Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
(5) Notation: Sequences are written in various ways:
abc
〈a, b, c〉
a, b, c
[a, b, c]
The programming language prolog requires the last format; otherwise, I try to choose the notation to
minimize confusion.
Similarly, the empty sequence is sometimes represented ɛ, but the prolog notation is [].
A stack is a sequence too, but with limitations on how we can access its elements: elements can only
be read or written on the “top” of the sequence. We adopt the convention that the top of a stack is on
the left, it is the “front” of the sequence.
(6) Notation: Context free grammars are commonly written in the familiar rewrite notation, which we will
use extensively in these notes:
S → NP VP
NP → DN VP→ VNP
NP → N VP → V
N → students V → sang
N → songs V → knew
D → some
D → all
These grammars are sometimes written in the more succinct Backus-Naur Form (BNF) notation:
S ::= NP VP
NP ::= D N | N VP ::= V NP | V
N ::= students | songs V ::= sang | knew
D ::= some | all
The categories on the left side of the ::= are expanded as indicated on the right, where the vertical bar
separates alternative expansions. (Sometimes in BNF, angle brackets or italics are used to distinguish
category from terminal symbols, rather than the capitalization that we have used here.) This kind of
BNF notation is often used by logicians, and we will use it in the following chapter.
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