ANTONIS ANASTASOPOULOS CS499 INTRODUCTION TO NLP PARSING WITH CFGS.

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PARSING WITH CFG ^S

ANTONIS ANASTASOPOULOS CS499 INTRODUCTION TO NLP

https://cs.gmu.edu/~antonis/course/cs499-spring21/

With adapted slides by David Mortensen and Alan Black

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LEVELS OF REPRESENTATION

2

discourse pragmatics

semantics syntax lexemes morphology

orthography phonology

phonetics

(text)

(speech)

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STRUCTURE OF THIS LECTURE

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1 Context-free 2 3

Grammars Parsing

Algorithms Chomsky

Normal Form ⁴ CKY

Algorithm

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CONTEXT-FREE GRAMMARS

Using grammars:

- recognition - parsing

Parsing algorithms:

- top down - bottom up

Chomsky Normal Form (CNF)

CKY Algorithm (Cocke-Younger-Kasami)

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PARSING VS WORD MATCHING

Consider the following sentence:

The student who was taught be Dr. A won the prize.

Who won the prize?

String matching:

“Dr. A won the prize.”

Parsing based

[[The student [who was taught be Dr. A]] won the prize.]

“the student won the prize”

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CONTEXT-FREE GRAMMARS

Vocabulary of terminal symbols:

Set of non-terminal symbols (aka variables):

Special start symbols:

Production rules of the form , where

Σ

N S ∈ N

X → α X ∈ N

α ∈ (N ∪ Σ)*

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TWO RELATED PROBLEMS

Input: sentence and CFG Output (recognition): tree iff

Output (parsing) one or more derivations for , under .

w = (w ₁ , …w _n ) 𝒢

w ∈ ^Language (𝒢)

w 𝒢

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PARSING AS SEARCH

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S

w

₁

w

₂

… w

_n

top down bottom-up

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RECOGNIZERS AS SEARCH

Agenda = { state0 }

while(Agenda not empty)

s = pop a state from Agenda

if s is a success-state return s // valid parse tree else if s is not a failure-state:

generate new states from s

push new states onto Agenda return nil

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EXAMPLE GRAMMAR AND LEXIVON

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CHOMSKY NORMAL

FORM

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CONTEXT-FREE GRAMMARS IN CHOMSKY NORMAL FORM

Vocabulary of terminal symbols:

Set of non-terminal symbols (aka variables):

Special start symbols:

Production rules of the form , where

Σ

N S ∈ N

X → α X ∈ N

α ∈ N, N ∪ Σ

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CONVERTING CFGS TO CNF

For each rule

Rewrite as:

(Introduces a new non-terminal)

X → A B C

X → A X ₂ X ₂ → B C

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CKY ALGORITHM

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CKY ALGORITHM

For

For // width

For // left boundary // right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w _i } l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

C[i, k] = C[i, k] ∪ {V ∣ V → YZ, Y ∈ C[i, j], Z ∈ C[j, k]}

S ∈ C[0,n]

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CKY: CHART

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For

For // width

For // left boundary

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

C[i, k] = C[i, k] ∪ {V ∣ V → YZ, Y ∈ C[i, j], Z ∈ C[j, k]}

S ∈ C[0,n]

Book

this

flight

through

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun Book

this

flight

through

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun, Verb Book

this

flight

through

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun, Verb

Book Det

this Noun

flight Prep

through PNoun

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun, Verb

Book Det

this Noun

flight Prep

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det

this Noun

flight Prep

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det NP

this Noun

flight Prep

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det NP

this Noun ——

flight Prep

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det NP ——

this Noun ——

flight Prep

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det NP ——

this Noun ——

flight Prep PP

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det NP ——

this Noun —— ——

flight Prep PP

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb ——

Book Det NP —— NP

this Noun —— ——

flight Prep PP

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb —— VP

Book Det NP —— NP

this Noun —— ——

flight Prep PP

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb —— VP,

S

Book Det NP —— NP

this Noun —— ——

flight Prep PP

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb —— VP,

S ——

Book Det NP —— NP

this Noun —— ——

flight Prep PP

through PNoun, NP

Houston

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CKY: CHART

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For

For // width

// right boundary

For // midpoint

Return true if

i = [1 … n]

C[i − 1, i] = {V ∣ V → w_i} l = 2 … n :

i = 0 … n − l : k = i + l

j = i + 1 … k − 1 :

S ∈ C[0,n]

Noun,

Verb —— VP,

S —— S

Book Det NP —— NP

this Noun —— ——

flight Prep PP

through PNoun, NP

Houston

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CKY EQUATIONS

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CKY COMPLEXITY

Worst case:

Best is also worst case

(Others better in average case)

𝒪(n ³ ⋅ |𝒢|)

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NEXT CLASS

Treebanks and probabilistic CFGs

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ANTONIS ANASTASOPOULOS CS499 INTRODUCTION TO NLP PARSING WITH CFGS.

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PARSING WITH CFG S