Top Down Parsing Questions and Answers

Q: Match each recovery technique with its characteristic action.List IA. panic modeB. phrase levelC. error productionsD. global correctionList II1. recognise an anticipated mistake2. skip tokens to a synchronising point3. find the minimal repair to the input4. insert or delete a symbol locally

Correct answer: A-2, B-4, C-1, D-3Four strategies of increasing ambition. Panic mode discards tokens until it reaches a synchronising symbol such as a semicolon. Phrase level recovery makes a local repair, inserting or deleting a single symbol. Error productions build the anticipated mistake into the grammar so it can be recognised and reported precisely. And global correction seeks the smallest set of changes making the input legal - theoretically attractive and too costly to use.

41.

Parsing expression grammars differ from context free grammars in that their choice operator is

A.ordered, so the first successful alternative is taken
B.unordered, so all alternatives are explored in parallel
C.restricted to at most two alternatives per rule
D.applied only to terminals and never to nonterminals

Answer: A

Correct answer: ordered, so the first successful alternative is taken

In a parsing expression grammar the choice operator is ordered: alternatives are tried in sequence and the first that succeeds is committed to. This makes every PEG unambiguous by construction, since there is exactly one parse - but it also means a rule can silently shadow a later alternative if the order is wrong.

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42.

An error entry in a predictive parsing table means

A.the input has been accepted and parsing is complete
B.the lookahead token cannot legally appear at that point
C.the grammar contains an error that must be corrected
D.the parser has run out of stack space to continue

Answer: B

Blank cells outnumber filled ones in a typical table. Reaching one invokes the error routine, which reports the fault and attempts to bring the parser back into step with the input. Good recovery is what distinguishes a usable compiler from a merely correct one.

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43.

Panic mode recovery in a predictive parser typically

A.puts in the token the table was hoping for and carries on
B.stops work entirely the instant a single fault has been announced
C.drops tokens until it reaches one that lets parsing resume safely
D.pops the entire stack and restarts the parse from the beginning

Answer: C

Synchronising sets are usually built from the FOLLOW sets of the nonterminals on the stack. Skipping to such a token lets the parser resume at a well defined point. The technique is crude but it never loops and it is trivial to implement.

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44.

Placing the FOLLOW set of a nonterminal in its error cells as synchronising tokens allows the parser to

A.insert the missing nonterminal and then carry on parsing as before
B.collapse the whole input down to the start symbol at once
C.detect that the grammar itself is not LL(1)
D.pop the nonterminal and continue when such a token appears

Answer: D

If the lookahead can follow A, then A has evidently been completed or omitted, so popping it is a reasonable repair. The technique costs nothing and improves recovery markedly. Filling the error cells this way is standard practice in generated parsers.

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45.

Phrase level recovery in a predictive parser inserts or deletes symbols in order to

A.allow parsing to continue past the point of the error
B.correct the source program so it compiles successfully
C.ensure that no further errors will be reported at all
D.convert the grammar into an LL(1) grammar automatically

Answer: A

The repair is local and aimed only at continuing. There is a risk of an infinite loop if the correction consumes no input, so implementations guard against that explicitly. A common guard is to require that at least one token be consumed per repair.

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46.

Match each recovery technique with its characteristic action.
List I
A. panic mode
B. phrase level
C. error productions
D. global correction
List II
1. recognise an anticipated mistake
2. skip tokens to a synchronising point
3. find the minimal repair to the input
4. insert or delete a symbol locally

A.A-3, B-1, C-4, D-2
B.A-2, B-4, C-1, D-3
C.A-4, B-2, C-1, D-3
D.A-2, B-4, C-3, D-1

Answer: B

Correct answer: A-2, B-4, C-1, D-3

Four strategies of increasing ambition. Panic mode discards tokens until it reaches a synchronising symbol such as a semicolon. Phrase level recovery makes a local repair, inserting or deleting a single symbol. Error productions build the anticipated mistake into the grammar so it can be recognised and reported precisely. And global correction seeks the smallest set of changes making the input legal - theoretically attractive and too costly to use.

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47.

Compared with a table driven parser, a hand written recursive descent parser usually offers

A.guaranteed linear time even on ambiguous grammars
B.automatic detection of conflicts in the grammar
C.better error messages and easier integration of semantic actions
D.the ability to parse a strictly larger class of grammars than before

Answer: C

Diagnostics can be written for each construct at exactly the point of failure. The cost is that no tool reports conflicts, so the author must verify the grammar properties by hand. A thorough test suite is the usual substitute for that missing static check.

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48.

A significant risk of writing a recursive descent parser by hand is

A.the parser running in exponential time on all inputs
B.a complete inability to parse any recursive construct of the language
C.the requirement to compute FIRST and FOLLOW at runtime
D.silently accepting a language different from the intended grammar

Answer: D

Correct answer: silently accepting a language different from the intended grammar

Nothing checks a hand-written parser against the grammar it was meant to implement, so a mistake in one procedure quietly changes the language accepted. A generated parser at least reports conflicts and is derived from the grammar mechanically - this is the strongest argument for using a tool.

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49.

A recursive descent parser is written for a grammar with 12 nonterminals. How many parsing procedures does it normally contain?

A.12
B.24
C.6
D.1

Answer: A

The usual structure is one procedure per nonterminal, so twelve are needed. Terminals are matched inline by a helper rather than by procedures of their own. The correspondence between grammar and code is what makes the style so readable.

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50.

The lookahead in a recursive descent parser is normally held in

A.the symbol table entry for the enclosing procedure
B.a single variable holding the current token from the scanner
C.an array containing every token of the whole input
D.the call stack alongside the procedure activation records

Answer: B

One variable suffices for LL(1). Advancing it means calling the scanner for the next token, which is why the two phases are so tightly coupled in this design. Extra lookahead would require buffering tokens the scanner has already produced.

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