sets.java

Standord Classifier实现了一个基于Java的最大熵分类器。用于模式识别

package edu.stanford.nlp.util;

import java.util.*;

/** Utilities for sets
 * @author Roger Levy
 */

public class Sets {
  private Sets() {
  }


  /** returns the set cross product of s1 and s2, as
   * <code>Pair</code>s */
  public static Set cross(Set s1, Set s2) {
    Set s = new HashSet();
    for(Iterator i = s1.iterator(); i.hasNext();){
      Object o1 = i.next();
      for(Iterator j = s2.iterator(); j.hasNext();){
      Object o2 = j.next();
      s.add(new Pair(o1,o2));
      }
    }
    return s;
  }

  /** set difference s1 - s2*/
  public static Set diff(Set s1, Set s2) {
    Set s = new HashSet();
    for(Iterator i = s1.iterator(); i.hasNext();){
      Object o = i.next();
      if(! s2.contains(o))
	s.add(o);
    }
    return s;
  }

  /** returns the powerset of a set */
  public static Set powerSet(Set s) {
    if(s.isEmpty()) {
      Set h = new HashSet();
      Set h0 = new HashSet(0);
      h.add(h0);
      return h;
    }
    else {
      Iterator i = s.iterator();
      Object elt = i.next();
      s.remove(elt);
      Set pow = powerSet(s);
      Set pow1 = powerSet(s);
      for(Iterator j = pow1.iterator(); j.hasNext();) {
	Set t = new HashSet((Set) j.next());
	t.add(elt);
	pow.add(t);
      }
      s.add(elt);
      return pow;
    }
  }

  public static void main(String[] args) {
    Set h = new HashSet();
    h.add("a");
    h.add("b");
    h.add("c");
    Set pow = powerSet(h);
    System.out.println(pow);
  }

}