Lexical Analysis Questions and Answers

Q: Consider the following statements about scanner generators.1. They resolve ties between equal length matches by rule order.2. They apply the longest match rule when patterns overlap.3. They require the token patterns to be context free grammars.

Correct answer: 1 and 2 onlyStatements 1 and 2 are the two disambiguation rules, and they are applied in that order: longest match first, then rule order to settle a tie. Statement 3 is wrong - the patterns are regular expressions, not context free grammars, which is precisely what makes the mechanical construction of an automaton possible.

41.

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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42.

In a Lex action the variable holding the matched text is conventionally

A.yylineno
B.yytext, with its length available in yyleng
C.yylval
D.yyin

Answer: B

yytext points at the matched lexeme and yyleng gives its length. yylval carries the attribute value to the parser and yylineno tracks the line, so each has a distinct role. Confusing yytext with yylval is a frequent source of bugs in hand edited scanners.

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43.

Consider the following statements about scanner generators.
1. They resolve ties between equal length matches by rule order.
2. They apply the longest match rule when patterns overlap.
3. They require the token patterns to be context free grammars.

A.1 only
B.All of them
C.1 and 2 only
D.3 only

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are the two disambiguation rules, and they are applied in that order: longest match first, then rule order to settle a tie. Statement 3 is wrong - the patterns are regular expressions, not context free grammars, which is precisely what makes the mechanical construction of an automaton possible.

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44.

A table driven scanner stores

A.the complete source program in memory before the scanning begins
B.each token produced along with the line on which it appeared
C.the grammar productions used by the parser during reduction
D.the transition function of the automaton as a two dimensional array

Answer: D

Correct answer: the transition function of the automaton as a two dimensional array

A table-driven scanner keeps the automaton as a table indexed by current state and input character, whose entry is the next state. The driver is then a short loop: look up, move, repeat. The approach separates the automaton from the code that runs it, which is why generated scanners take this form and why table compression matters.

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45.

Compressing the transition table of a scanner is worthwhile because

A.the table is large and mostly filled with identical error entries
B.an uncompressed table cannot represent all possible transitions
C.compression makes the scanner recognise a larger class of languages
D.the parser requires the table in a compressed form to operate

Answer: A

A row for every state crossed with every character in the alphabet grows quickly. Techniques such as default rows and displacement tables cut the space greatly at a small cost in access time. The saving matters most for scanners with a full byte alphabet and many states.

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46.

Lexical errors are those in which

A.the types of two operands are incompatible with the operator
B.a sequence of characters forms no valid lexeme of the language
C.the tokens are individually valid but violate the grammar
D.an identifier is used before it has been declared anywhere

Answer: B

An illegal character or an unterminated literal is a lexical error. Ordering faults are syntactic and type faults semantic, and each is caught by the corresponding later phase. Assigning each error to the earliest phase able to detect it gives the best diagnostics.

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47.

The following are all lexical errors EXCEPT

A.a numeric literal containing two decimal points in one lexeme
B.an illegal character such as a stray hash sign in the source
C.an if statement whose closing brace has been omitted
D.an unterminated string literal running to the end of the file

Answer: C

The first three concern the formation of individual lexemes. A missing brace leaves every lexeme valid and is detected by the parser when the token sequence no longer matches the grammar. The scanner happily produced every token; only their arrangement is at fault.

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48.

Panic mode recovery in a lexical analyser means

A.giving up on the whole compilation the moment a fault is found
B.asking the parser to decide how the error should be handled
C.inserting the characters needed to complete the current lexeme
D.deleting characters until a well formed lexeme can be recognised

Answer: D

Correct answer: deleting characters until a well formed lexeme can be recognised

Panic mode is the simplest recovery a scanner can offer: on encountering characters that form no legal lexeme, discard them one at a time until the remaining input begins something recognisable, report the error, and carry on. It is crude but it lets the compiler find later errors instead of stopping at the first.

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49.

The minimum distance correction strategy for lexical errors seeks

A.the smallest number of character changes that yields a valid lexeme
B.the lexeme that is alphabetically closest to the erroneous text found
C.the shortest valid lexeme regardless of how much it differs
D.the token whose pattern has the fewest states in its automaton

Answer: A

Insertions, deletions, replacements and transpositions are counted and the cheapest repair is chosen. It gives good diagnostics but is expensive, so most scanners use simpler heuristics. The extra cost is hard to justify when a plain message names the offending character.

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50.

Why is lexical analysis separated from parsing rather than combined?

A.combining them would make it impossible to detect any errors
B.it simplifies the design and lets each use its most suitable notation
C.the two phases must run on different machines within a compiler suite
D.the language standard requires them to be separate components

Answer: B

Correct answer: it simplifies the design and lets each use its most suitable notation

Separating the two lets the scanner use regular expressions, which are simple and fast, for the parts of the language that are regular, and lets the parser use a context free grammar for the parts that are not. It also confines the handling of white space, comments and buffering to one place, and it makes both halves easier to test and to reuse.

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