MAS.632 Conversational Computer Systems - MIT OpenCourseWare

Higher Levels of ingutk Kno~edge
(the subject) and a verb and can be represented as in Figure 9.1, e.g., "Cats purr."
Trees capture the order and grouping of words and also indicate the unique label
(noun, verb, etc.) associated with each word.
To generalize to more complex sentences we must introduce the concepts of
noun phrase and verb phrase. A noun phrase is a group of one or more words
that act as a noun in a sentence. A verb phrase similarly allows multiple words to
act as the verb. Because they can contain multiple component words, these
phrases introduce another level of structure to our tree representation. A sentence
such as "Orange cats purr loddly." is represented as in Figure 9.2. The
branching ofthis tree structure indicates that the adjective "orange" modifies the
noun "cats," while the adverb "loudly" is likewise related to the verb "purr."
While trees such as these examples convey the structure of particular sentences
or classes of sentences, they do not tell us how to derive the structure for a
S
Figure 9.1. The figure shows the root node representing the entire sentence
above two leaf nodes: one for the noun and one for the verb.
S
NP VP
Adiec ive Noun Verb Adverb
orange cats
loudly
Figure 9.2. The tree representation expanded to account for a noun
phrase and a verb phrase.
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