Feature structure
Encyclopedia
In phrase structure grammar
s, such as generalised phrase structure grammar
, head-driven phrase structure grammar
and lexical functional grammar
, a feature structure is essentially a set of attribute-value pair
s. For example the attribute named number might have the value singular. The value of an attribute may be either atomic, e.g. the symbol singular, or complex (most commonly a feature structure, but also a list or a set).
A feature structure can be represented as a directed acyclic graph
(DAG), with the nodes corresponding to the variable values and the paths to the variable names. Operations defined on feature structures, e.g. unification
, are used extensively in phrase structure grammars. In most theories (e.g. HPSG), operations are strictly speaking defined over equations describing feature structures and not over feature structures themselves, though feature structures are usually used in informal exposition.
Often, feature structures are written like this:
Here there are the two features category and agreement. Category has the value noun phrase whereas the value of agreement is indicated by another feature structure with the features number and person being singular and third.
This particular notation is called attribute value matrix (AVM).
The matrix
has two columns, one for the feature names and the other for the values. In this sense a feature structure is a list of key-value pairs. The value might be atomic or another feature structure. This leads to another notation for feature structures: the use of tree
s. In fact, some systems (such as PATR-II
) use S-expression
s to represent feature structures.
Phrase structure grammar
The term phrase structure grammar was originally introduced by Noam Chomsky as the term for grammars as defined by phrase structure rules, i.e. rewrite rules of the type studied previously by Emil Post and Axel Thue...
s, such as generalised phrase structure grammar
Generalised phrase structure grammar
Generalised phrase structure grammar is a framework for describing the syntax and semantics of natural languages. It is a type of phrase structure grammar, as opposed to a dependency grammar. GPSG was initially developed in the late 1970s by Gerald Gazdar. Other contributors include Ewan Klein,...
, head-driven phrase structure grammar
Head-driven phrase structure grammar
Head-driven phrase structure grammar is a highly lexicalized, non-derivational generative grammar theory developed by Carl Pollard and Ivan Sag. It is the immediate successor to generalized phrase structure grammar. HPSG draws from other fields such as computer science and uses Ferdinand de...
and lexical functional grammar
Lexical functional grammar
Lexical functional grammar is a grammar framework in theoretical linguistics, a variety of generative grammar. It is a type of phrase structure grammar, as opposed to a dependency grammar. The development of the theory was initiated by Joan Bresnan and Ronald Kaplan in the 1970s, in reaction to...
, a feature structure is essentially a set of attribute-value pair
Attribute-value pair
A name–value pair, key–value pair, field–value pair or attribute–value pair is a fundamental data representation in computing systems and applications. Designers often desire an open-ended data structure that allows for future extension without modifying existing code or data...
s. For example the attribute named number might have the value singular. The value of an attribute may be either atomic, e.g. the symbol singular, or complex (most commonly a feature structure, but also a list or a set).
A feature structure can be represented as a directed acyclic graph
Directed acyclic graph
In mathematics and computer science, a directed acyclic graph , is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of...
(DAG), with the nodes corresponding to the variable values and the paths to the variable names. Operations defined on feature structures, e.g. unification
Semantic unification
Semantic unification, in philosophy, linguistics, and computer science, is the process of unifying lexically different concept representations that are judged to have the same semantic content ....
, are used extensively in phrase structure grammars. In most theories (e.g. HPSG), operations are strictly speaking defined over equations describing feature structures and not over feature structures themselves, though feature structures are usually used in informal exposition.
Often, feature structures are written like this:
Here there are the two features category and agreement. Category has the value noun phrase whereas the value of agreement is indicated by another feature structure with the features number and person being singular and third.
This particular notation is called attribute value matrix (AVM).
The matrix
Matrix (mathematics)
In mathematics, a matrix is a rectangular array of numbers, symbols, or expressions. The individual items in a matrix are called its elements or entries. An example of a matrix with six elements isMatrices of the same size can be added or subtracted element by element...
has two columns, one for the feature names and the other for the values. In this sense a feature structure is a list of key-value pairs. The value might be atomic or another feature structure. This leads to another notation for feature structures: the use of tree
Tree (data structure)
In computer science, a tree is a widely-used data structure that emulates a hierarchical tree structure with a set of linked nodes.Mathematically, it is an ordered directed tree, more specifically an arborescence: an acyclic connected graph where each node has zero or more children nodes and at...
s. In fact, some systems (such as PATR-II
PATR-II
PATR-II is a linguistic formalism used in computational linguistics, developed by Stuart M. Shieber. It uses context-free grammar rules and feature constraints on these rules.-External links:* , an implementation of PATR-II for PC and Unix systems....
) use S-expression
S-expression
S-expressions or sexps are list-based data structures that represent semi-structured data. An S-expression may be a nested list of smaller S-expressions. S-expressions are probably best known for their use in the Lisp family of programming languages...
s to represent feature structures.
External links
- Feature Structures section of an online PrologPrologProlog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
course - Feature Structures in Text Encoding for Interchange (TEIText Encoding InitiativeThe Text Encoding Initiative is a text-centric community of practice in the academic field of digital humanities. The community runs a mailing list, meetings and conference series, and maintains a technical standard, a wiki and a toolset....
)