GATE CSE 2021 Set 1 Q14: Procedure calls and returns vs activation tree traversals

Consider the following statements.

$S_1$ : The sequence of procedure calls corresponds to a preorder traversal of the activation tree.

$S_2$ : The sequence of procedure returns corresponds to a postorder traversal of the activation tree.

Which one of the following options is correct?

Try it: multiple choice: exactly one correct option

Wrong answer costs 1/3 mark in GATE.

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Answer: C (official GATE 2021 final answer key)

Concept used: In an activation tree a node is entered before its children and returns only after all of them return.

First step: A call happens when its node is first visited (preorder); a return happens after all callees return (postorder).

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Source: official GATE 2021 Set 1 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key