The Gateway to Computer Science Excellence
First time here? Checkout the FAQ!
+16 votes

Which of the following statements is true?

  1. If a language is context free it can always be accepted by a deterministic push-down automaton
  2. The union of two context free languages is context free
  3. The intersection of two contest free languages is a context free
  4. The complement of a context free language is a context free
asked in Theory of Computation by Veteran (59.5k points)
edited by | 1.1k views

2 Answers

+21 votes
Best answer

Answer is (B).

(A) is wrong as a language can be context free even if it is being accepted by non-deterministic PDA for ex- $\{ ww^r: w \in \Sigma^*$ and $w^r$  is reverse of $w\}$

(C) and (D) not always true as Context free languages are not closed under Complement or Intersection.

answered by Active (2.2k points)
edited by

I think example of A is wrong ... it will be { WWr  :  W ∈(a,b)and W is reverse} ... correct me if i am wrong ...

Edit : { WWr  :  W ∈(a,b)and W is reverse}  ... both r npda .... My mistake ...

+2 votes
Ans: B not closed under intersection and complement
answered by Loyal (7.3k points)

Quick search syntax
tags tag:apple
author user:martin
title title:apple
content content:apple
exclude -tag:apple
force match +apple
views views:100
score score:10
answers answers:2
is accepted isaccepted:true
is closed isclosed:true

39,481 questions
46,655 answers
57,348 users