Question 1
Consider the following languages. Which one of the following statements is FALSE?
 A L2 is context-free. B L1 intersection L2 is context-free. C Complement of L2 is recursive. D Complement of L1 is context-free but not regular.
GATE CS 2013    Context free languages and Push-down automata
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Question 1 Explanation:
(D) is false. L1 is regular, so its complement would also be regular. L1 is a regular language of the form 0^* 1^* 0^*. L2 on the other hand is a CFL as it can be derived from the following CFG L2 = { 0^p 1^q 0^r | p,q,r>0   And p notEqualTo r } S -> AC|CA C -> 0C0|B A -> 0A|0 B -> 1B|epsilon If coming up with a CFG for L2 is difficult, one can intuitively see that by reducing it to a simpler problem. L2 is very similar to a known CFL L3 = { a^m b^l | m notEqualTo n } (A) L2 is context free, which is true [CORRECT] (B) L1 intersection L2 is context free, which is again true because L1 is a regular language and L2 is a CFL. RL union CFL is always a CFL. Hence [CORRECT] (C) Complement of L2 is recursive, which is true due to the fact that complement of a CFL is CSL for sure (Context sensitive language), which in turn (CSL) is a subset of recursive languages. Hence [CORRECT] (D) Complement of L1 is context free but not regular, which is false due to closure laws of regular languages. Complement of a RL is always a RL. Hence [INCORRECT]   This solution is contributed by Vineet Purswani .
 Question 2
Which of the following pairs have DIFFERENT expressive power?
 A Deterministic finite automata(DFA) and Non-deterministic finite automata(NFA) B Deterministic push down automata(DPDA)and Non-deterministic push down automata(NPDA) C Deterministic single-tape Turing machine and Non-deterministic single-tape Turing machine D Single-tape Turing machine and multi-tape Turing machine
GATE CS 2011    Context free languages and Push-down automata
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Question 2 Explanation:

NDPDA can handle languages or grammars with ambiguity, but DPDA cannot handle languages with ambiguity and any context-free grammar.

 Question 3
1) 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 {pnqn|n N}). Then which of the following is ALWAYS regular? (A) P Q (B) P - Q (C) * - P (D) * - Q
 A A B B C C D D
GATE CS 2011    Context free languages and Push-down automata
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Question 3 Explanation:
1. P ∩ Q would be Q, due to the given fact that Q ⊆ P, hence context free but not regular. 2. P − Q = P ∩ Q might not even be a context free language, due to the closure properties of context free languages. 3. Σ∗ − P is equivalently complement of P, hence regular. Refer to closure laws of regular languages. 4. Σ∗ − Q is equivalently complement of Q, hence it might not even be a context free language. Refer to closure laws of CFLs. Reference: http://quiz.geeksforgeeks.org/theory-of-computation-closure-properties-of-context-free-languages/ See http://www.geeksforgeeks.org/automata-theory-set-4/ This solution is contributed by Vineet Purswani.
 Question 4
2) Consider the language L1,L2,L3 as given below. L1={ | p,q N} L2={ | p,q N and p=q} L3={ | p,q,r N and p=q=r} Which of the following statements is NOT TRUE?
 A Push Down Automata (PDA) can be used to recognize L1 and L2 B L1 is a regular language C All the three languages are context free D Turing machine can be used to recognize all the three languages
GATE CS 2011    Context free languages and Push-down automata
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Question 4 Explanation:
L1 is regular. Its DFA is given as

L2 is not regular, can be proved using pumping lemma (refer to Ullman). But L2 is CFL.
        S → AB
A → 0A|ε
B → 1B|ε
L3 is not CFL, can be proved using pumping lemma (refer to Ullman). But L3 is Recursive. Every regular language is also a CFL. So PDA can be used to recognized L1 and L2. As a CFL and Regular language is algo a Recursive language. Hence, Turing machine can be used to recognise L1, L2 and L3. L2 is not regular, can be proved using pumping lemma (refer to Ullman). But L2 is CFL.
        S → AB
A → 0A|ε
B → 1B|ε
L3 is not CFL, can be proved using pumping lemma (refer to Ullman). But L3 is Recursive. Every regular language is also a CFL. So PDA can be used to recognised L1 and L2. As a CFL and Regular language is algo a Recursive language. Hence, turing machine can be used to recognise L1, L2 and L3. Source: http://clweb.csa.iisc.ernet.in/rahulsharma/gate2011key.html
 Question 5
Consider the languages L1 = {0i1j | i != j}. L2 = {0i1j | i = j}. L3 = {0i1j | i = 2j+1}. L4 = {0i1j | i != 2j}.
 A Only L2 is context free B Only L2 and L3 are context free C Only L1 and L2 are context free D All are context free
GATE CS 2010    Context free languages and Push-down automata
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Question 5 Explanation:
All these languages have valid CFGs that can derive them. Hence, all of them are CFLs. Intuitively, (A) & (B) are well known CFLs and CFGs for (C) & (D) could be made by little modifications in A & B’s CFGs.     This solution is contributed by vineet purswani.
 Question 6
S -> aSa|bSb|a|b; The language generated by the above grammar over the alphabet {a,b} is the set of
 A All palindromes B All odd length palindromes. C Strings that begin and end with the same symbol D All even length palindromes
GATE-CS-2009    Context free languages and Push-down automata
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Question 6 Explanation:
The possible palindrome generated by above grammar can be of odd length only as there is no rule for S -> [Tex]\epsilon[/Tex] For example generated palindromes are aba, aaa, bab, ababa, aaaaa, ..
 Question 7
Let L = L1  L2, where L1 and L2 are languages as defined below:
L1 = { | m, n >= 0 }
L2 = { | i, j, k >= 0 }
Then L is
 A Not recursive B Regular C Context free but not regular D Recursively enumerable but not context free.
GATE-CS-2009    Context free languages and Push-down automata
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Question 7 Explanation:
The language L1 accept strings {c, abc, abcab, aabbcab, aabbcaabb, …} and L2 accept strings {a, b, c, ab, abc, aabc, aabbc, … }. Intersection of these two languages is L1 \cap L2 = {a^{k}b^{k}c | k >= 0} which is context free, but not regular.
 Question 8
The language L= {0i21i | i≥0 } over the alphabet {0,1, 2} is:
 A not recursive B is recursive and is a deterministic CFL. C is a regular language. D is not a deterministic CFL but a CFL.
Context free languages and Push-down automata    GATE-CS-2007
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Question 8 Explanation:
Let us first design a deterministic pushdown automata for the given language.
• For each occurrence of ‘0’ , we PUSH X in the stack.
• When ‘2’ appears, no stack operation is performed. But, state of the automata is changed.
• For each occurrence of ‘1’ , we POP X from the stack.
• If at the end Z0 is on the stack top then input string is accepted
We also design a Turing machine for the given language.
• When ‘0’ appears in the input string , we replace it with X .Then, traverse to the rightmost corner and replace ‘1’ with Y.
• We go back to the leftmost ‘0’ and repeat the above process.
• While traversing rightwards from the beginning of the input string, if after X, ‘2’ appears and after ‘2’, Y appears then we reach the HALT state. Thus, the given language is recursive. Every recursive language is a CFL. Thus, option (B) is the answer. Please comment below if you find anything wrong in the above post.
 Question 9
Consider the CFG with {S,A,B) as the non-terminal alphabet, {a,b) as the terminal alphabet, S as the start symbol and the following set of production rules
S --> aB        S --> bA
B --> b         A --> a
B --> bS        A --> aS
B --> aBB       A --> bAA
Which of the following strings is generated by the grammar?
 A aaaabb B aabbbb C aabbab D abbbba
Context free languages and Push-down automata    GATE-CS-2007
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Question 9 Explanation:
Given below production rules.
S --> aB        S --> bA
B --> b         A --> a
B --> bS        A --> aS
B --> aBB       A --> bAA
We can derive aabbab using below sequence
S  -> aB      [Using S --> aB]
-> aaBB    [Using B --> aBB]
-> aabB    [Using B --> b]
-> aabbS   [Using B --> bS]
-> aabbaB  [Using S --> aB]
-> aabbab  [Using B --> b]
 Question 10
For the correct answer strings to above question, how many derivation trees are there?
 A 1 B 2 C 3 D 4
Context free languages and Push-down automata    GATE-CS-2007
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Question 10 Explanation:
When it asks about the no of derivations tree, we should consider either left most derivation(LMD) or right most derivations(RMD), but not both. Here two left most derivations are possible for the correct string of the previous question "aabbab" from the given grammar. LMD-1 S -> aB [Using S --> aB] -> aaBB [Using B --> aBB] -> aabB [Using B --> b] -> aabbS [Using B --> bS] -> aabbaB [Using S --> aB] -> aabbab [Using B --> b] LMD-2 S -> aB [Using S --> aB] -> aaBB [Using B --> aBB] -> aabSB [Using B --> bS] -> aabbAB [Using S --> bA] -> aabbaB [Using A --> a] -> aabbab [Using B --> b] The Derivation tress are shown below :
 Question 11
Here, wr is the reverse of the string w. Which of these languages are deterministic Context-free languages?
 A None of the languages B Only L1 C Only L1 and L2 D All the three languages
Context free languages and Push-down automata    GATE-CS-2014-(Set-3)
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 Question 12
 A L1 only B L3 Only C L1 and L2 D L2 and L3
Context free languages and Push-down automata    GATE-CS-2006
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Question 12 Explanation:
A PDA can be built only for L1. It is not possible to build PDA for L2 and L3.
 Question 13
Consider the following statements about the context free grammar
G = {S → SS, S → ab, S → ba, S → Ε}
I. G is ambiguous
II. G produces all strings with equal number of a’s and b’s
III. G can be accepted by a deterministic PDA.
Which combination below expresses all the true statements about G?
 A I only B I and III only C II and III only D I, II and III
Context free languages and Push-down automata    GATE-CS-2006
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Question 13 Explanation:
Statement I: G is ambiguous because, as shown in the image below there can be two decision tree for string S = ababab [TRUE] Statement II: G produces all strings with equal number of a’s and b’s [FALSE] string 'aabb' cannot be produced by G Statement III: G can be accepted by a deterministic PDA [TRUE] Assume there is a PDA which pushes if top of the stack is \$ (bottom most alphabet of the stack) and pops otherwise. A string is rejected while popping if the current letter and top of the stack are same. This PDA can derive G. Hence, correct answer should be (B) I and III only Reference: Ambiguity in Context free Grammar and Context free Languages This solution is contributed by Vineet Purswani.
 Question 14
Which one of the following grammars generates the language L = {aibj | i ≠ j}


 A A B B C C D D
Context free languages and Push-down automata    GATE-CS-2006
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Question 14 Explanation:
Language L contains the strings : {abb, aab, abbb, aabbb, aaabb, aa, bb, .......}, i.e, all a's appear before b's in a string, and "number of a's" is not equal to "number of b's", So i ≠ j.
Here Grammar A, B & C also generate the string "ab", where i = j, and many more strings with i = j, hence these grammars do not generate the language L, because for a string that belongs to language L, exponent i should not be equal to exponent  j.
Grammar D : This Grammar never generates a string with equal no of a's and b's, i.e. i=j. Hence this grammar generates the language L. Hence Option D.
 Question 15
In the correct grammar of above question, what is the length of the derivation (number of steps starring from S) to generate the string albm with l ≠ m?
 A max(l,m) + 2 B l + m + 2 C l + m + 3 D max(l, m) + 3
Context free languages and Push-down automata    GATE-CS-2006
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Question 15 Explanation:
Correct grammar of the last question was (D), which is:
S -> AC|CB
C -> aCb|epsilon
A -> aA|a
B -> Bb|b

Now, the most optimal and intuitive way to generate a string of the form albm would be to first use "C -> aCb|epsilon" production rule to get as many a and b as we can, which would be min(l,m). To get the rest of the string, we could just use latter two production rules accordingly. Formally deriving the string of the general format albm from the above grammar -
1.	  S -> AC
2.          -> A(aCb)
3.          -> ....
4.          -> ....
5.          -> A(am C bm)
6.          -> A(am bm)
7.          -> aA(am bm)
8.          -> ....
9.          -> ....
10.         -> a(l-m-1)A(am bm)
11.         -> al bm


From above set of derivation steps we can count the total steps as follows:

Production 1 took 1 step       : 1                   [using S->AC]
Production 2-5 took steps      : min(l,m)            [using C->aCb]
Production 6 took 1 step       : 1                   [using C->epsilon]
Production 7-11 took steps     : max(l,m)-min(l,m)   [using A -> aA|a or B -> Bb|b]

Total steps   : max(l,m) + 2

Hence, answer should be (A): max(l,m) + 2 This explanation is contributed by Vineet Purswani.
 Question 16
Consider the languages:
L1 = {anbncm | n, m > 0}
L2 = {anbmcm | n, m > 0} 
Which one of the following statements is FALSE?
 A L1 ∩ L2 is a context-free language B L1 U L2 is a context-free language C L1 and L2 are context-free language D L1 ∩ L2 is a context sensitive language
Context free languages and Push-down automata    GATE-CS-2005
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Question 16 Explanation:
L1 and L2 are context free languages. See this for closure properties.
 Question 17
Consider the languages:
L1 = {wwR |w ∈ {0, 1}*}
L2 = {w#wR | w ∈ {0, 1}*}, where # is a special symbol
L3 = {ww |  w ∈  (0, 1}*)
Which one of the following is TRUE?
 A L1 is a deterministic CFL B L2 is a deterministic CFL C L3 is a CFL, but not a deterministic CFL D L3 is a deterministic CFL
Context free languages and Push-down automata    GATE-CS-2005
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Question 17 Explanation:
L1: {ww^R | w belongs {0,1}*} This is a CFL but not a DCFL. It can be derived from the following grammar S -> aSa | bSb | epsilon But it can't be derived from any deterministic pushdown automaton, because there is no way to figure out where a word w ends and its reverse starts. L2: {w#w^R | w belongs {0,1}*} This is a CFL, due to the same reason as described above. This is a deterministic CFL because we have a marker to help us find out the end of the word w and start of its reverse. Thus a PDA where all the alphabets are pushed until we get # and afterwards pop only if the top of the stack matches the current alphabet and reject otherwise - will derive L2. L3: {ww | w belongs {0,1}*} This is not even a CFL. Above claim could be proved using pumping lemma - Consider a string z of the form (0^n 1^n 0^n 1^n). Assuming L3 is a CFL, and z obviously satisfies L3 - thus z should also satisfy pumping lemma. We will take n such that n = p, where p is the pumping length of L3, hence forcing our string to be of length greater than pumping length. Now, according to pumping lemma, there must exist u,v,w,x,y such that z = uvwxy, |vwx| <= p, |vx| > 0 and u{v^i}x{y^i}z belongs L3 for all i>=0. There doesn't exist any such configuration of u,v,w,x,y such that u{v^0}x{y^0}z belongs L3. Hence z doesn't satisfy pumping lemma. Hence L3 is not a CFL. Considering all the above conclusions, only correct option comes out to be (B) L2 is a deterministic CFL. Reference ; https://courses.engr.illinois.edu/cs373/sp2013/Lectures/lec17.pdf This solution is contributed by Vineet Purswani.
 Question 18
The language {am bn Cm+n | m, n ≥ 1} is
 A regular B context-free but not regular C context sensitive but not context free D type-0 but not context sensitive
Context free languages and Push-down automata    GATE-CS-2004
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Question 18 Explanation:

We construct a PDA for the given language.
PUSH Z0 in the stack initially. PUSH X in the stack for each occurrence of ‘a’ . PUSH Y in the stack for each occurrence of ‘b’. POP X and Y from the stack for each occurrence of ‘c’.
If after popping all X and Y from the stack , no input element is left in the string and we get Z0 on the top of the stack then the string is accepted.

Thus, option (B) is correct.

Please comment below if you find anything wrong in the above post.
 Question 19
If the strings of a language L can be effectively enumerated in lexicographic (i.e., alphabetic) order, which of the following statements is true ?
 A L is necessarily finite B L is regular but not necessarily finite C L is context free but not necessarily regular D L is recursive but not necessarily context free
Context free languages and Push-down automata    GATE-CS-2003
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Question 19 Explanation:

The strings of a language L can be effectively enumerated means a Turing machine exists for language L which will enumerate all valid strings of the language.
If the string is in lexicographic order then TM will accept the string and halt in the final state. But, if the string is not lexicographic order then TM will reject the string and halt in non-final state.
Thus, L is recursive language.
We can not construct PDA for language L. So, the given language is not context free.

Thus, option (D) is correct.

Please comment below if you find anything wrong in the above post.
 Question 20
Let G = ({S}, {a, b} R, S) be a context free grammar where the rule set R is S → a S b | SS | ε Which of the following statements is true?
 A G is not ambiguous B There exist x, y, ∈ L (G) such that xy ∉ L(G) C There is a deterministic pushdown automaton that accepts L(G) D We can find a deterministic finite state automaton that accepts L(G)
Context free languages and Push-down automata    GATE-CS-2003
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Question 20 Explanation:
An ambiguous grammar can be converted to unambiguous one.

Here we can get grammar in partial GNF form as
S -> ab | abS | aSb | aSbS

We can convert this into GNF too but no need for PDA reasoning
so, above grammar is not a ambiguous thus a definite PDA possible

Trick for this is but just deriving 3-4 strings from grammar, we
can easily understand its (anbn)* above
expression anbn is in CFL thus closure of DCFG is a DCFG
i.e., you can get L = {ε, ab, abab, aabb, aabbab, abaabb,
ababab,......}
PDA will push "a" until "b" is read, start popping "a" for the "b" read.

If "a" is read again from the tape then push only when stack is empty
else terminate.

Repeat this until string is read.

Remember fastest way to get answer is by elimination other options.
 Question 21
The language accepted by a Pushdown Automation in which the stack is limited to 10 items is best described as
 A Context Free B Regular C Deterministic Context Free D Recursive
Context free languages and Push-down automata    GATE-CS-2002
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Question 21 Explanation:
Pushdown automata is used for context free languages, i.e., languages in which the length of elements is unrestricted and length of one element is related to other. To resolve this problem, we use a stack with no restrictions on length.

But in the given case, length of stack is restricted. Thus, this pushdown automata can only accept languages which can also be accepted by finite state automata and a finite state automata accepts only regular languages.

Thus, B is the correct choice.

Please comment below if you find anything wrong in the above post.
 Question 22
Which of the following statements is true?
 A If a language is context free it can always be accepted by a deterministic push-down automaton B The union of two context free languages is context free C The intersection of two context free languages is context free D The complement of a context free language is context free
Context free languages and Push-down automata    GATE-CS-2001
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Question 22 Explanation:
 Question 23
Consider the NPDA 〈Q = {q0, q1, q2}, Σ = {0, 1}, Γ = {0, 1, ⊥}, δ, q0, ⊥, F = {q2}〉, where (as per usual convention) Q is the set of states, Σ is the input alphabet, Γ is stack alphabet, δ is the state transition function, q0 is the initial state, ⊥ is the initial stack symbol, and F is the set of accepting states, The state transition is as follows: Which one of the following sequences must follow the string 101100 so that the overall string is accepted by the automaton?
 A 10110 B 10010 C 01010 D 01001
Context free languages and Push-down automata    GATE-CS-2015 (Set 1)
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Question 23 Explanation:
In q0 state for '1', a '1' is pushed and for a '0' a '0' is pushed. In q1 state, for a '0' a '1' is popped and for a '1' a '0' is popped. So, the given PDA is accepting all strings of of the form x0x'r or x1x'r or xx'r, where x'r is the reverse of the 1's complement of x. The given string 101100 has 6 letters and we are given 5 letter strings. So, x0 is done, with x = 10110. So, x'r = (01001)r = 10010. Hence option B is correct.
 Question 24
Which of the following languages are context-free?
L1 = {ambnanbm ⎪ m, n ≥ 1}
L2 = {ambnambn ⎪ m, n ≥ 1}
L3 = {ambn ⎪ m = 2n + 1} 
 A L1 and L2 only B L1 and L3 only C L2 and L3 only D L3 only
Context free languages and Push-down automata    GATE-CS-2015 (Set 3)
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Question 24 Explanation:
We can build a push down automata for L1 and L3, but can not build push down automata for L@. Note that a PDA can uses a stack. L1 and L3 can be identified using a single stack, but L2 can't be.
 Question 25
Which one of the following statements is FALSE?
 A There exist context-free languages such that all the context-free grammars generating them are ambiguous B An unambiguous context free grammar always has a unique parse tree for each string of the language generated by it. C Both deterministic and non-deterministic pushdown automata always accept the same set of languages D A finite set of string from one alphabet is always a regular language.
Context free languages and Push-down automata    GATE-IT-2004
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Question 25 Explanation:

A) For real-world programming languages, the reference CFG is often ambiguous, due to issues such as the dangling else problem. //Wikipedia

B) A string is ambiguous if it has two distinct parse trees;The grammar is unambiguous,if a string has distinct parse trees.

C) Deterministic pushdown automata can recognize all deterministic context-free languages while        nondeterministic ones can recognize all context-free languages

Therefore it's FALSE

D)Properties of Regular Language:

• The set of regular languages over an alphabet is closed under operations union, concatenation and Kleene star.
• Finite languages are regular
 Question 26
If we use internal data forwarding to speed up the performance of a CPU (R1, R2 and R3 are registers and M[100] is a memory reference), then the sequence of operations
 A A B B C C D D
Context free languages and Push-down automata    GATE-IT-2004
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Question 26 Explanation:

Data Forwarding : In figure(2),ADD and SUB instructions have data dependency due to R1 registers,2nd and 3rd instructions read the register R1 value at ID stage but 1st instruction updates the value of R1 after WB stage .So 2nd SUB instruction is stalled for next two cycles to get updated value of R1 register.

Internal data forwarding is a mechanism to reduces the stalls due to data dependency, it uses hardware technique to forward the result of interstage buffer register (IBR) to next instruction's buffer register. As soon as result is available after ALU operation (in 1st instruction), result is transferred as input to ALU unit, then updated value of R1 gets available after ALU operation (otherwise it is available after WB satge),so no stalls are there.

The ALU result from the EX/MEM register is always fed back to the ALU input latches. If the forwarding hardware detects that the previous ALU operation has written the register corresponding to the source for the current ALU operation, con- trol logic selects the forwarded result as the ALU input rather than the value read from the register file.

As given in Ques, It is straight from question,register R1 value is copied(or better say loaded) to memory location M[100] then M[100]'s value is stored to registers R2 and R3 .Options A,B and C are wrong since they do not produce same result as desired.

Let's suppose register R1,R2,R3 and memory reference M[100] have initial values 10,20,30 and 40 respectively then after the execution of sequence of operation,registers R2,R3 and memory references M[100] have values 10,10 and 10 respectively.

In option A ,after the execution of operations,registers R2,R3 and memory references M[100] have values 20,10 and 20 respectively. In option B, registers R2 , R3 and memory references M[100] have values 10,10 and 40 respectively and option C ,reg- isters R2,R3 and memory references M[100] have values 20,20 and 10 respectively . But option D produces ,all registers and memory reference have value same value 10 as desired , so option (D) is correct only.

This solution is contributed by Nirmal Bharadwaj.

 Question 27
Consider the following context-free grammars: Which one of the following pairs of languages is generated by G1 and G2, respectively
 A A B B C C D D
Context free languages and Push-down automata    GATE-CS-2016 (Set 1)
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Question 27 Explanation:
In G1, there will be atleast 1 b becase S->B and B->b. But no of A’s can be 0 as well and no of A and B are independent. In G2, either we can take S->aA or S->bB. So it must have atleast 1 a or 1 b. So option D is correct.
 Question 28
Consider the pushdown automaton (PDA) below which runs over the input alphabet (a, b, c). It has the stack alphabet {Z0, X} where Z0 is the bottom-of-stack marker. The set of states of the PDA is (s, t, u, f} where s is the start state and f is the final state. The PDA accepts by final state. The transitions of the PDA given below are depicted in a standard manner. For example, the transition (s, b, X) → (t, XZ0) means that if the PDA is in state s and the symbol on the top of the stack is X, then it can read b from the input and move to state t after popping the top of stack and pushing the symbols Z0 and X (in that order) on the stack.
(s, a, Z0) → (s, XXZ0)
(s, ϵ, Z0) → (f, ϵ)
(s, a, X) → (s, XXX)
(s, b, X) → (t, ϵ)
(t, b, X) → (t,.ϵ)
(t, c, X) → (u, ϵ)
(u, c, X) → (u, ϵ)
(u, ϵ, Z0) → (f, ϵ)
The language accepted by the PDA is
 A {albmcn | l = m = n} B {albmcn | l = m} C {albmcn | 2l = m+n} D {albmcn | m=n}
Context free languages and Push-down automata    GATE IT 2006
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 Question 29
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?
 A L1 is not a CFL but L2 is B L1 ∩ L2 = ∅ and L1 is non-regular C L1 ∪ L2 is not a CFL but L2 is D There is a 4-state PDA that accepts L1, but there is no DPDA that accepts L2
Regular languages and finite automata    Context free languages and Push-down automata    Gate IT 2008
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Question 29 Explanation:
A: Both L1 and L2 are CFL
B: L1 ∩ L2 = ∅ is true
L1is not regular => true
=> B is true
C: L1 ∪ L2 is not a CFL nut L2 is CFL is closed under Union
=> False
D: Both L1 and L2 accepted by DPDA
 Question 30
Consider a CFG with the following productions. S → AA | B A → 0A | A0 | 1 B → 0B00 | 1 S is the start symbol, A and B are non-terminals and 0 and 1 are the terminals. The language generated by this grammar is
 A {0n 102n | n ≥ 1} B {0i 10j 10k | i, j, k ≥ 0} ∪ {0n 102n | n ≥ l} C {0i 10j | i, j ≥ 0} ∪ {0n 102n | n ≥ l} D The set of all strings over {0, 1} containing at least two 0's E None of the above
Context free languages and Push-down automata    Gate IT 2008
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Question 30 Explanation:
A− > 0A|A0|1 This production rule individually produces a CFL of the form {0i 10j |i, j ≥ 0} B− > 0B00|1 This production rule individually produces a CFL of the form {0n102n |n ≥ 0} S− > AA|B This production rule concatenates A’s CFL with itself and unions it with B’s CFL. Hence, CFL accepted by the given CFG will be {0n102n|n ≥ 0} ∪ {0i 10j 10k |i, j, k ≥ 0} According to our derivation, as none of the given CFL matches to our derived CFL, correct option should be (E) None of the above. This solution is contributed by vineet purswani.
 Question 31
A CFG G is given with the following productions where S is the start symbol, A is a non-terminal and a and b are terminals.
S→aS∣A
A→aAb∣bAa∣ϵ
Which of the following strings is generated by the grammar above?
 A aabbaba B aabaaba C abababb D aabbaab
Context free languages and Push-down automata    Gate IT 2008
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Question 31 Explanation:
 Question 32
A CFG G is given with the following productions where S is the start symbol, A is a non-terminal and a and b are terminals.
S→aS∣A
A→aAb∣bAa∣ϵ
For the correct answer in Q75, how many steps are required to derive the string and how many parse trees are there?
 A 6 and 1 B 6 and 2 C 7 and 2 D 4 and 2
Context free languages and Push-down automata    Parsing and Syntax directed translation    Gate IT 2008
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Question 32 Explanation:
 Question 33
Consider 2 scenarios:
C1: For DFA (ϕ, Ʃ, δ, qo, F),
if F = ϕ, then L = Ʃ*
C2: For NFA (ϕ, Ʃ, δ, qo, F),
if F = ϕ, then L = Ʃ*
Where F = Final states set
ϕ = Total states set


Choose the correct option ?
 A Both are true B Both are False C C1 is true, C2 is false D C1 is false, C2 is true
Context free languages and Push-down automata    GATE 2017 Mock
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Question 33 Explanation:
In case of a NFA even if F = ϕ, there may be some states where we have no transitions defined for a particular symbol i.e Dead state rejections in NFA. Due to which, L ≠ Ʃ*.
 Question 34
Let G be the CFG, l be the number of left most derivations, r be the number of right most derivations and P be the number of parse trees. Assume l , r and P are computed for a particular string. For a given CFG ‘G’ and given string ‘w’, what is the relation between l , P , r ?
 A l ≤ P ≥ r B l = P = r C l ≥ P ≤ r D none of these
Context free languages and Push-down automata    GATE 2017 Mock
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Question 34 Explanation:
For any particular string :
# parse tree = # LMD’s = # RMD’s
Therefore l = P = r.
There are 34 questions to complete.

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