Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

psychological aspects of understanding the natural languages. The first step to
understand natural languages is parsing, which analyses the syntactic structure
of sentences. It serves two basic purposes. First it determines the linguistic
relations such as subject-verb, etc. and finally checks the syntactical
correctness of the sentences. Generally, the parser learns the sentence as a
structured object (tree/graph) for subsequent semantic analysis.
18.2 Syntactic Analysis
In this section we shall discuss three distinct methods for syntactic analysis of
sentences. These are: i) by using context free grammar, ii) by employing
transition network and iii) by using context sensitive grammar.
18.2.1 Parsing Using Context
Free Grammar
A context free grammar G is generally defined by a 4 tuple, given by G =
{Vn,Vt,S,P } , where Vn is a set of non-terminal symbols, Vt denotes a set of
terminal symbols, S denotes the starting symbol and P represents a set of
production rules that cause a change. It may further be noted that Vn ∪ Vt
denotes the entire set of symbols. The above definition of context free
grammar is prevalent in automata theory and compilers. The tuples in G are
illustrated below with an example.
Example 18.1: Let us consider the following sentence and illustrate the
parameters involved in the definition of G in connection with the problem of
natural language understanding.
Sentence: The man killed a deer.
Here, Vt = {the, man, killed, a, deer}
Vn = {noun-phrase, verb-phrase, article, noun, ---}.
S = initial symbol
P: 1) S → noun-phrase verb-phrase,
2) noun-phrase →noun,
3) noun-phrase → article noun,
4) verb-phrase → verb,
5) verb-phrase → verb noun-phrase,