData("cs", "computer");
                cl.addData("java", "computer");
                cl.addData("soccer", "sport");
                cl.addData("snowboard", "sport");

                double[] result = cl.classifyMessage("java");
                System.out.println("====== RESULT ====== \tCLASSIFIED AS:\t" + Arrays.toString(result));

                result = cl.classifyMessage("asdasdasd");
                System.out.println("====== RESULT ======\tCLASSIFIED AS:\t" + Arrays.toString(result));
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        private Instances trainingData;
        private StringToWordVector filter;
        private Classifier classifier;
        private boolean upToDate;
        private FastVector classValues;
        private FastVector attributes;
        private boolean setup;

        private Instances filteredData;

        public TextClassifier(Classifier classifier) throws FileNotFoundException {
            this(classifier, 10);
        }

        public TextClassifier(Classifier classifier, int startSize) throws FileNotFoundException {
            this.filter = new StringToWordVector();
            this.classifier = classifier;
            // Create vector of attributes.
            this.attributes = new FastVector(2);
            // Add attribute for holding texts.
            this.attributes.addElement(new Attribute("text", (FastVector) null));
            // Add class attribute.
            this.classValues = new FastVector(startSize);
            this.setup = false;

        }

        public void addCategory(String category) {
            category = category.toLowerCase();
            // if required, double the capacity.
            int capacity = classValues.capacity();
            if (classValues.size() > (capacity - 5)) {
                classValues.setCapacity(capacity * 2);
            }
            classValues.addElement(category);
        }

        public void addData(String message, String classValue) throws IllegalStateException {
            if (!setup) {
                throw new IllegalStateException("Must use setup first");
            }
            message = message.toLowerCase();
            classValue = classValue.toLowerCase();
            // Make message into instance.
            Instance instance = makeInstance(message, trainingData);
            // Set class value for instance.
            instance.setClassValue(classValue);
            // Add instance to training data.
            trainingData.add(instance);
            upToDate = false;
        }

        /**
         * Check whether classifier and filter are up to date. Build i necessary.
         * @throws Exception
         */
        private void buildIfNeeded() throws Exception {
            if (!upToDate) {
                // Initialize filter and tell it about the input format.
                filter.setInputFormat(trainingData);
                // Generate word counts from the training data.
                filteredData = Filter.useFilter(trainingData, filter);
                // Rebuild classifier.
                classifier.buildClassifier(filteredData);
                upToDate = true;
            }
        }

        public double[] classifyMessage(String message) throws Exception {
            message = message.toLowerCase();
            if (!setup) {
                throw new Exception("Must use setup first");
            }
            // Check whether classifier has been built.
            if (trainingData.numInstances() == 0) {
                throw new Exception("No classifier available.");
            }
            buildIfNeeded();
            Instances testset = trainingData.stringFreeStructure();
            Instance testInstance = makeInstance(message, testset);

            // Filter instance.
            filter.input(testInstance);
            Instance filteredInstance = filter.output();
            return classifier.distributionForInstance(filteredInstance);

        }

        private Instance makeInstance(String text, Instances data) {
            // Create instance of length two.
            Instance instance = new Instance(2);
            // Set value for message attribute
            Attribute messageAtt = data.attribute("text");
            instance.setValue(messageAtt, messageAtt.addStringValue(text));
            // Give instance access to attribute information from the dataset.
            instance.setDataset(data);
            return instance;
        }

        public void setupAfterCategorysAdded() {
            attributes.addElement(new Attribute("class", classValues));
            // Create dataset with initial capacity of 100, and set index of class.
            trainingData = new Instances("MessageClassificationProblem", attributes, 100);
            trainingData.setClassIndex(trainingData.numAttributes() - 1);
            setup = true;
        }

    }

Btw, found a good page:

http://www.hakank.org/weka/TextClassifierApplet3.html

Answer 1

The Bayes classifier gives you a (weighted) probability that a word belongs to a category. This will almost never be exactly 0 or 1. You can either set a hard cutoff (e.g. 0.5) and decide membership for a class based on this, or inspect the calculated probabilities and decide based on that (i.e. the highest map to 1, the lowest to 0).