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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Context-free language ： ウィキペディア英語版 Context-free language In formal language theory, a context-free language (CFL) is a language generated by some context-free grammar (CFG). Different CF grammars can generate the same CF language. It is important to distinguish properties of the language (intrinsic properties) from properties of a particular grammar (extrinsic properties). The set of all context-free languages is identical to the set of languages accepted by pushdown automata, which makes these languages amenable to parsing. Indeed, given a CFG, there is a direct way to produce a pushdown automaton for the grammar (and corresponding language), though going the other way (producing a grammar given an automaton) is not as direct. Context-free languages have many applications in programming languages; for example, the language of all properly matched parentheses is generated by the grammar $S\backslash to\; SS\; ~|~\; (S)\; ~|~\; \backslash varepsilon$. Also, most arithmetic expressions are generated by context-free grammars. ==Examples== An archetypal context-free language is $L\; =\; \backslash$, the language of all non-empty even-length strings, the entire first halves of which are $a$'s, and the entire second halves of which are $b$'s. $L$ is generated by the grammar $S\backslash to\; aSb\; ~|~\; ab$. This language is not regular. It is accepted by the pushdown automaton $M=(\backslash ,\; \backslash ,\; \backslash ,\; \backslash delta,\; q\_0,\; z,\; \backslash )$ where $\backslash delta$ is defined as follows:〔meaning of $\backslash delta$'s arguments and results: $\backslash delta(\backslash mathrm\_1,\; \backslash mathrm,\; \backslash mathrm)\; =\; (\backslash mathrm\_2,\; \backslash mathrm)$〕 $\backslash delta(q\_0,\; a,\; z)\; =\; (q\_0,\; az)$ $\backslash delta(q\_0,\; a,\; a)\; =\; (q\_0,\; aa)$ $\backslash delta(q\_0,\; b,\; a)\; =\; (q\_1,\; \backslash varepsilon)$ $\backslash delta(q\_1,\; b,\; a)\; =\; (q\_1,\; \backslash varepsilon)$ Unambiguous CFLs are a proper subset of all CFLs: there are inherently ambiguous CFLs. An example of an inherently ambiguous CFL is the union of $\backslash$ with $\backslash$. This set is context-free, since the union of two context-free languages is always context-free. But there is no way to unambiguously parse strings in the (non-context-free) subset $\backslash$ which is the intersection of these two languages. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Context-free language」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.