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UGC NET Computer Science Previous Year Questions (PYQs)

UGC NET Computer Science Compiler Design PYQ

UGC NET Computer Science PYQ
Arrange the following parsers in increasing order of their power (least → most):
  1. LR(0)
  2. LR(1)
  3. LALR(1)
  4. LL(0)
  5. SLR
Choose the correct answer:





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UGC NET Computer Science UGC NET PYQ UGC NET Computer Science UGC NET Computer Science 26 June 2025 (Paper II) PYQ

Solution

  • LL(0) is the weakest (no lookahead, top-down).
  • LR(0) is stronger than LL(0) (bottom-up, no lookahead).
  • SLR(1) adds 1-symbol lookahead using FOLLOW sets → accepts more grammars than LR(0).
  • LALR(1) merges LR(1) states; more powerful than SLR(1) but below LR(1).
  • LR(1) is the most powerful among these deterministic bottom-up parsers.
UGC NET Computer Science PYQ
Consider the following DFA that generates set of strings over Σ = {a, b, c}.
Identify the best description of the language for the DFA.





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UGC NET Computer Science UGC NET PYQ UGC NET Computer Science UGC NET Computer Science 26 June 2025 (Paper II) PYQ

Solution

The automaton has two “phases” connected by the path recognizing a→b→c. Each time the substring “abc” is seen, the DFA moves to the other phase, toggling parity. The start phase (no “abc” yet) is accepting (count 0), and after every additional “abc” the parity flips; therefore the accepting states correspond to an even number of occurrences of “abc”. Options 1 and 2 describe Σ* (all strings), and option 3 does not match the transitions.
UGC NET Computer Science PYQ

Match List I with List II

List I (Grammar Type) List II (Production Form)
A. Type 3 Grammar IV. $V \rightarrow V\Sigma \mid \Sigma$
B. Type 2 Grammar I. $V \rightarrow (V \cup \Sigma)^{*}$
C. Type 1 Grammar II. $\lambda A \beta \rightarrow \lambda\,\gamma\,\beta,\;(\lambda,\beta)\in (V\cup\Sigma)^{*}$
D. Type 0 Grammar III. $(V\cup\Sigma)^{*} \rightarrow (V\cup\Sigma)^{*}$





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UGC NET Computer Science UGC NET PYQ UGC NET Computer Science UGC NET Computer Science 26 June 2025 (Paper II) PYQ

Solution

Why: Type-3 (regular) → right-linear (IV); Type-2 (context-free) → $V \to (V\cup\Sigma)^*$ (I); Type-1 (context-sensitive) → context-preserving form (II); Type-0 (unrestricted) → any string to any string (III).

UGC NET Computer Science PYQ

Consider the following DFA

Which of the following NFA is valid for the given DFA?




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UGC NET Computer Science UGC NET PYQ UGC NET Computer Science UGC NET Computer Science 26 June 2025 (Paper II) PYQ

Solution


UGC NET Computer Science PYQ
Which of the following Grammars is/are only Context-Free?
LabelProductions Note
A S → Ab
aS → aA
A → a 		LHS “aS” has a terminal → not CFG
B S → Ab
A → Sa
A → a 		All rules are left-linear (V→Vt or V→t) → regular
C S → AS
S → aA
A → a 		CFG, not regular (non-linear: V→VV)
D S → Ab
S → aA
A → a 		CFG; mixes left/right-linear → not regular
E S → Sb
A → Aa
A → ε 		From start S no terminal string → L(S)=∅ (regular)





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UGC NET Computer Science UGC NET PYQ UGC NET Computer Science UGC NET Computer Science 26 June 2025 (Paper II) PYQ

Solution

A is not even CFG (terminal on LHS).
B is a regular (left-linear) grammar.
C and D are context-free but not regular (“only” context-free).
E generates the empty language from start symbol S, which is regular.

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