Features and augmented grammars
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Features and Augmented Grammars. Allen ’ s Chapter 4 J&M ’ s Chapter 11. Augmenting CFGs with Features. Certain linguistic constraints are not naturally described via CFGs Example: Number Agreement between constituents - “a boys” Possible to describe using refined CF rules:

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Features and Augmented Grammars

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Features and augmented grammars

Features and Augmented Grammars

Allen’s Chapter 4

J&M’s Chapter 11


Augmenting cfgs with features

Augmenting CFGs with Features

  • Certain linguistic constraints are not naturally described via CFGs

  • Example: Number Agreement between constituents - “a boys”

  • Possible to describe using refined CF rules:

    NP-Sing --> ART-Sing N-Sing

    NP-Plural --> ART-Plural N-Plural

  • Much more natural to describe via a single feature Number in an augmented CF rule:

    NP --> ART N (only when Number1 = Number2)


Feature structures

Feature Structures

  • Constituents can be defined as feature structures that map features to values

  • Features can be shared between constituents

  • Some basic features for English:

    – Number, Gender and Person agreement

    – Verb form features and sub-categorizations

  • Complex Feature Structures: Feature values can themselves be feature structures


An example

An Example

ART1: (CAT ART

ROOT a

NUMBER s)

Represents a particular use of word a

Often summarized as:

ART1: (ART ROOT a NUMBER s)


Larger constituents

Larger constituents:

Representing an NP constituent for the phrase a fish

NP1: (NP NUMBER s

1 (ART ROOT a

NUMBER s)

2 (N ROOT fish

NUMBER s))


Feature structure

Feature Structure


Augmented rules

Augmented Rules

(NP NUMBER ?n) 

(ART NUMBER ?n) (N NUMBER ?n)

*(NP 1 (ART NUMBER s)

2 (N NUMBER s))

*(NP NUMBER s

1 (ART NUMBER s)

2 (N NUMBER p))


Using variables to express ambiguities

Using Variables to Express ambiguities

(N ROOT fish NUMBER ?n)

(N ROOT fish NUMBER ?b {s, p})

(N ROOT fish NUMBER {s, p})


Person and number features

Person and Number Features


Pform feature

PForm Feature


Verb sub categorizations subcat feature

Verb Sub categorizations:SUBCAT Feature


Subcat examples

SUBCAT Examples:

(VP)  (V SUBCAT _np_vp:inf)

(NP)

(VP VFORM inf)

(VP)  (V SUBCAT _np_pp:loc)

(NP)

(PP PFORM LOC)


Binary features

Binary Features

Whether the constituent does or does not have a feature

INV feature has two values + and –

Jack Laughed

has an INV feature with value – = (-INV)

Did Jack Laugh

has an INV feature with value + = (+INV)


Morphological analysis

Morphological Analysis


Sample grammar abbreviated form

Sample Grammar (Abbreviated Form)


Sample grammar detailed form

Sample Grammar (Detailed Form)


Parsing with features

Parsing with Features

  • (NP AGR ?a)   (ART AGR ?a) (N AGR ?a)

  • (ART ROOT A AGR 3s)

  • (NP AGR 3s)   (ART AGR 3s) (N AGR 3s)

  • (NP AGR 3s)  (ART AGR 3s)  (N AGR 3s)

  • (N ROOT DOG1 AGR 3s)

  • (NP AGR 3s)  (ART AGR 3s) (N AGR 3s) 


Parsing with features1

Parsing with Features

  • Given an arc A, and NEXT as the constituent following , and a new constituent X, which is used to extend A

  • Find an instantiation of variables such that all features specified in NEXT are found in X.

  • Create a new arc A’, which is a copy of A except for the instantiations of the variables determined in step 1

  • Update A’ as usual in a Chart parser


Parse trees with features

Parse Trees with Features


Chart with features

Chart with Features


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