Context-free Language
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Context-free Language

In formal language theory, a context-free language (CFL) is a language generated by a context-free grammar (CFG).

Context-free languages have many applications in programming languages, in particular, most arithmetic expressions are generated by context-free grammars.

Background

Context-free grammar

Different context-free grammars can generate the same context-free language. Intrinsic properties of the language can be distinguished from extrinsic properties of a particular grammar by comparing multiple grammars that describe the language.

Automata

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. Further, for a given CFG, there is a direct way to produce a pushdown automaton for the grammar (and thereby the corresponding language), though going the other way (producing a grammar given an automaton) is not as direct.

Examples

A model context-free language is , 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 . This language is not regular. It is accepted by the pushdown automaton where is defined as follows:[note 1]

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 with . 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 which is the intersection of these two languages.[1]

Dyck language

The language of all properly matched parentheses is generated by the grammar .

Properties

Context-free parsing

The context-free nature of the language makes it simple to parse with a pushdown automaton.

Determining an instance of the membership problem; i.e. given a string , determine whether where is the language generated by a given grammar ; is also known as recognition. Context-free recognition for Chomsky normal form grammars was shown by Leslie G. Valiant to be reducible to boolean matrix multiplication, thus inheriting its complexity upper bound of O(n2.3728639).[2][note 2] Conversely, Lillian Lee has shown O(n3-?) boolean matrix multiplication to be reducible to O(n3-3?) CFG parsing, thus establishing some kind of lower bound for the latter.[3]

Practical uses of context-free languages require also to produce a derivation tree that exhibits the structure that the grammar associates with the given string. The process of producing this tree is called parsing. Known parsers have a time complexity that is cubic in the size of the string that is parsed.

Formally, the set of all context-free languages is identical to the set of languages accepted by pushdown automata (PDA). Parser algorithms for context-free languages include the CYK algorithm and Earley's Algorithm.

A special subclass of context-free languages are the deterministic context-free languages which are defined as the set of languages accepted by a deterministic pushdown automaton and can be parsed by a LR(k) parser.[4]

See also parsing expression grammar as an alternative approach to grammar and parser.

Closure

The class of context-free languages is closed under the following operations. That is, if L and P are context-free languages, the following languages are context-free as well:

  • the union of L and P[5]
  • the reversal of L[6]
  • the concatenation of L and P[5]
  • the Kleene star of L[5]
  • the image of L under a homomorphism [7]
  • the image of L under an inverse homomorphism [8]
  • the circular shift of L (the language )[9]
  • the prefix closure of L (the set of all prefixes of strings from L)[10]
  • the quotient L/R of L by a regular language R[11]

Nonclosure under intersection, complement, and difference

The context-free languages are not closed under intersection. This can be seen by taking the languages and , which are both context-free.[note 3] Their intersection is , which can be shown to be non-context-free by the pumping lemma for context-free languages. As a consequence, context-free languages cannot be closed under complementation, as for any languages A and B, their intersection can be expressed by union and complement: . In particular, context-free language cannot be closed under difference, since complement can be expressed by difference: .[12]

However, if L is a context-free language and D is a regular language then both their intersection and their difference are context-free languages.[13]

Decidability

In formal language theory, questions about regular languages are usually decidable, but ones about context-free languages are often not. It is decidable whether such a language is finite, but not whether it contains every possible string, is regular, is unambiguous, or is equivalent to a language with a different grammar.[14]

The following problems are undecidable for arbitrarily given context-free grammars A and B:

  • Equivalence: is ?[15]
  • Disjointness: is  ?[16] However, the intersection of a context-free language and a regular language is context-free,[17][18] hence the variant of the problem where B is a regular grammar is decidable (see "Emptiness" below).
  • Containment: is  ?[19] Again, the variant of the problem where B is a regular grammar is decidable,[] while that where A is regular is generally not.[20]
  • Universality: is  ?[21]

The following problems are decidable for arbitrary context-free languages:

  • Emptiness: Given a context-free grammar A, is  ?[22]
  • Finiteness: Given a context-free grammar A, is finite?[23]
  • Membership: Given a context-free grammar G, and a word , does  ? Efficient polynomial-time algorithms for the membership problem are the CYK algorithm and Earley's Algorithm.

According to Hopcroft, Motwani, Ullman (2003),[24] many of the fundamental closure and (un)decidability properties of context-free languages were shown in the 1961 paper of Bar-Hillel, Perles, and Shamir[25]

Languages that are not context-free

The set is a context-sensitive language, but there does not exist a context-free grammar generating this language.[26] So there exist context-sensitive languages which are not context-free. To prove that a given language is not context-free, one may employ the pumping lemma for context-free languages[25] or a number of other methods, such as Ogden's lemma or Parikh's theorem.[27]

Notes

  1. ^ meaning of 's arguments and results:
  2. ^ In Valiant's paper, O(n2.81) was the then-best known upper bound. See Matrix multiplication#Algorithms for efficient matrix multiplication and Coppersmith-Winograd algorithm for bound improvements since then.
  3. ^ A context-free grammar for the language A is given by the following production rules, taking S as the start symbol: S -> Sc | aTb | ?; T -> aTb | ?. The grammar for B is analogous.

References

  1. ^ Hopcroft & Ullman 1979, p. 100, Theorem 4.7.
  2. ^ Valiant, Leslie G. (April 1975). "General context-free recognition in less than cubic time". Journal of Computer and System Sciences. 10 (2): 308-315. doi:10.1016/s0022-0000(75)80046-8. Archived from the original on 10 November 2014.
  3. ^ Lee, Lillian (January 2002). "Fast Context-Free Grammar Parsing Requires Fast Boolean Matrix Multiplication" (PDF). J ACM. 49 (1): 1-15. arXiv:cs/0112018. doi:10.1145/505241.505242.
  4. ^ Knuth, D. E. (July 1965). "On the translation of languages from left to right" (PDF). Information and Control. 8 (6): 607-639. doi:10.1016/S0019-9958(65)90426-2. Archived from the original (PDF) on 15 March 2012. Retrieved 2011.
  5. ^ a b c Hopcroft & Ullman 1979, p. 131, Corollary of Theorem 6.1.
  6. ^ Hopcroft & Ullman 1979, p. 142, Exercise 6.4d.
  7. ^ Hopcroft & Ullman 1979, p. 131-132, Corollary of Theorem 6.2.
  8. ^ Hopcroft & Ullman 1979, p. 132, Theorem 6.3.
  9. ^ Hopcroft & Ullman 1979, p. 142-144, Exercise 6.4c.
  10. ^ Hopcroft & Ullman 1979, p. 142, Exercise 6.4b.
  11. ^ Hopcroft & Ullman 1979, p. 142, Exercise 6.4a.
  12. ^ Stephen Scheinberg (1960). "Note on the Boolean Properties of Context Free Languages" (PDF). Information and Control. 3: 372-375. doi:10.1016/s0019-9958(60)90965-7. Cite has empty unknown parameter: |month= (help)
  13. ^ Beigel, Richard; Gasarch, William. "A Proof that if L = L1 ? L2 where L1 is CFL and L2 is Regular then L is Context Free Which Does Not use PDA's" (PDF). University of Maryland Department of Computer Science. Retrieved 2020.
  14. ^ Wolfram, Stephen (2002). A New Kind of Science. Wolfram Media, Inc. p. 1138. ISBN 1-57955-008-8.
  15. ^ Hopcroft & Ullman 1979, p. 203, Theorem 8.12(1).
  16. ^ Hopcroft & Ullman 1979, p. 202, Theorem 8.10.
  17. ^ Salomaa (1973), p. 59, Theorem 6.7
  18. ^ Hopcroft & Ullman 1979, p. 135, Theorem 6.5.
  19. ^ Hopcroft & Ullman 1979, p. 203, Theorem 8.12(2).
  20. ^ Hopcroft & Ullman 1979, p. 203, Theorem 8.12(4).
  21. ^ Hopcroft & Ullman 1979, p. 203, Theorem 8.11.
  22. ^ Hopcroft & Ullman 1979, p. 137, Theorem 6.6(a).
  23. ^ Hopcroft & Ullman 1979, p. 137, Theorem 6.6(b).
  24. ^ John E. Hopcroft; Rajeev Motwani; Jeffrey D. Ullman (2003). Introduction to Automata Theory, Languages, and Computation. Addison Wesley. Here: Sect.7.6, p.304, and Sect.9.7, p.411
  25. ^ a b Yehoshua Bar-Hillel; Micha Asher Perles; Eli Shamir (1961). "On Formal Properties of Simple Phrase-Structure Grammars". Zeitschrift für Phonetik, Sprachwissenschaft und Kommunikationsforschung. 14 (2): 143-172.
  26. ^ Hopcroft & Ullman 1979.
  27. ^ How to prove that a language is not context-free?

Works cited

Further reading


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