What is a context sensitive language in TOC?
A context-sensitive grammar whose productions are of the form
αAβ → αγβ
Where α, β ∈ (N ∪ T)*, A ∈ N; γ ∈ (N ∪ T)+ and a rule of the form S → λ is allowed if the start symbol S do not appear on the right hand side of any rule.
The language generated by such a grammar is called a context-sensitive language.
Every context-free grammar is also context-sensitive =⇒ the context-free languages are a subset of the context-sensitive languages (see Chomsky Normal Form).
But, not every context-sensitive language is context-free.
Example
The language {anbncn, n > 1} is context-sensitive but not context free.
A grammar for this language is given by:
S → aSBC | aBC,
CB → HB,
HB → HC,
HC → BC,
aB → ab,
bB → bb,
bC → bc,
cC → cc
A derivation e.g. aabbcc, using this grammar is:
S ⇒ aSBC
⇒ aaBCBC (using S → aBC)
⇒ aabCBC (using aB → ab)
⇒ aabHBC (using CB → HB)
⇒ aabHCC (using HB → HC)
⇒ aabBCC (using HC → BC)
⇒ aabbCC (using bB → bb)
⇒ aabbcC (using bC → bc)
⇒ aabbcc (using cC → cc)
The context-sensitive languages can also be generated by a monotonic grammar, any production is allowed, as long as there are no rules for making strings shorter (except one S → ∧)!
Published on 15-Jun-2021 13:17:53
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