Syntax Analysis and Parsing Questions and Answers

Q: A grammar in which every production has two symbols on the right side derives a sentence of 8 tokens. How many reduction steps does a bottom-up parse perform?

Correct answer: 7Each reduction combines two symbols into one, so it reduces the count on the stack by one. Starting from 8 tokens and finishing with the single start symbol requires \[ 8 - 1 = 7 \] reductions. Equivalently, a binary tree with 8 leaves has 7 internal nodes, and each internal node is one reduction.

61.

Precedence and associativity declarations in a parser generator are used to

A.resolve conflicts without rewriting the grammar into levels
B.define the tokens that the scanner is expected to produce
C.specify the order in which semantic actions will be executed
D.select which of several parsing algorithms will be generated

Answer: A

Correct answer: resolve conflicts without rewriting the grammar into levels

Declaring the precedence and associativity of the operators lets the generator settle shift-reduce conflicts in a short ambiguous expression grammar, instead of the grammar being expanded into one nonterminal per precedence level. The result is a smaller grammar, a smaller table and a specification that reads more directly.

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62.

An ambiguous expression grammar with precedence declarations produces

A.exactly the same table as the unambiguous grammar would
B.a smaller parse table than the equivalent unambiguous grammar
C.a parser that rejects some expressions the language permits
D.a parser that no available tool is able to generate at all

Answer: B

Correct answer: a smaller parse table than the equivalent unambiguous grammar

The unambiguous grammar needs a nonterminal for each precedence level and a chain of single-symbol productions linking them, and every one of those levels adds states to the table. The ambiguous grammar has one nonterminal for expressions and lets the declarations resolve the conflicts, so the table is smaller and the parse does less work.

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63.

Panic mode recovery in a parser discards input tokens until

A.the stack has been emptied of all grammar symbols
B.the same token appears twice in succession in the input
C.a synchronising token from a chosen set is encountered
D.the end of the input file has finally been reached

Answer: C

Synchronising tokens are typically statement terminators or closing brackets. The parser pops stack symbols until a state accepting the found token is reached, then continues. The method never loops and is simple to implement, which is why it is ubiquitous.

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64.

Error productions in a grammar allow the parser to

A.correct the source program automatically before compiling it
B.continue parsing without reporting the error to the user
C.generate code for the erroneous construct as if it were valid
D.recognise a common mistake and issue a precise diagnostic

Answer: D

By writing a production for an anticipated error the author can attach a tailored message. The construct is recognised and reported, but no code is generated for the faulty program. Anticipating the most common mistakes pays for itself in support time.

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65.

A good parser error message should

A.name the position and describe what was expected there
B.list every production of the grammar that failed to apply
C.print the entire parser stack at the moment of the error
D.suggest a complete rewriting of the offending statement

Answer: A

Position and expectation are what a programmer needs. Dumping internal state helps nobody outside the compiler team, and confident rewriting suggestions are often wrong. The best messages name the construct in the terms the programmer used.

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66.

Cascading errors after a single mistake are best reduced by

A.parsing the program a second time from the beginning
B.recovering to a synchronising point before reporting again
C.reporting only the very first error and then stopping
D.suppressing all error messages after the first ten are issued

Answer: B

Once the parser is out of step, subsequent messages are usually spurious. Resynchronising at a statement boundary before resuming keeps later reports meaningful. A compiler that floods the screen after one typing slip is worse than useless.

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67.

A parse tree with n leaves for a grammar in which every production has exactly two symbols on the right has

A.two n internal nodes in every possible case
B.a number of internal nodes that cannot be determined
C.n minus 1 internal nodes
D.n internal nodes, one for each leaf of the tree

Answer: C

Each internal node combines two subtrees into one, reducing the count by one. Starting from n leaves and ending at a single root therefore requires exactly n minus 1 combinations. The same identity underlies the node count of any full binary tree.

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68.

A grammar in which every production has two symbols on the right side derives a sentence of 8 tokens. How many reduction steps does a bottom-up parse perform?

A.8
B.9
C.16
D.7

Answer: D

Correct answer: 7

Each reduction combines two symbols into one, so it reduces the count on the stack by one. Starting from 8 tokens and finishing with the single start symbol requires \[ 8 - 1 = 7 \] reductions. Equivalently, a binary tree with 8 leaves has 7 internal nodes, and each internal node is one reduction.

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69.

An abstract syntax tree is preferred to a parse tree for later phases because

A.it omits punctuation and chains of single productions
B.it contains more nodes and therefore more information
C.it can be built without reference to the grammar at all
D.it records the exact sequence of productions applied

Answer: A

The abstract tree keeps operators and operands and discards syntactic scaffolding. Semantic analysis and code generation both become simpler on the smaller structure. The parse tree is often never materialised at all, only the abstract one.

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70.

Syntax directed definitions attach to grammar productions

A.the target machine instructions for the construct parsed
B.semantic rules computing attribute values for the symbols
C.the precedence and associativity of the operators involved
D.the error messages to be issued if the production fails

Answer: B

Correct answer: semantic rules computing attribute values for the symbols

A syntax directed definition pairs each production with rules that compute attributes of the symbols in it - a type, a value, an address, a piece of generated code. The parse then carries the semantic work along with it, which is how meaning is attached to structure without a separate traversal.

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