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07/05/2005CSA2050: DCG31 CSA2050 Introduction to Computational Linguistics Lecture DCG3 Handling Subcategorisation Handling Relative Clauses.

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Presentation on theme: "07/05/2005CSA2050: DCG31 CSA2050 Introduction to Computational Linguistics Lecture DCG3 Handling Subcategorisation Handling Relative Clauses."— Presentation transcript:

1 07/05/2005CSA2050: DCG31 CSA2050 Introduction to Computational Linguistics Lecture DCG3 Handling Subcategorisation Handling Relative Clauses

2 07/05/2005CSA2050: DCG32 Subcategories of Verbs English includes the following sentences: –John disappeared –John broke the window –John gave Mary the book In each case the verb phrase has a different structure corresponding to the number of noun phrases that follow.

3 07/05/2005CSA2050: DCG33 Subcategories of VP VP V disappeared VP V NP broke the window VP V NP NP gave Mary the book To handle this we need separate rules for VP

4 07/05/2005CSA2050: DCG34 Some Rules for Subcategories of Verb vp -> v. vp -> v, np. vp -> v, np, np. Checkpoint: how good are these rules for handling English?

5 07/05/2005CSA2050: DCG35 Counterexamples The following sentences obey the above rules: –John broke –Mary disappeared the window –Mike told Mary the book All of them violate the subcategorisation constraints of the individual verbs. How can we make sure that the right rule goes with the right verb?

6 07/05/2005CSA2050: DCG36 Handling Subcategorisation - I vp --> v1. vp --> v2, np. vp --> v3 np, np. This solution has the problem that the v i are distinct symbols with no relationship between them. We thus miss the fact that they are all verbs

7 07/05/2005CSA2050: DCG37 Handling Subcategorisation - II A slightly better method is to adopt rules of the following shape: vp --> v(intrans) vp --> v(trans), np. vp --> v(ditrans), np, np. This is essentially the same method that we used to avoid multiplying categories when handling agreement.

8 07/05/2005CSA2050: DCG38 Handling Subcategorisation: The Lexicon Assume that the lexicon looks something like this: lex(disappeared, v, intrans). lex(broke,v, trans). lex(gave, v, ditrans).

9 07/05/2005CSA2050: DCG39 Two Approaches – One Lexicon Approach 1 v1 --> [X] {lex(X,v,intrans)}. v2 --> [X] {lex(X,v,trans)}. v2 --> [X] {lex(X,v,ditrans)}. Approach 2 v(Type) --> [X], {lex(X,v,Type)}.

10 07/05/2005CSA2050: DCG310 Other Verb Complements So far we have only discussed three possible VP rules. But in practice there are many more: –np, pp: John told Bill about the accident –np, vp: John persuaded Bill to come –np, pp,pp: John rented an apartment to Bill for Lm100 –np, s: John informed Bill that Jack was coming Taking prepositions into account, about 40 different patterns are required to handle verbs in English. This will lead to an undesirable proliferation of grammar rules.

11 07/05/2005CSA2050: DCG311 Subcategorisation: Approach III Shift responsibility towards the lexicon Key idea: lexical entry for each verb contains a complement list. lex(told,v, [np,pp]). lex(persuaded,v,[np,vp]). lex(rented,v,[np,pp,pp]. The list is then managed by the grammar.

12 07/05/2005CSA2050: DCG312 Grammar Rules for VP Basic idea is to avoid multiple grammar rules of the form vp --> v([]). vp --> v([np]), np(...). vp --> v([np,np]), np(...), np(...). vp --> v([np,pp]), np(...), pp(...). vp --> v([np,s]), np(...), s(...). Instead have a single rule of this form vp --> v(SubCatList), comps(SubCatList). which allows a verb phrase to be formed from a verb followed by a sequence of zero or more complements.

13 07/05/2005CSA2050: DCG313 The Grammar s --> np, vp. np --> n. np --> d, n. vp --> v(SC), comps(SC). comps([]) --> []. comps([X|R]) --> x(X), comps(R).

14 07/05/2005CSA2050: DCG314 The Comps Rule comps([]) --> []. comps([X|R]) --> x(X), comps(R) Challenge: how to write the definition of the "x" predicate.

15 07/05/2005CSA2050: DCG315 Relative Clauses Sentences like Terry runs a program that Bertrand wrote The man who came to dinner vomited include relative clauses (shown in boldface) Relative clauses are interesting because they involve "filler-gap dependency"

16 07/05/2005CSA2050: DCG316 Structure of Relative Clauses Relative clauses such as that Bertrand wrote are not well described by relclause --> [that], vp. This is because the underlying structure is not the concatenation of "that" and a VP, i.e. If this is not the structure, what is?

17 07/05/2005CSA2050: DCG317 The Structure of the Relative Clause To understand the structure we must first look at the wider context in which it occurs, in this case the noun phrase: a program that Bertrand wrote. This noun phrase is derived from the underlying sentence Bertrand wrote a program.

18 07/05/2005CSA2050: DCG318 Derivation of Relative Clause Basic sentence: Bertrand wrote a program Step 1: identify object NP Bertrand wrote [a program] Step 2: move to the front, leaving gap where object NP was. [A program] Bertrand wrote npgap Note that underlined structure is a sentence with a gap instead of an object. Step 3: insert relative pronoun just before sentence containing gap. [A program] that Bertrand wrote npgap N.B. Gap is invisible

19 07/05/2005CSA2050: DCG319 Structure of the Noun Phrase NP dN relSgap VPgap v NPgap aprogramthat n bertrandwrote RelCl filler gap

20 07/05/2005CSA2050: DCG320 Long Distance Dependencies A filler-gap dependency occurs in an NL sentence when a subpart of some phrase is missing from its normal location and another phrase, outside the incomplete one, stands in its place. Filler-gap dependencies are examples of long- distance dependencies. The amount of material between dependents (in this case filler and gap) is unbounded, at least in principle, e.g. Terry read a book that Bertrand told a student to ask a professor to write.

21 07/05/2005CSA2050: DCG321 Handling Gaps within a DCG Use a nonterminal argument position to indicate the presence or absence a gap, e.g. np(nogap) = an NP without a gap np(gap) = an NP with a gap Introduce the term gap(T) to indicate the presence or absence of a gap of category T. So gap(np) will indicate where an NP used to be. For each category c that admits gaps, introduce the rule c(gap(c)) rewrites nothing, e.g. np(gap(np)) --> [ ].

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