Lexical Analysis Practice Questions

Symbol table interaction during lexical analysis: When the lexer recognizes an identifier, it typically looks it up in the symbol table. If not found, it creates a new entry. At lexing time, the only attribute recorded is the lexeme (name) itself - type information, scope, and usage attributes are filled in by the semantic analyzer during later phases. Keywords may be pre-loaded into the symbol table or handled by checking a keyword table separately (so they don't get treated as identifiers). A is partially wrong - 'fully records attributes' is incorrect. C is wrong - the symbol table is used across all phases. D is wrong - keywords are typically not given symbol table entries.

A floating-point literal pattern: optional sign: (+|-)? - zero or one occurrence of + or -. Integer part: (digit)+ - one or more digits (required, must have at least one digit). Optional decimal part: (.(digit)+)? - a period followed by one or more digits, entire group optional. Combined: (+|-)?(digit)+(.(digit)+)? matches: 42, +3.14, -0.5, 123.456, -99. This covers integers and floats. A more complete pattern might also include scientific notation: (digit)+(.(digit)+)?(e(+|-)?(digit)+)?. Option B allows strings like '.5' or '.' (no integer part). Option C matches integers only. Option D requires a sign and both integer and fractional parts.

Trace the DFA: State A: initial - no 0s seen recently. On 1: stay at A (reset). On 0: go to B. State B: one 0 seen. On 1: go to A (reset). On 0: go to C (two consecutive 0s found). State C: accepting - '00' seen. On any input: stay at C (once accepted, always accept). The DFA accepts a string iff at some point two consecutive 0s appear. 'Containing 00 as a substring' ≡ 'having at least two consecutive 0s' - these are the same condition. So B and C express the same language, making D correct.

Lexical analyzers (lex/flex) use two rules to resolve conflicts: (1) Longest match (maximal munch): always prefer the longest match. Example: '==' should match the equality operator token, not two '=' tokens. '<=', '>=', '!=' should each be one token, not two. (2) Priority rule (first match): when two patterns match the SAME longest string, the pattern listed FIRST in the lex specification wins. This is critical for keywords: 'if', 'while', 'return' would match the identifier pattern too, but since keyword patterns are listed before the identifier pattern, they take priority. Together: (1) find the longest match, then (2) if tie, choose the first-listed pattern.