Lexical Analysis Questions and Answers

Q: State whether the following is true or false.The set of strings consisting of n opening brackets followed by n closing brackets is a regular language.

Correct answer: FalseThis is the standard non-regular language. A finite automaton would have to remember how many opening brackets it had seen in order to require the same number of closing ones, and with finitely many states it cannot count without bound. The pumping lemma makes the argument precise, and the same reasoning is why nested comments defeat a pure scanner.

41.

Blanks being insignificant in early Fortran made scanning harder because

A.the scanner must insert blanks into the source before it scans it at all
B.the parser then receives tokens in an unpredictable order
C.the symbol table cannot store names containing blank characters
D.the scanner cannot use white space to delimit one lexeme from the next

Answer: D

Correct answer: the scanner cannot use white space to delimit one lexeme from the next

If blanks are insignificant then DO 10 I = 1, 25 and DO10I=1,25 are the same statement, and worse, DO10I=1.25 is an assignment to a variable called DO10I. The scanner cannot tell which until it reaches the comma or the point, so it needs arbitrary lookahead - which is exactly what the design of a scanner is meant to avoid.

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42.

White space and comments are usually handled by the scanner by

A.recognising and discarding them without producing any token
B.passing them to the parser as tokens to be ignored later
C.storing them in the symbol table for the code generator
D.reporting them as errors if they appear between two tokens

Answer: A

Discarding them in the scanner keeps the grammar free of any mention of layout. The scanner still counts newlines so that later diagnostics can name the correct source line. Comments are therefore invisible to the grammar while remaining visible to the reader.

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43.

Nested comments are difficult for a conventional scanner because

A.a comment may span more than one line of the source program
B.counting the nesting depth is beyond the power of a finite automaton
C.comments may contain characters outside the alphabet of the language
D.the parser rather than the scanner must recognise a comment

Answer: B

Correct answer: counting the nesting depth is beyond the power of a finite automaton

A finite automaton has a fixed number of states and so cannot count to an arbitrary depth; matching nested comment delimiters is the same problem as matching nested brackets, which is not regular. Scanners therefore either forbid nesting, or handle it with an explicit counter written into the action code rather than in the pattern.

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44.

State whether the following is true or false.
The set of strings consisting of n opening brackets followed by n closing brackets is a regular language.

A.False only at small scales
B.True
C.False
D.True only at large scales

Answer: C

Correct answer: False

This is the standard non-regular language. A finite automaton would have to remember how many opening brackets it had seen in order to require the same number of closing ones, and with finitely many states it cannot count without bound. The pumping lemma makes the argument precise, and the same reasoning is why nested comments defeat a pure scanner.

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45.

The pumping lemma for regular languages is used to prove that a language

A.is regular by constructing some automaton that accepts all of it
B.is context free by exhibiting a suitable grammar for it
C.is accepted by some deterministic pushdown automaton
D.is not regular by exhibiting a string that cannot be pumped

Answer: D

Correct answer: is not regular by exhibiting a string that cannot be pumped

The lemma states a property that every regular language must have, so it is used contrapositively: choose a string in the language long enough that the lemma would apply, show that no way of pumping it stays inside the language, and conclude that the language is not regular. It can never be used to prove that a language is regular.

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46.

A transition diagram in scanner design is

A.a graph whose edges are labelled with input characters
B.the parse tree produced by the syntax analysis phase
C.a listing of the tokens that the scanner will recognise
D.the flow graph over which data flow analysis is performed

Answer: A

Nodes are states and edges are moves on input characters. Hand written scanners are often coded directly from such diagrams, with each state becoming a labelled section of code. Generated scanners encode the same diagram as a table rather than as explicit code.

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47.

In a hand written scanner, a retract or unget operation is needed when

A.the input buffer must be refilled from the source file
B.a character was read that belongs to the next lexeme rather than this
C.the symbol table already contains the identifier being scanned
D.the parser rejects the token that the scanner has only just produced

Answer: B

Correct answer: a character was read that belongs to the next lexeme rather than this

The scanner often knows a lexeme has ended only by reading the character after it - the digits of a number end at the first non-digit. That character belongs to the next lexeme, so it must be pushed back for the next call to read. This is why one character of lookahead, and a way of retracting it, is part of every scanner.

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48.

A scanner generator such as Lex or Flex produces

A.a machine independent form of the program ready for optimisation
B.the symbol table implementation used by the rest of the compiler
C.a program that recognises the specified tokens in an input stream
D.a parser that checks the input against a context free grammar

Answer: C

Correct answer: a program that recognises the specified tokens in an input stream

Lex takes a specification of patterns with actions and emits source code for a scanner - typically a table-driven automaton together with the driver that runs it. The scanner it produces recognises the specified tokens and executes the corresponding action for each. Producing a parser is the job of a separate tool such as Yacc.

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49.

A Lex specification is divided into

A.a lexical part and a syntactic part separated by a blank line
B.one section per token that the scanner is to recognise
C.a grammar section followed by the semantic actions for each rule
D.a declarations section, a rules section and a user code section

Answer: D

Correct answer: a declarations section, a rules section and a user code section

The three sections are separated by lines containing %%. The declarations hold regular definitions and any C declarations needed; the rules pair each pattern with the action to run when it matches; and the user code section is copied verbatim into the generated file, and is where supporting functions go.

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50.

The action attached to a Lex rule is executed

A.when the corresponding pattern matches the longest available lexeme
B.once at the very start of the scan
C.only if the parser subsequently accepts the token produced
D.after the whole input has been read and tokenised completely

Answer: A

Correct answer: when the corresponding pattern matches the longest available lexeme

Lex applies two rules in order: match the longest possible lexeme, and where two patterns match a lexeme of the same length, prefer the one written earlier. Only then is the action for the winning rule executed. This is why keyword patterns are written before the identifier pattern - both match if, and order decides.

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