jurafsky&martin_3rdEd_17 (1).pdf

The bulk of this chapter is devoted to the topic of

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simpler approaches. The bulk of this chapter is devoted to the topic of context-free grammars. Context- free grammars are the backbone of many formal models of the syntax of natural language (and, for that matter, of computer languages). As such, they are integral to many computational applications, including grammar checking, semantic interpreta- tion, dialogue understanding, and machine translation. They are powerful enough to express sophisticated relations among the words in a sentence, yet computationally tractable enough that efficient algorithms exist for parsing sentences with them (as we show in Chapter 12). In Chapter 13, we show that adding probability to context- free grammars gives us a powerful model of disambiguation. And in Chapter 20 we show how they provide a systematic framework for semantic interpretation. In addition to an introduction to this grammar formalism, this chapter also pro- vides a brief overview of the grammar of English. To illustrate our grammars, we have chosen a domain that has relatively simple sentences, the Air Traffic Informa- tion System (ATIS) domain (Hemphill et al., 1990) . ATIS systems were an early example of spoken language systems for helping book airline reservations. Users try to book flights by conversing with the system, specifying constraints like I’d like to fly from Atlanta to Denver . 11.1 Constituency The fundamental notion underlying the idea of constituency is that of abstraction — groups of words behaving as a single units, or constituents. A significant part of developing a grammar involves discovering the inventory of constituents present in the language.
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11.2 C ONTEXT -F REE G RAMMARS 169 How do words group together in English? Consider the noun phrase , a sequence Noun phrase of words surrounding at least one noun. Here are some examples of noun phrases (thanks to Damon Runyon): Harry the Horse a high-class spot such as Mindy’s the Broadway coppers the reason he comes into the Hot Box they three parties from Brooklyn What evidence do we have that these words group together (or “form constituents”)? One piece of evidence is that they can all appear in similar syntactic environments, for example, before a verb. three parties from Brooklyn arrive . . . a high-class spot such as Mindy’s attracts . . . the Broadway coppers love . . . they sit But while the whole noun phrase can occur before a verb, this is not true of each of the individual words that make up a noun phrase. The following are not grammat- ical sentences of English (recall that we use an asterisk (*) to mark fragments that are not grammatical English sentences): *from arrive . . . *as attracts . . . *the is . . . *spot sat . . . Thus, to correctly describe facts about the ordering of these words in English, we must be able to say things like “ Noun Phrases can occur before verbs ”.
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