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<Paper uid="W98-0508">
  <Title>I I l I I i I I i I I II I I I I I I ! On Parsing Binary Dependency Structures Deterministically in Linear Time Hard ARNOLA I Kielikone Oy</Title>
  <Section position="1" start_page="0" end_page="68" type="abstr">
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Abstract
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    <Paragraph position="0"> In this paper we demonstrate that it is possible to parse dependency structures deterministically in linear time using syntactic heuristic choices. We in'st prove theoretically that deterministic, linear parsing of dependency structures is possible under certain conditions. We then discuss a fully implemented parser and argue that those conditions hold for at least one natural language. Empirical data demonstrates that the parsing time is indeed linear. The present quality of the parser in terms of finding the right dependency structure for sentences is about 85%.</Paragraph>
    <Paragraph position="1"> Introduction Natural language sentences have ambiguities at many levels of abstraction. Since present computational algorithms can handle only partial structures, one after another, these ambiguities cause problems for parsing. A common solution is to create ahemative structures in parallel, and explore a forest of possible trees in hope that the right parse tree will appear among them. This solution for processing ambiguities in parsing creates two new problems. Which tree is the right one among many in a forest.'? Furthermore, in the process of creating alternative structures, the number of partial trees tends to grow exponentially or at least polynomially with the number of words of a sentence. That in turn implies similar growth in the processing time.</Paragraph>
    <Paragraph position="2"> If a parsing algorithm were able to make confidently only the right local structural choices for a sentence, it would deterministically produce only a single, correct tree. The benefits would be obvious: there would be no search for the right tree in a forest, and the processing time could be benign. However, to our best knowledge, no one has yet been able to produce a deterministic parser for a constituent analysis of sentences.</Paragraph>
    <Paragraph position="3"> A dependency theory of syntactic structure indicates syntactic relations directly between the words of a sentence (e.g., Hays, 1964; Hudson, 1976, Hellwig, 1986; Mel'chuk, 1988; Robinson, 1970; Schubert, 1986; Starosta, 1988). We have studied the parsing of dependency structures over several years (Nelimarkka et al. 1984, Jiippinen et al, 1986, Valkonen et al., 1987). In this paper we discuss the final version of our fully implementod dependency parser and show that h is possible to design a heuristic deterministic dependency parser that parses sentences in linear time. The parser chooses heuristically only one direct governor among alternatives for each word in a sentence. Such a deterministic parser runs a great potential risk that at some point a wrong choice is made and the right parse tree is missed. We demonslrate empirically that the quality of the deterministic l Formerly Hard Jfippinen Current address: Ganesa Oy, It. Teatterik. 1 D 22, 00100 Helsinki, Finland. E-mail: harri@kielikone.fi  parser can be maintained on a satisfactory level. We first discuss deterministic parsing theoretically and then proceed to discuss the implemented parser.</Paragraph>
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