CSA3180: NLP Algorithms

October 2008 CSA3180: Sentence Parsing 1

CSA3180: NLP Algorithms

Sentence Parsing Algorithms 2

Problems with DFTD Parser


Problems withDFTD Parser

• Left Recursion• Handling Ambiguity• Inefficiency


Left Recursion

• A grammar is left recursive if it contains at least one non-terminal A for whichA * A and *

(n.b. * is the transitive closure of )• Intuitive idea: derivation of that category

includes itself along its leftmost branch.

NP NP PP NP NP and NP

NP DetP Nominal DetP NP ' s


Left RecursionLeft recursion can lead to an infinite loop

[nltk demo


Dealing with Left Recursion

• Use different parsing strategy• Reformulate the grammar to

eliminate LRA A |

is rewritten asA A' A' A' |


Rewriting the Grammar

NP → NP ‘and’ NP

NP → D N | D N PP




β

NP → D N | D N PP

α




β

NP → D N | D N PP

α

New Grammar

NP → α NP1

NP1 → β NP1 | ε




β

NP → D N | D N PP

α

New Grammar

NP → α NP1

NP1 → β NP1 | ε

α → D N | D N PP

β → ‘and’ NP


New Parse Tree

D N

the cat

α

NP

NP1

ε



• Different parse tree• Unnatural parse tree?





Handling Ambiguity

• Coordination Ambiguity: different scope of conjunction:Hot curry and ice taste nice with riceHot curry and rice taste nice with ice

• Attachment Ambiguity: a constituent can be added to the parse tree in different places:I shot an elephant in my trousers

• VP → VP PPNP → NP PP


Real sentences are full of ambiguities

President Kennedy today pushed aside other White House business to devote all his time and attention to working on the Berlin crisis address he will deliver tomorrow night to the American people over nationwide television and radio


Prepositional Phrase Ambiguity

No of PPs # parses

2 2

3 5

4 14

5 132

6 469

7 1430

8 4867

he will deliver- to the American people- over nationwide TV- in New York- during September- for very good reasons


Growth of Number of Ambiguities

The nth Catalan number counts the ways of dissecting a polygon with n+2 sides into triangles by drawing nonintersecting diagonals. No of PPs # parses

2 2

3 5

4 14

5 132

6 469

7 1430

8 4867


Handling Ambiguities

• Statistical disambiguation– which is the most probable interpretation?

• Semantic knowledge– which is the most sensible interpretation?– Subatomic particles such as positively charged

protons and electrons





Repeated Parsing of Subtrees

• Local versus global ambiguity.– NP → Det Noun– NP → NP PP

• Because of the top down depth first, left to right policy, the parser builds trees that fail because they do not cover all of the input.

• Successive parses cover larger segments of the input, but these include structures that have already been built before.


Repeated Parsing ofSubtrees

a flight 4

from Indianapolis 3

to Houston 2

on TWA 1

A flight from Indianapolis 3

A flight from Indianapolis to Houston

2

A flight from Indianapolis to Houston on TWA

1

NP Nom

Det Noun

a flight

NP

NP PP Nom

Det Noun P Noun

a flight from Indianapolis


Repeated Parsing ofSubtrees

a flight 4

from Indianapolis 3

to Houston 2

on TWA 1

A flight from Indianapolis 3

A flight from Indianapolis to Houston

2

A flight from Indianapolis to Houston on TWA

1

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CSA3180: NLP Algorithms