the cyk parsing method (2) cornelia kempa carla parra escartín ws 2006-2007

THE CYK PARSING METHOD (2)

Cornelia Kempa

Carla Parra Escartín

WS 2006-2007

Tübingen, 4th December 2006 2

AGENDA

1. Building the recognition table2. Undoing the effect of the CNF transformation

2.1. Overview2.2. Adding removed elements2.3. Parsing

2.3.1. Methodology2.3.2. Left-most derivation

3. A short retrospective of CYK4. Chart Parsing


AGENDA






2. Undoing the effect of the CNF transformation

2.1. Overview (1)



2.1. Overview (2)



2.1. Overview (3)


AGENDA


2.1. Overview

2.2. Adding removed elements2.3. Parsing




2.2. Adding removed elements (1)



CYK does not require all non-terminals reachable.

Rule added!!!

+




New line




Set Rsi, 0

non-terminals that can be considered as possibly occurring just in front of symbol zi

Set Rsn +1,0

non-terminals that can occur at the end of the sentence.




AGENDA


2.1. Overview2.2. Adding removed elements

2.3. Parsing2.3.1. Methodology2.3.2. Left-most derivation



Derivation starts with start-symbol S.Is A1A2···Am a right hand side of S?

Does A1A2···Am derive S1,n?

We check this starting with A1

2 cases:

2. Undoing the effect of the CNF transformation2.3. Parsing

2.3.1. Methodology (1)


A) A1 is a terminal symbol

it must be the first symbol of s1,n

then check if A2···Am derives s2,n-1

B) A1 is a non-terminal symbol

it must be a member of Rs1,k

then check if A2···Am derives s2,k+1,n-k




Now: determined whether the rule is applicable

if so, which parts of the rule derive which substrings.

Next step: determine how the substrings can be derived.




AGENDA



2.3.1. Methodology

2.3.2. Left-most derivation




2.3.2. Left-most derivation (1)


This is what we saw so far:

If we continue... The derivation is:




AGENDA






First: recognition table using the original grammar.

Then: transforming grammar to CNF.

3. A short retrospective of CYK (1)


For efficiency, CNF is a bit too restrictive

Disadvantage of CNF: • Resulting recognition table lacks the

information we need to construct a derivation using the original grammar!



In the transformation process, some non-terminals were thrown away

(non-productive)Missing information could be added.



Result: almost the same recognition table.• Extra information on non-terminals

• Obtained in a simpler and much more efficient way.



AGENDA




3. A short retrospective of CYK

4. Chart Parsing


CYK algorithm also known as chart parsing.

Both techniques variants

(some of them identical among the techniques)

Difference: implementation.

4. Chart parsing (1)


A chart is just a recognition table.

4. Chart parsing (2)


FINAL OVERVIEW (1)

1. Building the recognition table

2. Possible Derivations

3. Undoing the effect of the CNF transformation.

4. A short retrospective of CYK

5. Chart Parsing


PARSING METHODS

Non-directional methods

Top-down Bottom-up

Unger Parser CYK Parser

FINAL OVERVIEW (2)

THANK YOU VERY MUCH

the cyk parsing method (2) cornelia kempa carla parra escartín ws 2006-2007

Documents

cnf transformation2

cnf transformationtbingen

removed elements2

recognition tableundoing

nonterminal symbol

symbol of s1

startsymbol s

nonterminals reachable