60 solved GATE CS questions on Compiler Design, drawn from 10 exam years and grouped by year. Every question shows the official answer and a step-by-step solution.
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Trace how source text becomes tokens, parse trees, and intermediate code. These selected foundations connect lexical analysis, parsing, syntax-directed translation, and basic optimization. Each phase's output is the next phase's input; errors in one phase propagate forward.
Our study notes and original examples support the PYQs below; they are not official exam questions or a replacement for the current syllabus.
A lexer converts a character stream into tokens using patterns typically described by regular expressions. Longest match and priority rules resolve ambiguity when multiple patterns match. The lexer does not check grammatical structure.
Check yourself: Does the longest-match rule apply before the priority rule?
A predictive parser chooses a production using the current non-terminal and lookahead token. An LL(1) grammar requires: no left recursion, and disjoint FIRST sets for alternatives (using FOLLOW when FIRST contains ε). Left recursion must be eliminated; left factoring resolves common prefixes.
Check yourself: Do FIRST sets of alternative productions overlap for any non-terminal?
LR parsers shift tokens onto a stack and reduce when the top matches a production's right side. SLR uses FOLLOW sets to decide reductions; CLR and LALR use more precise lookahead sets. An LR(1) grammar is unambiguous, but not every unambiguous grammar is LR(1). Shift-reduce and reduce-reduce conflicts indicate the grammar is not in the parser's class.
Check yourself: Is the conflict a genuine grammar ambiguity or a limitation of the parser class?
Attributes attached to grammar symbols compute values during parsing. Synthesized attributes flow up the parse tree; inherited attributes flow down. An S-attributed definition uses only synthesized attributes and can be evaluated in a single bottom-up pass.
Check yourself: Does the evaluation order respect attribute dependencies?
Three-address code represents expressions using at most one operator per statement. Basic blocks are maximal sequences with no branches except at entry and exit. Common subexpression elimination avoids recomputing the same value; dead code elimination removes results that are never used.
Check yourself: Is the value from this computation used on any path to the exit?
Original teaching example · not a PYQ
Original mini-example: compute FIRST and FOLLOW sets for the grammar S → aAB, A → b | ε, B → c.
FIRST(S)={a}, FIRST(A)={b,ε}, FIRST(B)={c}. FOLLOW(S)={$}, FOLLOW(A)={c}, FOLLOW(B)={$}.
Is the grammar S → Sa | b left-recursive? If so, eliminate the left recursion.
Yes. After elimination: S → bS', S' → aS' | ε.
S → Sa is direct left recursion with α = a and β = b. Applying the transformation: S → βS' and S' → αS' | ε gives S → bS', S' → aS' | ε. This generates the same language: b followed by zero or more a's.
Supplemental reading, not an official GATE reading list or an endorsement of these notes.
Free supplemental lecture handouts covering lexical analysis, parsing, semantic analysis, and code generation from Stanford's undergraduate compilers course.
This page shows 60 recent questions from the released archive, newest first. For older questions and complete papers, browse all GATE CS papers. Questions can carry more than one subject tag; counts are not marks weightage.
S → DHTU |…MCQ · +2 marks · MediumA → PQ and A → XY. Each of the five non-terminals and has two attributes: is a synthesized attribute,…MCQ · +2 marks · MediumA → BD Let a, b, d, and $ be indexed as follows: | a | b |…NAT · +1 marks · Mediumò refers to the empty statement, and if, then,…NAT · +2 marks · MediumContinue learning with Success Tracker
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