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

One important issue in the process of parsing is the selection of the rewrite
rules at any step of expanding the parse tree. One simple way to explore
this is to expand a non-terminal by the first possible means and check whether
the expansion supports the part of the sentence. If yes, the selection of the rewrite
rule is done correctly. If no, then a backtracking has to be made upto the
node in the tree, just expanded, delete the older expansions from that node and
select a second alternative re-write rule for expansion. The process is to be
continued recursively until all the productions terminate to terminal nodes.
Such type of parsing is called recursive descent parsing. An alternative
approach is to check the input string prior to selection of the re-write rule,
called look-ahead parsing. It is also prevalent in the literature [6]. For the
purpose of computing, the parse tree can be realized by a structure with
pointers and list. The representation of the parse tree of fig.18.2 by pointers to
structures and list is presented below.
N
the
killed
S
VP
ART
NP
man a deer
Fi Pointer to structure representation of the parse tree of fig. 18.2.
Since the re-write rules have at most two symbols at the right hand
side, we kept two pointer fields and one name field of each structure. The null
pointers have been denoted by crossed lines within the field.
now.
ART
NP
V
The list representation of the parse tree, shown in fig.18.2 is presented
N