/*
* 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.
*/
/*
* MarginCurve.java
* Copyright (C) 2002 University of Waikato, Hamilton, New Zealand
*
*/
package weka.classifiers.evaluation;
import weka.core.Attribute;
import weka.core.FastVector;
import weka.core.Instance;
import weka.core.DenseInstance;
import weka.core.Instances;
import weka.core.RevisionHandler;
import weka.core.RevisionUtils;
import weka.core.Utils;
/**
* Generates points illustrating the prediction margin. The margin is defined
* as the difference between the probability predicted for the actual class and
* the highest probability predicted for the other classes. One hypothesis
* as to the good performance of boosting algorithms is that they increaes the
* margins on the training data and this gives better performance on test data.
*
* @author Len Trigg (len@reeltwo.com)
* @version $Revision: 5987 $
*/
public class MarginCurve
implements RevisionHandler {
/**
* Calculates the cumulative margin distribution for the set of
* predictions, returning the result as a set of Instances. The
* structure of these Instances is as follows:<p> <ul>
* <li> <b>Margin</b> contains the margin value (which should be plotted
* as an x-coordinate)
* <li> <b>Current</b> contains the count of instances with the current
* margin (plot as y axis)
* <li> <b>Cumulative</b> contains the count of instances with margin
* less than or equal to the current margin (plot as y axis)
* </ul> <p>
*
* @return datapoints as a set of instances, null if no predictions
* have been made.
*/
public Instances getCurve(FastVector predictions) {
if (predictions.size() == 0) {
return null;
}
Instances insts = makeHeader();
double [] margins = getMargins(predictions);
int [] sorted = Utils.sort(margins);
int binMargin = 0;
int totalMargin = 0;
insts.add(makeInstance(-1, binMargin, totalMargin));
for (int i = 0; i < sorted.length; i++) {
double current = margins[sorted[i]];
double weight = ((NominalPrediction)predictions.elementAt(sorted[i]))
.weight();
totalMargin += weight;
binMargin += weight;
if (true) {
insts.add(makeInstance(current, binMargin, totalMargin));
binMargin = 0;
}
}
return insts;
}
/**
* Pulls all the margin values out of a vector of NominalPredictions.
*
* @param predictions a FastVector containing NominalPredictions
* @return an array of margin values.
*/
private double [] getMargins(FastVector predictions) {
// sort by predicted probability of the desired class.
double [] margins = new double [predictions.size()];
for (int i = 0; i < margins.length; i++) {
NominalPrediction pred = (NominalPrediction)predictions.elementAt(i);
margins[i] = pred.margin();
}
return margins;
}
/**
* Creates an Instances object with the attributes we will be calculating.
*
* @return the Instances structure.
*/
private Instances makeHeader() {
FastVector fv = new FastVector();
fv.addElement(new Attribute("Margin"));
fv.addElement(new Attribute("Current"));
fv.addElement(new Attribute("Cumulative"));
return new Instances("MarginCurve", fv, 100);
}
/**
* Creates an Instance object with the attributes calculated.
*
* @param margin the margin for this data point.
* @param current the number of instances with this margin.
* @param cumulative the number of instances with margin less than or equal
* to this margin.
* @return the Instance object.
*/
private Instance makeInstance(double margin, int current, int cumulative) {
int count = 0;
double [] vals = new double[3];
vals[count++] = margin;
vals[count++] = current;
vals[count++] = cumulative;
return new DenseInstance(1.0, vals);
}
/**
* Returns the revision string.
*
* @return the revision
*/
public String getRevision() {
return RevisionUtils.extract("$Revision: 5987 $");
}
/**
* Tests the MarginCurve generation from the command line.
* The classifier is currently hardcoded. Pipe in an arff file.
*
* @param args currently ignored
*/
public static void main(String [] args) {
try {
Utils.SMALL = 0;
Instances inst = new Instances(new java.io.InputStreamReader(System.in));
inst.setClassIndex(inst.numAttributes() - 1);
MarginCurve tc = new MarginCurve();
EvaluationUtils eu = new EvaluationUtils();
weka.classifiers.meta.LogitBoost classifier
= new weka.classifiers.meta.LogitBoost();
classifier.setNumIterations(20);
FastVector predictions
= eu.getTrainTestPredictions(classifier, inst, inst);
Instances result = tc.getCurve(predictions);
System.out.println(result);
} catch (Exception ex) {
ex.printStackTrace();
}
}
}