Converting Dependency Treebank to Constituency Treebank for Persian

There are two major types of treebanks: dependency-based and constituency-based. Both of them have applications in natural language processing and computational linguistics. Several dependency treebanks have been developed for Persian. However, there is no available big size constituency treebank for this language. In this paper, we aim to propose an algorithm for automatic conversion of a dependency treebank to a constituency treebank for Persian. Our method is based on an existing method. However, we make modification to enhance its accuracy. The base algorithm constructs a constituency structure according to a set of conversion rules. Each rule maps a dependency relation to a constituency subtree. The constituency structure is built by combining these subtrees. We investigate the effects of the order in which dependency relations are processed on the output constituency structure. We show that the best order depends on the charactersitics of the target language. We also make modification in the algorithm for matching the conversion rules. To match a dependency relation to a conversion rule, we start with detailed infromation and if no match was found, we decrease the details and also change the method for matching. We also make modification in the algorithm used for combining the constituency subtrees. We use statistical data derived from a treebank to find a proper position for attaching a constituency subtree to the projection chain of the head. The expremental results show that these modifications provide an improvement of 16.48% in the accuracy of the conversion algorithm.