Question

I have some sentences that I want to parse. Here is what I have and what I need: I have sentences like these:

I was in the hospital.

I was going from home to Canada.

What I want is to know the head of “in the hospital”, “from home”, and “to Canada” phrases.

I am using Berkley parser, but what it gives me is the parsing result of all the sentence, and if I want to extract the head of phrases manually, I should develop another parser! The file that I want to parse is a very big file, so if I develop a parser myself, it may have many errors. Is there any parser that can give me the result I am looking for?

By the way, as parsing the phrases separately, may result in a different parsing compared with sentence parsing, I insist on parsing the sentences and then extract the phrase heads.

Answer 1

The Stanford Parser and the OpenNLP parsers will give you Part-of-Speech and dependency information, which you can use to determine the head of phrases

For example, using the Stanford parser, you would get:

(S
  (NP (PRP I))
  (VP (VBD was)
      (PP (IN in)
          (NP (DT the)
              (NN hospital)))))

Which tells you that the sentence (S) consists of a noun-phrase (NP) and a verb-phrase (VP); the verb-phrase is a verb (V*) + prepositional phrase (PP), which is the preposition in and a noun-phrase; the second noun-phrase is a determiner (DT) and a noun (NN).

If I understand the question properly, you are looking for the heads of noun-phrases (and possibly the verb-phrases). You can identify the head from this information already, but the parser gives you the following dependency information as well:

nsubj(was, I)
prep_in(was, hospital)
det(hospital, the)

This tells you that the words was and I are in an nominal-subject nsubj relationship (I is the subject of the verb was); the words was and hospital are in an “in” preposition (prep-in) relationship; the words “hospital” and “the” are in a determiner (det) relationship. Using the previous parsing and the dependency information, you can tell that the head of the first noun-phrase is “I” (trivial), and the head of the second noun-phrase is “hospital” (as that is the “top” element of the relations within the noun-phrase)