Topic 4: Grammar & Parsing

Question 1

What is syntax?

Answer 1

Refers to the way words are arranged together

Question 2

Constituency

Answer 2

group of words behaving as a single unit or constituent
fundamentals of developing grammar

example
noun phrase, a sequence of words surrounding at least one noun

Question 3

Evidence for constituency

Answer 3

words can all appear in a similar syntactic environments.
give example

noun phrase can occur before verbs

preposed or postposed constructions
example of prepositional phrase “ on September seventeenth” can be placed in ….

Question 4

Context Free Grammar

Answer 4

formal system for modeling constituent structure in English and other natural languages

also known as phrase-structure grammars

consists of a set of rules or productions

the rule expresses ways symbol of the language grouped and ordered together and lexicon of words and symbols

Question 5

Some rules for noun phrase

Answer 5

NP -> Det Nominal
NP -> ProperNoun
Nominal -> Noun | Nominal Noun

example derivation
NP -> Det Nom -> the Nom Noun -> the Noun Noun -> the morning noun -> the morning flight

Question 6

2 Classes of symbols

Answer 6

terminal - corresponds to words in the language
non-terminal - express abstractions over the terminals

item on left of the arrow is a single non-terminal symbol.
right side is an ordered list of one or more terminals or non-terminals

Question 7

Function of CFG

Answer 7

• ways for generating structures

- ways to assign structure to a given sentence.

Question 8

Derivation

Answer 8

the sequence of rule expansions is called derivation of the string of words.

Question 9

Parse Tree

Answer 9

derivation can be represented with parse tree

Question 10

More rules on Eng CFG

Answer 10

Give examples

S -> NP VP
VP -> VERB NP
VP -> VERB PP
PP -> PREPOSITION NP

Question 11

Sample Lexicon and Sample Grammar

Answer 11

examples …

Question 12

Bracketed Notation

Answer 12

used to represent parse tree in a more compact format

give example

Question 13

Grammatical Sentence

Answer 13

sentences that can be derived by a grammar defined by the grammar in the formal language.

Question 14

Ungrammatical sentences

Answer 14

sentences that cannot be derived by a given formal grammar

Question 15

Four parameters in CFG

Answer 15

non-terminal symbols
terminal symbols
rules
start symbol

Question 16

Sentence-Level construction

Answer 16

Declarative - S -> NP VP

Imperative - S -> VP (VP and no subject)

Yes-no question - S -> AV NP VP

Wh-subject question - same with declarative except that noun phrase contains some wh-word S -> wh NP VP

wh-non-subject question - auxiliary appears before subject NP just as in the yes-no question S -> Wh-NP Aux NP VP

Question 17

Noun Phrase

Answer 17

Role of determiner can be filled by a possesive expression
Det -> NP ‘s

before head noun
relative pronoun
prepositional phrase

Question 18

Verb Phrase

Answer 18

consist of the verb and number of other constituents
VP -> Verb
VP -> Verb NP
VP -> Verb NP PP
VP -> Verb PP
VP -> Verb S

Question 19

Example possible constituents in VP

Answer 19

not every verb compatible with every VP

Question 20

More VP Rules

Answer 20

VP -> verb with no complement
VP -> verb with NP complement NP
VP -> verb with S complement S

Question 21

Coordination

Answer 21

Major phrases types discussed here can be cojoined with conjunctions to form larger construction of the same type.

example
NP -> NP and NP
VP -> VP and VP
S -> S and S

Question 22

Tree bank

Answer 22

syntactically annotated corpus

important role in parsing

used by parsers to automatically parse each sentence followed by human corrected the parses.

sufficiently robust grammars consists of CFG rules can be used to assign parse tree to any sentence.

Question 23

Constituency Parsing

Answer 23

syntactic parsing is a task of recognizing a sentence and assigning a syntactic structure to it

goal is to produce the correct tree

Question 24

Importance of parse tree

Answer 24

used in grammatical checking.

used as intermedia representation for semantic analysis

Question 25

Ambiguity

Answer 25

structural ambiguity occurs when grammar can assign more than one parse to a sentence

Question 26

2 types of ambiguity

Answer 26

attachment ambiguity
- constituent can be attached to a parse tree at more than one place

coordination ambiguity
- different set of phrases conjoined by conjunction
example
[old [ men and women ] ]
[old men] and [women]

Question 27

Syntactic disambiguation

Answer 27

to choose a single correct parse from multitude possible parses

algo requires statistical, semantic and contextual knowledge sources

Question 28

Statistical parsing

Answer 28

compute probability of each interpretation and choose most probable

Question 29

Probabilistic CFG

Answer 29

assigned rule based on probability
also known as stochastic context free grammar
used for disambiguation

PCFG differs from standard CFG by augmenting each rule in R with a conditional probability
A -> b [p]
P ( A -> b | A )
sum of all possible expansions of non-terminal probabilities must be one

Question 30

exercise PCFG

Answer 30

give probabilities of rules
give multiple parse tree
compute the probabilities of rules from the parse trees and select the PCFG with highest probability

Question 31

Dependency Parsing

Answer 31

Phrasal constituents and phrase-structure rules do not play a direct role

Question 32

Dependency Style Analysis

Answer 32

relations among words are illustrated with directed, labelled arcs from head to dependents.

known as typed dependency structure because labels are drowned from a fixed inventory of grammatical relations

Question 33

Advantages of dependency grammars

Answer 33

ability to deal with languages that are morphologically rich and have relatively free word order

the approach abstracts away from word-order information

Question 34

Core relations

Answer 34

clausal relations - syntactic roles with respect to a predicate

modifier relations - categorize ways the word can modify heads

give examples. . .