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Marrying words and trees

Published:11 June 2007Publication History

ABSTRACT

Traditionally, data that has both linear and hierarchical structure, such as annotated linguistic data, is modeled using ordered trees and queried using tree automata. In this paper, we argue that nested words and automata over nested words offer a better way to capture and process the dual structure. Nested words generalize both words and ordered trees, and allow both word and tree operations. We study various classes of automata over nested words, and show that while they enjoy expressiveness and succinctness benefits over word and tree automata, their analysis complexity and closure properties are analogous to the corresponding word and tree special cases. In particular, we show that finite-state nested word automata can be exponentially more succinct than tree automata, and pushdown nested word automata include the two incomparable classes of context-free word languages and context-free tree languages.

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          • Published in

            cover image ACM Conferences
            PODS '07: Proceedings of the twenty-sixth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems
            June 2007
            328 pages
            ISBN:9781595936851
            DOI:10.1145/1265530

            Copyright © 2007 ACM

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            Association for Computing Machinery

            New York, NY, United States

            Publication History

            • Published: 11 June 2007

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