Syntax Directed Translation Questions and Answers

Q: Match each attribute usage with the direction of information flow.List IA. the value of an arithmetic expressionB. the type declared for a list of namesC. the width of a generated code fragmentD. the nesting level of the enclosing blockList II1. upward, synthesised2. upward, synthesised3. downward, inherited4. downward, inherited

Correct answer: A-2, B-4, C-1, D-3The direction follows from where the information comes from. An expression’s value and a code fragment’s width are both built up from the parts below, so they are synthesised. A declared type must be passed down to the list of names it applies to, and a nesting level comes from the enclosing block, so both are inherited.

Q: A syntax directed definition is S-attributed when

Correct answer: every attribute in it is synthesisedS-attributed definitions use no inherited attributes at all, so information flows only upward. That makes them exactly what a bottom up parser can evaluate during the parse, with the attribute values held on a stack parallel to the parsing stack and no extra machinery at all.

Q: Consider the following statements about attribute classes.1. Every S-attributed definition is also L-attributed.2. Every L-attributed definition is also S-attributed.3. L-attributed definitions can be evaluated in one depth first pass.

Correct answer: 1 and 3 onlyStatements 1 and 3 are correct: an S-attributed definition has no inherited attributes at all, so it satisfies the L-attributed condition vacuously, and L-attributed definitions evaluate in one depth-first pass. Statement 2 reverses the containment - an L-attributed definition may use inherited attributes, which an S-attributed one by definition does not.

A syntax directed definition attaches to each production of a grammar

A.a set of semantic rules relating the attributes of its symbols
B.a pattern describing which tokens the production is allowed to contain
C.how tightly the operators it mentions bind and which way they group
D.the target machine instructions that implement the construct

Answer: A

Correct answer: a set of semantic rules relating the attributes of its symbols

A syntax directed definition pairs each production with rules saying how the attributes of the symbols in it are related - the value of the left side from those of the right, or the reverse. It says what is to be computed without saying when, which is what distinguishes it from a translation scheme.

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An attribute of a grammar symbol is

A.the position of the symbol within the right side of a production
B.a value associated with an occurrence of that symbol in a tree
C.a terminal that may appear immediately after the symbol
D.one of the productions in which the symbol appears on the left

Answer: B

Correct answer: a value associated with an occurrence of that symbol in a tree

An attribute belongs to a node rather than to the symbol in the abstract, so the same nonterminal appearing twice in a tree carries two independent sets of values. Attributes hold whatever the translation needs - a type, a value, a piece of generated code, an address.

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A synthesised attribute of a node is computed from

A.the attributes of every node appearing anywhere in the tree
B.the tokens remaining in the input at the time of evaluation
C.the attributes of that node's children and of the node itself
D.the attributes of that node's parent and left siblings only

Answer: C

Correct answer: the attributes of that node's children and of the node itself

A synthesised attribute flows upward, computed from below. That direction is what makes S-attributed definitions so convenient for bottom up parsing: by the time a production is reduced, every child has already been reduced and its attributes are available on the stack.

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Where does an inherited attribute of a node get its value from?

A.the attributes of that node's own children exclusively
B.the tokens that the scanner has yet to produce
C.the target machine on which the program will eventually run
D.the attributes of that node's parent and its siblings

Answer: D

Information flows downward or sideways across the tree. Inherited attributes express context, such as the type declared at the head of a declaration being passed to each name in the list. Without inherited attributes that construct cannot be handled in one pass.

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Match each attribute usage with the direction of information flow.
List I
A. the value of an arithmetic expression
B. the type declared for a list of names
C. the width of a generated code fragment
D. the nesting level of the enclosing block
List II
1. upward, synthesised
2. upward, synthesised
3. downward, inherited
4. downward, inherited

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

Answer: A

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

The direction follows from where the information comes from. An expression’s value and a code fragment’s width are both built up from the parts below, so they are synthesised. A declared type must be passed down to the list of names it applies to, and a nesting level comes from the enclosing block, so both are inherited.

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A syntax directed definition is S-attributed when

A.it contains no attributes at all on any of its symbols
B.every attribute in it is synthesised
C.every attribute in it is inherited from the parent node
D.it uses both synthesised and inherited attributes together

Answer: B

Correct answer: every attribute in it is synthesised

S-attributed definitions use no inherited attributes at all, so information flows only upward. That makes them exactly what a bottom up parser can evaluate during the parse, with the attribute values held on a stack parallel to the parsing stack and no extra machinery at all.

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An S-attributed definition can be evaluated during

A.a separate pass after the whole tree has been discarded
B.the lexical analysis phase before parsing has begun
C.a bottom up parse, computing values at each reduction
D.a top down parse only, since attributes flow downward

Answer: C

Every attribute is synthesised, so the children are complete when the parent is reduced. Yacc-style value stacks implement exactly this evaluation with no extra machinery. Nothing beyond the parallel stack is needed to realise the whole definition.

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A syntax directed definition is L-attributed when each inherited attribute of a right side symbol depends only on

A.the attributes of symbols appearing anywhere in the production
B.the synthesised attributes of the symbols to its right
C.attributes computed in a completely different production
D.the left side's inherited attributes and symbols to its left

Answer: D

Correct answer: the left side's inherited attributes and symbols to its left

The restriction is to information already available when the symbol is reached in a left-to-right traversal - what came down from the parent, and what has been computed from the siblings already passed. That is precisely what makes a single depth-first left-to-right pass sufficient.

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Consider the following statements about attribute classes.
1. Every S-attributed definition is also L-attributed.
2. Every L-attributed definition is also S-attributed.
3. L-attributed definitions can be evaluated in one depth first pass.

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

Answer: A

Correct answer: 1 and 3 only

Statements 1 and 3 are correct: an S-attributed definition has no inherited attributes at all, so it satisfies the L-attributed condition vacuously, and L-attributed definitions evaluate in one depth-first pass. Statement 2 reverses the containment - an L-attributed definition may use inherited attributes, which an S-attributed one by definition does not.

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10.

Assertion (A): Every S-attributed definition is also L-attributed.
Reason (R): The L condition constrains only inherited attributes, of which an S-attributed definition has none.

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 the containment is strict. An L-attributed definition passing a declared type down a list of identifiers uses an inherited attribute and so lies outside the S-attributed class.

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