patternngram.java

Boosting算法软件包

package jboost.examples.ngram;

import java.util.Enumeration;
import java.util.NoSuchElementException;

/**
 * An enumeration object that creates a stream of all ngrams with 
 * specified pattern defined on a given string of words.
 *
 * @author Rob Schapire  (rewritten by Aaron Arvey)
 */

class PatternNgram implements Enumeration {

    // current position in the list of words
    private int pos = 0; 
    private String[] words;
    private boolean[] pattern;
    private String prepend = "";

    private static final String[] prependChar = {"0", "1"};


    /**
     * The constructor for this class.  The stream of ngrams is
     * defined using tuples from the given array of words.  Tuples are
     * created using the pattern defined by the boolean array.  For
     * instance, a pattern of the form 1101 indicates that triples
     * should be created of the form word[i] word[i+1] word[i+3]
     * (skipping all places in which the pattern is false).
     */
    PatternNgram(String[] words, boolean[] pattern) {
	this.words = words;
	this.pattern = pattern;
	for (int i = 0; i < pattern.length; i++)
	    prepend += prependChar[pattern[i] ? 1 : 0];
    }

    /**
     * Returns true if and only if there are more elements in the
     * stream.
     */
    public boolean hasMoreElements() {
	int eltsInStream = words.length - pattern.length;
	return (pos <= eltsInStream);
    }

    /**
     * Next tuple in the stream.  Tuples are concatenated, separated
     * by spaces.
     */
    public Object nextElement() {
	if (!hasMoreElements())
	    throw new NoSuchElementException();
	String ret = prepend;
	for (int i = 0; i < pattern.length; i++) {
	    if (pattern[i])
		ret += " " + words[pos + i];
	}
	pos++;
	return ret;
    }
}