Parsing Based on Scattered Context Grammars

Tato práce se zabývá novými možnostmi překladu na základě pokročilých formálních modelů. Konkrétně využitím gramatik s rozptýleným kontextem v syntaktické analýze. Byl vytvořen kontextový jazyk, na kterém jsou porovnávány dvě vybrané metody syntaktické analýzy, kde jsou tyto gramatiky využívány. Závěrem je diskutováno možné využití v praxi, paralelní analýza, výhody, nevýhody a možný další vývoj.This work deals with new possibilities in parsing based on advanced formal models, specifically using scattered context grammars in syntactic analysis. Context-sensitive language was created for comparing different methods of syntactic analysis, where these grammars are used. In conclusion is discussed possible use in practise, paralell parsing, pros and cons and further investigation.

Descriptional Complexity of Three-Nonterminal Scattered Context Grammars: An Improvement

Recently, it has been shown that every recursively enumerable language can be generated by a scattered context grammar with no more than three nonterminals. However, in that construction, the maximal number of nonterminals simultaneously rewritten during a derivation step depends on many factors, such as the cardinality of the alphabet of the generated language and the structure of the generated language itself. This paper improves the result by showing that the maximal number of nonterminals simultaneously rewritten during any derivation step can be limited by a small constant regardless of other factors

An infinite hierarchy of language families generated by scattered context grammars with n-limited derivations

AbstractThis paper introduces scattered context grammars without erasing productions, in which an application of a production always occurs within the first n nonterminals of the current sentential form. It demonstrates that this restriction gives rise to an infinite hierarchy of language families each of which is properly included in the family of context-sensitive languages. In addition, it proves analogous results for unordered scattered context grammars. Some consequences of these results are derived and open problems formulated

Accepting grammars and systems

We investigate several kinds of regulated rewriting (programmed, matrix, with regular control, ordered, and variants thereof) and of parallel rewriting mechanisms (Lindenmayer systems, uniformly limited Lindenmayer systems, limited Lindenmayer systems and scattered context grammars) as accepting devices, in contrast with the usual generating mode. In some cases, accepting mode turns out to be just as powerful as generating mode, e.g. within the grammars of the Chomsky hierarchy, within random context, regular control, L systems, uniformly limited L systems, scattered context. Most of these equivalences can be proved using a metatheorem on so-called context condition grammars. In case of matrix grammars and programmed grammars without appearance checking, a straightforward construction leads to the desired equivalence result. Interestingly, accepting devices are (strictly) more powerful than their generating counterparts in case of ordered grammars, programmed and matrix grammars with appearance checking (even programmed grammarsm with unconditional transfer), and 1lET0L systems. More precisely, if we admit erasing productions, we arrive at new characterizations of the recursivley enumerable languages, and if we do not admit them, we get new characterizations of the context-sensitive languages. Moreover, we supplement the published literature showing: - The emptiness and membership problems are recursivley solvable for generating ordered grammars, even if we admit erasing productions. - Uniformly limited propagating systems can be simulated by programmed grammars without erasing and without appearance checking, hence the emptiness and membership problems are recursively solvable for such systems. - We briefly discuss the degree of nondeterminism and the degree of synchronization for devices with limited parallelism

Derivation in Scattered Context Grammar via Lazy Function Evaluation

This paper discusses scattered context grammars (SCG) and considers the application of scattered context grammar production rules. We use function that represents single derivation step over the given sentential form. Moreover, we define this function in such a way, so that it represents the delayed execution of scattered context grammar production rules using the same principles as a lazy evaluation in functional programming. Finally, we prove equivalence of the usual and the delayed execution of SCG production rules

Generation of sentences with their parses : the case of propagating scattered context grammars

Propagating scattered context grammars are used to generate their sentences together with their parses - that is, the sequences of labels denoting productions whose use lead to the generation of the corresponding sentences. It is proved that for every recursively enumerable language L, there exists a propagating scattered context grammar whose language consists of L's sentences followed by their parses

Two power-decreasing derivation restrictions in generalized scattered context grammars

The present paper introduces and discusses generalized scattered context grammars that are based upon sequences of productions whose left-hand sides are formed by nonterminal strings, not just single nonterminals. It places two restrictions on the derivations in these grammars. More specifically, let k be a positive integer. The first restriction requires that all rewritten symbols occur within the first k symbols of the first continuous block of nonterminals in the sentential form during every derivation step. The other restriction defines derivations over sentential forms containing no more than k occurrences of nonterminals. As its main result, the paper demonstrates that both restrictions decrease the generative power of these grammars to the power of context-free grammars

Scattered Context Grammars and Their Applications in Natural Language Analysis

Obsahem práce je vytvoření metody, která by umožnila automatizované zpracování přirozeného jazyka s využitím gramatik s rozptýleným kontextem. Toho je dosaženo zavedením upravené metody syntaktické analýzy shora dolů, přičemž součástí práce je i implementace algoritmu v jazyce Python. K tomu je využita knihovna NLTK, která slouží k získávání větných členů z anglických vět. Výsledný program je modulární a je schopen zpracovat libovolnou gramatiku v korektním formátu. The purpose of this thesis is to create a method, based on scattered context grammars, for natural language analysis. The method proposed in this thesis, is similiar to syntax analysis for context-free grammars. Implementation of the proposed algorithm is attached. The programm is written in Python 2.7 and the module NLTK is needed. The purpose of NLTK is to parse English sentence and return the lexems