Syntax Directed Translation Questions and Answers

Q: State whether the following is true or false.Semantic rules in a syntax directed definition may have side effects.

Correct answer: TrueRules may print output, enter names in the symbol table, emit code or report errors, and most practical translations depend on their doing so. The price is that the order of evaluation now matters in a way it does not for pure attribute computation, which is why the restricted classes are used.

21.

The reason a mid-production action cannot simply be executed when the parser reaches that point in a bottom up parse is that

A.the parser does not yet know which production it is working on
B.the value stack has already been emptied by that stage
C.actions are forbidden anywhere but at the end of a right side
D.the lookahead token has not been read from the scanner yet

Answer: A

Until a reduction occurs the parser has committed to nothing, so there is no moment at which it could know that this particular action applies. Inserting an empty nonterminal manufactures a reduction at the wanted point, which is how generators implement such actions and also why they sometimes introduce conflicts.

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22.

Assertion (A): Introducing a marker nonterminal into a grammar can create a parsing conflict that was not present before.
Reason (R): The parser must decide to reduce the empty marker before it has seen the symbols that follow it.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

Both are true and the reason gives the mechanism, and it is a common surprise in practice. A grammar that generated cleanly may report a conflict as soon as a mid-rule action is added, because the action silently introduces such a marker.

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23.

A top down parser evaluates an L-attributed definition by

A.using a value stack parallel to an explicit parsing stack
B.deferring every last piece of the evaluation until the parse has finished
C.passing inherited attributes as parameters and returning synthesised ones
D.building a dependency graph and topologically sorting it

Answer: C

Correct answer: passing inherited attributes as parameters and returning synthesised ones

In recursive descent each nonterminal is a procedure, so the natural mechanism is already present: inherited attributes go in as parameters and synthesised ones come back as results. This is why recursive descent handles L-attributed definitions so comfortably, and it is the argument most often made for writing the parser by hand.

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24.

The evaluation order that suffices for every L-attributed definition is

A.a breadth first traversal visiting each level in turn
B.a right to left traversal of each production's right side
C.any order at all
D.a depth first left to right traversal of the parse tree

Answer: D

The L condition was chosen precisely so that this one traversal always works. Inherited attributes are ready when a subtree is entered and synthesised ones when it is left. Recursive descent performs exactly this traversal without being asked to.

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25.

Converting a definition that uses inherited attributes into an S-attributed one sometimes requires

A.restructuring the grammar so the information flows upward instead
B.removing the semantic rules from the offending productions
C.adding still more inherited attributes to the remaining productions
D.abandoning syntax directed translation for that construct

Answer: A

Correct answer: restructuring the grammar so the information flows upward instead

Where a bottom up parser must evaluate a definition that uses inherited attributes, one remedy is to rewrite the grammar so the information is available from below - splitting a production so that the inherited value is synthesised by a marker nonterminal, for instance. The alternative is a marker with a mid-rule action, which amounts to the same thing.

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26.

In the declaration grammar D goes to T L, passing the type from T to L uses

A.no attribute at all
B.an inherited attribute on L set from the synthesised type of T
C.a synthesised attribute on L read by the rule for D
D.a synthesised attribute on T computed from those of L

Answer: B

Correct answer: an inherited attribute on L set from the synthesised type of T

The type is determined by T and needed by every name in L, so it must travel downward - an inherited attribute. Note that both directions appear in one rule: the type is first synthesised up from T, then inherited down into L. This is the standard example of an L-attributed definition.

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27.

The symbol table is usually updated by semantic rules that

A.run only after the entire parse has been completed
B.are attached to the terminal symbols rather than the productions
C.have side effects rather than merely computing attribute values
D.compute a synthesised attribute holding the whole table

Answer: C

Installing an entry is a side effect. Such rules are permitted in a translation scheme, but their order of execution then matters, which is why action placement must be specified precisely. A definition without placement leaves the behaviour of side effects undefined.

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28.

State whether the following is true or false.
Semantic rules in a syntax directed definition may have side effects.

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

Answer: D

Correct answer: True

Rules may print output, enter names in the symbol table, emit code or report errors, and most practical translations depend on their doing so. The price is that the order of evaluation now matters in a way it does not for pure attribute computation, which is why the restricted classes are used.

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29.

The dependency graph for an S-attributed definition always has edges

A.from child nodes upward toward their parents
B.from parent nodes downward toward their children
C.between sibling nodes at the same level of the tree
D.forming at least one cycle in every parse tree

Answer: A

Synthesised attributes depend only on children, so every edge points upward. A graph whose edges all point one way through a tree can never contain a cycle. This is why the S-attributed class needs no circularity test at all.

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30.

A production has 3 symbols on its right side, each contributing one synthesised attribute to the left side. How many dependency edges does this production contribute at each node where it is used?

Answer: B

One edge runs from each right side attribute to the single left side attribute, giving three edges. The count would rise if the left side carried more than one attribute of its own. Counting edges is a useful check when a definition is first written down.

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