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We introduce a new approach to transitionbased dependency parsing in which the parser does not directly construct a dependency structure, but rather an undirected graph, which is then converted into a directed dependency tree in a post-processing step. This alleviates error propagation, since undirected parsers do not need to observe the single-head constraint. Undirected parsers can be obtained by simplifying existing transition-based parsers satisfying certain conditions. We apply this approach to obtain undirected variants of the planar and 2-planar parsers and of Covington’s non-projective parser. . | Dependency Parsing with Undirected Graphs Carlos Gomez-Rodriguez Departamento de Computation Universidade da Coruna Campus de Elvina 15071 A Coruna Spain carlos.gomez@udc.es Daniel Fernandez-Gonzalez Departamento de Informatica Universidade de Vigo Campus As Lagoas 32004 Ourense Spain danifg@uvigo.es Abstract We introduce a new approach to transitionbased dependency parsing in which the parser does not directly construct a dependency structure but rather an undirected graph which is then converted into a directed dependency tree in a post-processing step. This alleviates error propagation since undirected parsers do not need to observe the single-head constraint. Undirected parsers can be obtained by simplifying existing transition-based parsers satisfying certain conditions. We apply this approach to obtain undirected variants of the planar and 2-planar parsers and of Covington s non-projective parser. We perform experiments on several datasets from the CoNLL-X shared task showing that these variants outperform the original directed algorithms in most of the cases. 1 Introduction Dependency parsing has proven to be very useful for natural language processing tasks. Data-driven dependency parsers such as those by Nivre et al. 2004 McDonald et al. 2005 Titov and Henderson 2007 Martins et al. 2009 or Huang and Sagae 2010 are accurate and efficient they can be trained from annotated data without the need for a grammar and they provide a simple representation of syntax that maps to predicateargument structure in a straightforward way. In particular transition-based dependency parsers Nivre 2008 are a type of dependency parsing algorithms which use a model that scores transitions between parser states. Greedy deterministic search can be used to select the transition to be taken at each state thus achieving linear or quadratic time complexity. 0 1 2 3 Figure 1 An example dependency structure where transition-based parsers enforcing the single-head constraint will incur .
