id3.java

:<<数据挖掘--实用机器学习技术及java实现>>一书的配套源程序

/*
 *    This program is free software; you can redistribute it and/or modify
 *    it under the terms of the GNU General Public License as published by
 *    the Free Software Foundation; either version 2 of the License, or
 *    (at your option) any later version.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU General Public License for more details.
 *
 *    You should have received a copy of the GNU General Public License
 *    along with this program; if not, write to the Free Software
 *    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

/*
 *    Id3.java
 *    Copyright (C) 1999 Eibe Frank
 *
 */

package weka.classifiers;

import weka.core.*;
import java.io.*;
import java.util.*;

/**
 * Class implementing an Id3 decision tree classifier. For more
 * information, see<p>
 *
 * R. Quinlan (1986). <i>Induction of decision
 * trees</i>. Machine Learning. Vol.1, No.1, pp. 81-106.<p>
 *
 * @author Eibe Frank ([e-mail])
 * @version $Revision: 1.7 $ 
 */
public class Id3 extends DistributionClassifier {

  /** The node's successors. */ 
  private Id3[] m_Successors;

  /** Attribute used for splitting. */
  private Attribute m_Attribute;

  /** Class value if node is leaf. */
  private double m_ClassValue;

  /** Class distribution if node is leaf. */
  private double[] m_Distribution;

  /** Class attribute of dataset. */
  private Attribute m_ClassAttribute;

  /**
   * Builds Id3 decision tree classifier.
   *
   * @param data the training data
   * @exception Exception if classifier can't be built successfully
   */
  public void buildClassifier(Instances data) throws Exception {


    if (!data.classAttribute().isNominal()) {
      throw new Exception("Id3: nominal class, please.");
    }
    Enumeration enumAtt = data.enumerateAttributes();
    while (enumAtt.hasMoreElements()) {
      Attribute attr = (Attribute) enumAtt.nextElement();
      if (!attr.isNominal()) {
        throw new Exception("Id3: only nominal attributes, please.");
      }
      Enumeration enum = data.enumerateInstances();
      while (enum.hasMoreElements()) {
        if (((Instance) enum.nextElement()).isMissing(attr)) {
          throw new Exception("Id3: no missing values, please.");
        }
      }
    }
    data = new Instances(data);
    data.deleteWithMissingClass(); 
    makeTree(data);
  }

  /**
   * Method building Id3 tree.
   *
   * @param data the training data
   * @exception Exception if decision tree can't be built successfully
   */
  private void makeTree(Instances data) throws Exception {

    // Check if no instances have reached this node.
    if (data.numInstances() == 0) {
      m_Attribute = null;
      m_ClassValue = Instance.missingValue();
      m_Distribution = new double[data.numClasses()];
      return;
    }

    // Compute attribute with maximum information gain.
    double[] infoGains = new double[data.numAttributes()];
    Enumeration attEnum = data.enumerateAttributes();
    while (attEnum.hasMoreElements()) {
      Attribute att = (Attribute) attEnum.nextElement();
      infoGains[att.index()] = computeInfoGain(data, att);
    }
    m_Attribute = data.attribute(Utils.maxIndex(infoGains));
    
    // Make leaf if information gain is zero. 
    // Otherwise create successors.
    if (Utils.eq(infoGains[m_Attribute.index()], 0)) {
      m_Attribute = null;
      m_Distribution = new double[data.numClasses()];
      Enumeration instEnum = data.enumerateInstances();
      while (instEnum.hasMoreElements()) {
	Instance inst = (Instance) instEnum.nextElement();
	m_Distribution[(int) inst.classValue()]++;
      }
      Utils.normalize(m_Distribution);
      m_ClassValue = Utils.maxIndex(m_Distribution);
      m_ClassAttribute = data.classAttribute();
    } else {
      Instances[] splitData = splitData(data, m_Attribute);
      m_Successors = new Id3[m_Attribute.numValues()];
      for (int j = 0; j < m_Attribute.numValues(); j++) {
	m_Successors[j] = new Id3();
	m_Successors[j].buildClassifier(splitData[j]);
      }
    }
  }

  /**
   * Classifies a given test instance using the decision tree.
   *
   * @param instance the instance to be classified
   * @return the classification
   */
  public double classifyInstance(Instance instance) {

    if (m_Attribute == null) {
      return m_ClassValue;
    } else {
      return m_Successors[(int) instance.value(m_Attribute)].
	  classifyInstance(instance);
    }
  }

  /**
   * Computes class distribution for instance using decision tree.
   *
   * @param instance the instance for which distribution is to be computed
   * @return the class distribution for the given instance
   */
  public double[] distributionForInstance(Instance instance) {

    if (m_Attribute == null) {
      return m_Distribution;
    } else { 
      return m_Successors[(int) instance.value(m_Attribute)].
	  distributionForInstance(instance);
    }
  }

  /**
   * Prints the decision tree using the private toString method from below.
   *
   * @return a textual description of the classifier
   */
  public String toString() {

    if ((m_Distribution == null) && (m_Successors == null)) {
      return "Id3: No model built yet.";
    }
    return "Id3\n\n" + toString(0);
  }

  /**
   * Computes information gain for an attribute.
   *
   * @param data the data for which info gain is to be computed
   * @param att the attribute
   * @return the information gain for the given attribute and data
   */
  private double computeInfoGain(Instances data, Attribute att) 
    throws Exception {

    double infoGain = computeEntropy(data);
    Instances[] splitData = splitData(data, att);
    for (int j = 0; j < att.numValues(); j++) {
      if (splitData[j].numInstances() > 0) {
	infoGain -= ((double) splitData[j].numInstances() /
		     (double) data.numInstances()) *
	  computeEntropy(splitData[j]);
      }
    }
    return infoGain;
  }

 /**
  * Computes the entropy of a dataset.
  * 
  * @param data the data for which entropy is to be computed
  * @return the entropy of the data's class distribution
  */
  private double computeEntropy(Instances data) throws Exception {

    double [] classCounts = new double[data.numClasses()];
    Enumeration instEnum = data.enumerateInstances();
    while (instEnum.hasMoreElements()) {
      Instance inst = (Instance) instEnum.nextElement();
      classCounts[(int) inst.classValue()]++;
    }
    double entropy = 0;
    for (int j = 0; j < data.numClasses(); j++) {
      if (classCounts[j] > 0) {
        entropy -= classCounts[j] * Utils.log2(classCounts[j]);
      }
    }
    entropy /= (double) data.numInstances();
    return entropy + Utils.log2(data.numInstances());
  }

  /**
   * Splits a dataset according to the values of a nominal attribute.
   *
   * @param data the data which is to be split
   * @param att the attribute to be used for splitting
   * @return the sets of instances produced by the split
   */
  private Instances[] splitData(Instances data, Attribute att) {

    Instances[] splitData = new Instances[att.numValues()];
    for (int j = 0; j < att.numValues(); j++) {
      splitData[j] = new Instances(data, data.numInstances());
    }
    Enumeration instEnum = data.enumerateInstances();
    while (instEnum.hasMoreElements()) {
      Instance inst = (Instance) instEnum.nextElement();
      splitData[(int) inst.value(att)].add(inst);
    }
    return splitData;
  }

  /**
   * Outputs a tree at a certain level.
   *
   * @param level the level at which the tree is to be printed
   */
  private String toString(int level) {

    StringBuffer text = new StringBuffer();
    
    if (m_Attribute == null) {
      if (Instance.isMissingValue(m_ClassValue)) {
        text.append(": null");
      } else {
        text.append(": "+m_ClassAttribute.value((int) m_ClassValue));
      } 
    } else {
      for (int j = 0; j < m_Attribute.numValues(); j++) {
        text.append("\n");
        for (int i = 0; i < level; i++) {
          text.append("|  ");
	}
        text.append(m_Attribute.name() + " = " + m_Attribute.value(j));
        text.append(m_Successors[j].toString(level + 1));
      }
    }
    return text.toString();
  }

  /**
   * Main method.
   *
   * @param args the options for the classifier
   */
  public static void main(String[] args) {

    try {
      System.out.println(Evaluation.evaluateModel(new Id3(), args));
    } catch (Exception e) {
      System.err.println(e.getMessage());
    }
  }
}