【问题和解决】NLTK was unable to find the megam file!(1)

在学到“训练基于分类器的分块器”这一小节的时候，在测试代码之后遇到了问题。

class ConsecutiveNPChunkTagger(nltk.TaggerI):
    def __init__(self, train_sents):
        train_set = []
        for tagged_sent in train_sents:
            untagged_sent = nltk.tag.untag(tagged_sent)
            history = []
            for i, (word, tag) in enumerate(tagged_sent):
                featureset = npchunk_features(untagged_sent, i, history) 
                train_set.append( (featureset, tag) )
                history.append(tag)
        self.classifier = nltk.MaxentClassifier.train(train_set, algorithm='megam', trace=0)
    def tag(self, sentence):
        history = []
        for i, word in enumerate(sentence):
            featureset = npchunk_features(sentence,i, history)
            tag = self.classifier.classify(featureset)
            history.append(tag)
        return zip(sentence, history)

class ConsecutiveNPChunker(nltk.ChunkParserI):
        def __init__(self, train_sents):
            tagged_sents = [[((w,t),c) for (w,t,c) in
            nltk.chunk.tree2conlltags(sent)]for sent in train_sents]
            self.tagger = ConsecutiveNPChunkTagger(tagged_sents)
        def parse(self, sentence):
            tagged_sents = self.tagger.tag(sentence)
            conlltags =[(w,t,c) for ((w,t),c) in tagged_sents]
            return nltk.chunk.conlltags2tree(conlltags)

def npchunk_features(sentence,i, history):
... word,pos= sentence[i]
... return {"pos": pos}
>>>chunker = ConsecutiveNPChunker(train_sents)
>>>print chunker.evaluate(test_sents)

以上是书上提供的代码，问题是，当在执行

chunker = ConsecutiveNPChunker(train_sents)并没有如期执行，反而出现了一个错误。

Traceback (most recent call last):
  File "<pyshell#119>", line 1, in <module>
    chunker = ConsecutiveNPChunker(train_sents)
  File "<pyshell#118>", line 5, in __init__
    self.tagger = ConsecutiveNPChunkTagger(tagged_sents)
  File "<pyshell#116>", line 11, in __init__
    self.classifier = nltk.MaxentClassifier.train(train_set, algorithm='megam', trace=0)
  File "D:\SpecialSoftware\Python25\Lib\site-packages\nltk\classify\maxent.py", line 319, in train
    gaussian_prior_sigma, **cutoffs)
  File "D:\SpecialSoftware\Python25\Lib\site-packages\nltk\classify\maxent.py", line 1522, in train_maxent_classifier_with_megam
    stdout = call_megam(options)
  File "D:\SpecialSoftware\Python25\Lib\site-packages\nltk\classify\megam.py", line 163, in call_megam
    config_megam()
  File "D:\SpecialSoftware\Python25\Lib\site-packages\nltk\classify\megam.py", line 59, in config_megam
    url='http://www.cs.utah.edu/~hal/megam/')
  File "D:\SpecialSoftware\Python25\Lib\site-packages\nltk\internals.py", line 528, in find_binary
    url, verbose)
  File "D:\SpecialSoftware\Python25\Lib\site-packages\nltk\internals.py", line 512, in find_file
    raise LookupError('\n\n%s\n%s\n%s' % (div, msg, div))
LookupError: 

===========================================================================
NLTK was unable to find the megam file!
Use software specific configuration paramaters or set the MEGAM environment variable.

  For more information, on megam, see:
    <http://www.cs.utah.edu/~hal/megam/>
===========================================================================

虽然说给出了相应的提示，但是并不完全。

通过对谷歌的搜索，找到了一些解决的眉目。

我的操作系统是Windows8.

nltk语言工具的官网给出了提示：

https://sites.google.com/site/naturallanguagetoolkit/download

Megam为可选包，将来使用的时候可以再来安装。下载的网址为：MegaM: http://hal3.name/megam/megam_src.tgz，直接下载我并没有下载成功，使用迅雷下载成功的。

但是打开之后发现，都是些源文件。但是在这个压缩包里面有一个README文件，给出了怎样使用的提示，发现，还需要装一个东西。

README中这样写到：ocaml(http://caml.inria.fr)

需要到这个网站下载源文件的编译器，于是我下载了和自己电脑系统相匹配的版本，但是看说明安装起来还是需要琢磨的，在安装过程中提示是病毒，但是我还是选择信任，要不然没办法继续。

 【现在Ocaml正在安装，我点的完全安装（可能实际当中没有必要完全安装吧），等安装完成后，再继续探索怎么解决这个问题】