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GATE Overflow Test Series | Mock GATE | Test 5 | Question: 17

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Which of the following sentences regarding Viable prefixes is/are CORRECT? (Mark all the appropriate choices)

  1. Viable prefixes are used in predictive parsing.
  2. Viable prefixes are the set of prefixes of right-sentential forms that do not extend past the end of the right-most handle.
  3. Set of all viable prefixes for a sentence forms a regular language.
  4. Viable prefixes have prefix-free property (no string is a prefix of any other string).
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Option A is false. Viable prefixes are used in shift-reduce parsing and not in predictive parsing.

Viable prefixes are the set of prefixes of right-sentential forms that can appear on the stack of a shift-reduce parser - as soon as the handle comes on top of stack, it gets reduced. So, options B is true.

The set of all viable prefixes of the right sentential forms of a grammar is a REGULAR LANGUAGE.

Viable prefixes are prefixes of the right sentential forms of a grammar and they do contain strings which are prefix of the other strings. That is viable prefixes do not obey the prefix-free property. Option D is false.

Correct Answer: B;C.

Reference : https://gatecse.in/viable-prefixes-and-handle-in-lr-parsing
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When we merge states in an $LR(1)$ parser to form an $LALR(1)$ parser, we may introduce Shift-reduce conflict. Reduce-reduce conflict. Neither shift-reduce nor reduce-reduce conflict. Both shift-reduce as well as reduce-reduce conflict.
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