Computational Linguistics Introduction

1
Computational Linguistics Introduction

• Approaches to Parsing

2
Grammar versus Parsing

• A grammar is a description of a language.
• A grammar abstractly associates structures with
  all and only the strings of the grammar.
• A parser is an implementation of an algorithm
  that actually discovers the structures assigned
  by a grammar to a sentence.
• Typically there may be several different parsing
  algorithms for achieving this.
• Top down strategy
• Bottom up strategy

3
Parse Tree

• A valid parse tree for a grammar G is a tree
• whose root is the start symbol for G
• whose interior nodes are nonterminals of G
• whose children of a node T (from left to right)
  correspond to the symbols on the right hand side
  of some production for T in G.
• whose leaf nodes are terminal symbols of G.
• Every sentence generated by a grammar has a
  corresponding parse tree
• Every valid parse tree exactly covers a sentence
  generated by the grammar

4
Parsing Problem

• Given grammar G and sentence A find all valid
  parse trees for G that exactly cover A

S
VP
NP
V
Nom
Det
book
N
that
flight
5
Top Down

• Top down parser tries to build from the root node
  S down to the leaves by replacing nodes with
  non-terminal labels with RHS of corresponding
  grammar rules.
• Nodes with pre-terminal (word class) labels are
  compared to input words.

6
Top Down Search Space
Start node ?
Goal node ?
7
Bottom Up

• Each state is a forest of trees.
• Start node is a forest of nodes labelled with
  pre-terminal categories (word classes derived
  from lexicon)
• Transformations look for places where RHS of
  rules can fit.
• Any such place is replaced with a node labelled
  with LHS of rule.

8
Bottom Up Search Space
fl
fl
fl
fl
fl
fl
fl
9
Top Down vs Bottom UpGeneral

• Top down
• For Never wastes time exploring trees that
  cannot be derived from S
• Against Can generate trees that are not
  consistent with the input

• Bottom up
• For Never wastes time building trees that cannot
  lead to input text segments.
• Against Can generate subtrees that can never
  lead to an S node.

10
Development of a Concrete Strategy

• Try to combine best features of both top down and
  bottom up strategies.
• Use the grammar to control the parsing process
• Top down, grammar directed control.
• Wherever possible use the text to eliminate
  impossible hypotheses
• Bottom up filtering.