GATE CS 2015 Set 1 — Question 28

MCQ+1 / -0.33MediumSyntax AnalysisCompiler Design

Compiler Design → Syntax Analysis

Last updated 5 September 2026

Question

Which one of the following is TRUE at any valid state in shift-reduce parsing?

Correct answer (C) The stack contains only a set of viable prefixes Solution In shift-reduce (bottom-up) parsing, a viable prefix is a prefix of a right-sentential form that does not extend past the right end of the handle. A viable prefix is exactly something that can appear on the parser's stack. At every valid configuration of a shift-reduce parser, the stack contents (read from bottom to top) form a viable prefix; and every prefix of that stack content (bottom up to any depth) is itself a viable prefix. So the stack holds only viable prefixes. (A) and (B) are wrong: viable prefixes are not restricted to just the bottom or just the top of the stack; the whole stack content from the bottom is a viable prefix. (D) is wrong: the stack always contains a viable prefix (the empty string included). (C) is right: the stack contains only (a set of) viable prefixes. Answer: (C) Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (C) The stack contains only a set of viable prefixes Solution In shift-reduce (bottom-up) parsing, a viable prefix is a prefix of a right-sentential form that does not extend past the right end of the handle. A viable prefix is exactly something that can appear on the parser's stack. At every valid configuration of a shift-reduce parser, the stack contents (read from bottom to top) form a viable prefix; and every prefix of that stack content (bottom up to any depth) is itself a viable prefix. So the stack holds only viable prefixes. (A) and (B) are wrong: viable prefixes are not restricted to just the bottom or just the top of the stack; the whole stack content from the bottom is a viable prefix. (D) is wrong: the stack always contains a viable prefix (the empty string included). (C) is right: the stack contains only (a set of) viable prefixes. Answer: (C)

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