MAS.632 Conversational Computer Systems - MIT OpenCourseWare

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Context-free grammars can describe many sentences in a natural language
such as English; they also specify most programming languages in their entirety.
Note that the abbreviated discussion of context-free grammars presented here
has avoided some of the issues of gender, number, and case agreement mentioned
earlier. For example, the modified verb phrase rule just described allows "Purr!"
which is a valid imperative sentence, as well as "Purrs," which is incomplete.
A second powerful representation for grammars is the transitionnetwork or
a slightly more general variation, the recursive transition network, RTN. A
simple transition network consists ofnodes and arcs as shown in Figure 9.4. Each
are is labelled with a word category, and each node is a state in the syntactic analysis
of a sentence. Starting at the beginning of the sentence, each word is compared
with the label on the are from the initial state, S. If the word and label
match, analysis transitions to the next node and word and label comparison continue
as before. The last are is labelled pop and always succeeds, indicating com­
pletion of the network.
This linear network does not convey the syntactic relationships
adjective-noun and adverb-verb pairs as well as do the equivalent tree or contextfree
grammar representations. The network can be enhanced by allowing ares to
be labelled with the names of other networks as well as individual words; with
this ability, the network has become recursive. For example, Figure 9.5 illustrates
a transition network describing a noun phrase as an arbitrary number of
adjectives followed by a single noun. Figure 9.6 shows how the noun phrase net-
adjective noun adverb verb
Figure 9.4. A simple transition network for sentences consisting of a single
adjective, a noun, a single adverb, and a verb. The states, al through
s4, are connected by arcs labelled with the classes of words required at
each transition.
adjective noun
NP: pop
Figure 9.5. A transition network for a noun phrase allowing an arbitrary
number of adjectives.
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