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Examine the following buggy Invoker code snippet implementing undo/redo stacks. Which line contains the subtle bug that can cause incorrect redo behavior after executing a new command?

medium🐞 Bug Identification Q14 of Q15

OOP & Design Patterns - Command Pattern - Undo/Redo, Request Queuing & Logging

Examine the following buggy Invoker code snippet implementing undo/redo stacks. Which line contains the subtle bug that can cause incorrect redo behavior after executing a new command?

ALine where redo_stack is not cleared after new command execution

BLine where redo_stack is appended in undo

CLine where command.execute() is called in execute_command

DLine where undo_stack is popped in undo

Step-by-Step Solution

Solution:

Step 1: Identify expected redo stack behavior
After executing a new command, redo stack must be cleared to avoid invalid redo commands.
Step 2: Locate missing redo_stack.clear()
The execute_command method lacks clearing redo_stack, so redo commands remain stale.
Final Answer:
Option A -> Option A
Quick Check:
Redo stack not cleared after new command breaks redo correctness [OK]

Quick Trick: Redo stack must clear on new command execution [OK]

Common Mistakes:

MISTAKES

Forgetting to clear redo stack
Undoing commands in wrong order

Trap Explanation:

PITFALL

The missing redo_stack.clear() is subtle but critical; other lines look suspicious but are correct.

Interviewer Note:

CONTEXT

Checks candidate's attention to subtle state management bugs in undo/redo implementations.

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