Editing Probabilistic context-free grammar (section)

==Definitions==
'''Derivation:'''  The process of recursive generation of strings from a grammar.
                                           
'''[[Parsing]]:''' Finding a valid derivation using an automaton.

'''[[Parse tree|Parse Tree:]]''' The alignment of the grammar to a sequence.

An example of a parser for PCFG grammars is the [[pushdown automaton]]. The algorithm parses grammar nonterminals from left to right in a [[Stack (abstract data type)#Applications|stack-like]] manner. This [[Brute-force search|brute-force]] approach is not very efficient. In RNA secondary structure prediction variants of the '''[[CYK algorithm|Cocke–Younger–Kasami (CYK) algorithm]]''' provide more efficient alternatives to grammar parsing than pushdown automata.<ref name="Durbin 1998" /> Another example of a  PCFG parser is the Stanford Statistical Parser which has been trained using [[Treebank]].<ref>{{cite journal|last=Klein|first=Daniel|author2=Manning, Christopher |title=Accurate Unlexicalized Parsing|journal=Proceedings of the 41st Meeting of the Association for Computational Linguistics|year=2003|pages=423–430|url=http://nlp.stanford.edu/~manning/papers/unlexicalized-parsing.pdf}}</ref>