有关Lucene的问题(2):stemming和lemmatization

问题：

我试验了一下文章中提到的 stemming 和 lemmatization

• 将单词缩减为词根形式，如“cars”到“car”等。这种操作称为：stemming。
• 将单词转变为词根形式，如“drove”到“drive”等。这种操作称为：lemmatization。

试验没有成功

代码如下：

public class TestNorms {   
    public void createIndex() throws IOException {   
        Directory d = new SimpleFSDirectory(new File("d:/falconTest/lucene3/norms"));   
        IndexWriter writer = new IndexWriter(d, new StandardAnalyzer(Version.LUCENE_30), 
                                                                                      true, IndexWriter.MaxFieldLength.UNLIMITED);   
        Field field = new Field("desc", "", Field.Store.YES, Field.Index.ANALYZED);   
        Document doc = new Document();   
        field.setValue("Hello students was drive");   
        doc.add(field);   
        writer.addDocument(doc);   
        writer.optimize();   
        writer.close();   
    }   
    public void search() throws IOException {   
        Directory d = new SimpleFSDirectory(new File("d:/falconTest/lucene3/norms"));   
        IndexReader reader = IndexReader.open(d);   
        IndexSearcher searcher = new IndexSearcher(reader);   
        TopDocs docs = searcher.search(new TermQuery(new Term("desc","drove")), 10);   
        System.out.println(docs.totalHits);   
    }   
    public static void main(String[] args) throws IOException {   
        TestNorms test= new TestNorms();   
        test.createIndex();   
        test.search();   
    }   
} 

不管是单复数，还是单词的变化，都是没有体现的

不知道是不是分词器的原因？

回答：

的确是分词器的问题，StandardAnalyzer并不能进行stemming和lemmatization，因而不能够区分单复数和词型。

文章中讲述的是全文检索的基本原理，理解了他，有利于更好的理解Lucene，但不代表Lucene是完全按照此基本流程进行的。

(1) 有关stemming

作为stemming，一个著名的算法是The Porter Stemming Algorithm，其主页为http://tartarus.org/~martin/PorterStemmer/，也可查看其论文http://tartarus.org/~martin/PorterStemmer/def.txt。

通过以下网页可以进行简单的测试：Porter's Stemming Algorithm Online[http://facweb.cs.depaul.edu/mobasher/classes/csc575/porter.html]

cars –> car

driving –> drive

tokenization –> token

然而

drove –> drove

可见stemming是通过规则缩减为词根的，而不能识别词型的变化。

在最新的Lucene 3.0中，已经有了PorterStemFilter这个类来实现上述算法，只可惜没有Analyzer向匹配，不过不要紧，我们可以简单实现：

public class PorterStemAnalyzer extends Analyzer
{
    @Override
    public TokenStream tokenStream(String fieldName, Reader reader) {
      return new PorterStemFilter(new LowerCaseTokenizer(reader));
    }
}

把此分词器用在你的程序中，就能够识别单复数和规则的词型变化了。

public void createIndex() throws IOException {
  Directory d = new SimpleFSDirectory(new File("d:/falconTest/lucene3/norms"));
  IndexWriter writer = new IndexWriter(d, new PorterStemAnalyzer(), true, IndexWriter.MaxFieldLength.UNLIMITED);

  Field field = new Field("desc", "", Field.Store.YES, Field.Index.ANALYZED);
  Document doc = new Document();
  field.setValue("Hello students was driving cars professionally");
  doc.add(field);

  writer.addDocument(doc);
  writer.optimize();
  writer.close();
}

public void search() throws IOException {
  Directory d = new SimpleFSDirectory(new File("d:/falconTest/lucene3/norms"));
  IndexReader reader = IndexReader.open(d);
  IndexSearcher searcher = new IndexSearcher(reader);
  TopDocs docs = searcher.search(new TermQuery(new Term("desc", "car")), 10);
  System.out.println(docs.totalHits);
  docs = searcher.search(new TermQuery(new Term("desc", "drive")), 10);
  System.out.println(docs.totalHits);
  docs = searcher.search(new TermQuery(new Term("desc", "profession")), 10);
  System.out.println(docs.totalHits);
}

(2) 有关lemmatization

至于lemmatization，一般是有字典的，方能够由"drove"对应到"drive".

在网上搜了一下，找到European languages lemmatizer[http://lemmatizer.org/]，只不过是在linux下面C++开发的，有兴趣可以试验一下。

首先按照网站的说明下载，编译，安装：

libMAFSA is the core of the lemmatizer. All other libraries depend on it. Download the last version from the following page, unpack it and compile:

# tar xzf libMAFSA-0.2.tar.gz
# cd libMAFSA-0.2/
# cmake .
# make
# sudo make install

After this you should install libturglem. You can download it at the same place.

# tar xzf libturglem-0.2.tar.gz
# cd libturglem-0.2
# cmake .
# make
# sudo make install

Next you should install english dictionaries with some additional features to work with.

# tar xzf turglem-english-0.2.tar.gz
# cd turglem-english-0.2
# cmake .
# make
# sudo make install

安装完毕后：

• /usr/local/include/turglem是头文件，用于编译自己编写的代码
• /usr/local/share/turglem/english是字典文件，其中lemmas.xml中我们可以看到"drove"和"drive"的对应，"was"和"be"的对应。
• /usr/local/lib中的libMAFSA.a  libturglem.a  libturglem-english.a  libtxml.a是用于生成应用程序的静态库

<l id="DRIVE" p="6" />

<l id="DROVE" p="6" />

<l id="DRIVING" p="6" />

在turglem-english-0.2目录下有例子测试程序test_utf8.cpp

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <turglem/lemmatizer.h>
#include <turglem/lemmatizer.hpp>
#include <turglem/english/charset_adapters.hpp>

int main(int argc, char **argv)
{
        char in_s_buf[1024];
        char *nl_ptr;

        tl::lemmatizer lem;

        if(argc != 4)
        {
                printf("Usage: %s words.dic predict.dic flexias.bin\n", argv[0]);
                return -1;
        }

        lem.load_lemmatizer(argv[1], argv[3], argv[2]);

        while (!feof(stdin))
        {
                fgets(in_s_buf, 1024, stdin);
                nl_ptr = strchr(in_s_buf, '\n');
                if (nl_ptr) *nl_ptr = 0;
                nl_ptr = strchr(in_s_buf, '\r');
                if (nl_ptr) *nl_ptr = 0;

                if (in_s_buf[0])
                {
                        printf("processing %s\n", in_s_buf);
                        tl::lem_result pars;
                        size_t pcnt = lem.lemmatize<english_utf8_adapter>(in_s_buf, pars);
                        printf("%d\n", pcnt);
                        for (size_t i = 0; i < pcnt; i++)
                        {
                                std::string s;
                                u_int32_t src_form = lem.get_src_form(pars, i);
                                s = lem.get_text<english_utf8_adapter>(pars, i, 0);
                                printf("PARADIGM %d: normal form '%s'\n", (unsigned int)i, s.c_str());
                                printf("\tpart of speech:%d\n", lem.get_part_of_speech(pars, (unsigned int)i, src_form));
                        }
                }
        }

        return 0;
}

编译此文件，并且链接静态库：注意链接顺序，否则可能出错。

g++ -g -o output test_utf8.cpp -L/usr/local/lib/ -lturglem-english -lturglem -lMAFSA –ltxml

运行编译好的程序：

./output /usr/local/share/turglem/english/dict_english.auto

/usr/local/share/turglem/english/prediction_english.auto

/usr/local/share/turglem/english/paradigms_english.bin

做测试，虽然对其机制尚不甚了解，但是可以看到lemmatization的作用：

drove
processing drove
3
PARADIGM 0: normal form 'DROVE'
        part of speech:0
PARADIGM 1: normal form 'DROVE'
        part of speech:2
PARADIGM 2: normal form 'DRIVE'
        part of speech:2

was
processing was
3
PARADIGM 0: normal form 'BE'
        part of speech:3
PARADIGM 1: normal form 'BE'
        part of speech:3
PARADIGM 2: normal form 'BE'
        part of speech:3