GATE CSE 2003 | Question: 18

Question

In a bottom-up evaluation of a syntax directed definition, inherited attributes can

• A. always be evaluated
• B. be evaluated only if the definition is L-attributed
• C. be evaluated only if the definition has synthesized attributes
• D. never be evaluated

Comments

Read all the available solutions and still confused ?

Doubt :- for inherited attribute , defination must be L attributed. and if defination is L attributed then evaluation is done using depth first traversal (Left to right ).

and if it is S attributed then evaluation is done using Bottom Up parsing .

I know only these two points .

Can somebody plz explain this que in simple word ??

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Please correct me if I am wrong I am completely confused by all the answers here is what I think,S-attributed SDT attributes are evaluated during Bottom Up parsing but since there is inheritance no chance of S-attribute and L-attributed is used for depth first,left to right making option D as the correct answer.Please Help

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@sripo

check my comment on the answer of arjun sir

Answer 1

Answer should be (B). 

$(\text{p only if q} )\leftrightarrow (p\rightarrow q)$

so B is, evaluation is possible implies SDD is L-attributed,
which us true. Not all L-attributed SDDs can be evaluated in bottum-up parser but all SDDs which can be evaluated are L-attributed

Answer 2

In a bottom-up evaluation of a syntax-directed definition, inherited attributes can be evaluated only if the definition is L-attributed.

Explanation:

1. Always be evaluated:

   • Incorrect. In a bottom-up evaluation, the inherited attributes are evaluated when the corresponding non-terminal is reduced during the bottom-up parsing process. However, this depends on whether the definition is L-attributed.

2. Be evaluated only if the definition is L-attributed:

   • Correct. L-attributed definitions ensure that the inherited attributes can be evaluated in a bottom-up manner. In L-attributed definitions, attributes associated with the leftmost derivations can be computed bottom-up.

3. Be evaluated only if the definition has synthesized attributes:

   • Incorrect. Inherited attributes are distinct from synthesized attributes. Both types of attributes can coexist in a syntax-directed definition, and the evaluation of inherited attributes depends on whether the definition is L-attributed.

4. Never be evaluated:

   • Incorrect. In a syntax-directed definition, inherited attributes are evaluated during the parsing process. However, the ability to evaluate them in a bottom-up manner depends on the characteristics of the definition.