Context free languages and Push-down automata

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Question 1

Consider the following languages. 

 

 Which one of the following statements is FALSE?

  • L2 is context-free.

  • L1 intersection L2 is context-free.

  • Complement of L2 is recursive.

  • Complement of L1 is context-free but not regular.

Question 2

Which of the following pairs have DIFFERENT expressive power?

  • Deterministic finite automata(DFA) and Non-deterministic finite automata(NFA)

  • Deterministic push down automata(DPDA)and Non-deterministic push down automata(NPDA)

  • Deterministic single-tape Turing machine and Non-deterministic single-tape Turing machine

  • Single-tape Turing machine and multi-tape Turing machine

Question 3

Let P be a regular language and Q be context-free language such that Q ⊆ P. (For example, let P be the language represented by the regular expression p*q* and Q be {pn qn  | n ∈ N}). Then which of the following is ALWAYS regular? 

(A) P ∩ Q 

(B) P - Q 

(C) ∑* - P 

(D) ∑* - Q

  • A

  • B

  • C

  • D

Question 4

Consider the languages L1, L2 and L3 as given below.

LI= {0P1q |p, q∈N}

L2={0p1q |p, q∈ N and p=q} and

L3= {0P1q0r |p, q, r∈ N and p =q = r}

Which of the following statements is NOT TRUE?

  • Push Down Automata (PDA) can be used to recognize L1 and L2

  • L1 is a regular language

  • All the three languages are context free

  • Turing machine can be used to recognize all the three languages

Question 5

Consider the languages - 
L1 = {0i1j | i != j}. 
L2 = {0i1j | i = j}. 
L3 = {0i1j | i = 2j+1}. 
L4 = {0i1j | i != 2j}.

  • Only L2 is context free

  • Only L2 and L3 are context free

  • Only L1 and L2 are context free

  • All are context free

Question 6

S -> aSa|bSb|a|b; The language generated by the above grammar over the alphabet {a,b} is the set of

  • All palindromes

  • All odd length palindromes.

  • Strings that begin and end with the same symbol

  • All even length palindromes

Question 7

Let L = L1∩L2, where L1 and L2 are languages as defined below:

L1 = {am bm can bn | m, n >= 0}

L2 = {ai bj ck | i, j, k >= 0}

Then L is

  • Not recursive

  • Regular

  • Context free but not regular

  • Recursively enumerable but not context free.

Question 8

Which of following statement(s) is/are not correct? (I) Languages generated by the grammar S→aSa ∣ aa is not regular. (II) Languages generated by the grammar S→aSb ∣ aa is not regular. (III) Languages generated by the grammar S→S1|S3, S1→aS1c |S2|λ, S2→aS2b|λ, S3→aS3b|S4| λ, S4→bS4c|λ is {a^nb^mc^k | k = |n - m|, n≥0, m≥0, k≥0}. (IV) Languages generated by the grammar S→S1S3, S1→aS1c |S2|λ, S2→aS2b|λ, S3→aS3b|S4| λ, S4→bS4c|λ is {a^nb^mc^k | k = |n - m|, n≥0, m≥0, k≥0}.

  • Only (II), (IV)

  • Only (I), (III), (IV)

  • Only (I), (II), (III)

  • All of the above

Question 9

Consider the following languages.



L1 = {ai bj ck | i = j, k ≥ 1}

L1 = {ai bj | j = 2i, i ≥ 0}

Which of the following is true?

  • L1 is not a CFL but L2 is

  • L1 ∩ L2 = ∅ and L1 is non-regular

  • L1 ∪ L2 is not a CFL but L2 is

  • There is a 4-state PDA that accepts L1, but there is no DPDA that accepts L2

Question 10

Match the following: 24

  • (1)
  • (2)
  • (3)
  • (4)

There are 93 questions to complete.

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