Need Help Debugging PopMiddle in Dafny’s BoundedStackWithMiddle

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Asked By CodeMaster42 On

I'm working on a Dafny assignment for CSSE3100/7100 where I need to implement and verify three data structures: BoundedStack (Q1), BoundedDeque (Q2), and BoundedStackWithMiddle (Q3). While Q1 and Q2 verify without issues, I'm stuck on Q3. The problem arises in the PopMiddle method of BoundedStackWithMiddle; I'm encountering verification errors that I'm struggling to fix.

The task requires a bounded stack that can remove and return the middle element (located at n/2 for n elements). This structure employs a BoundedStack for the second half of the elements and a BoundedDeque for the first half, with the top of the stack at s[0].

I've submitted my work in a single Dafny file (A3.dfy) to Gradescope, and it's due on May 27, 2025. However, I keep getting syntax errors for PopMiddle that manifest as large exclamation marks in VS Code, specifically at lines 315 and 316. I'm hoping someone can assist me in figuring out what's wrong!

2 Answers

Answered By SyntaxSleuth99 On

It sounds like you're dealing with a classic Dafny syntax error issue. It's common to see misleading errors in Dafny that highlight the wrong line. Make sure to check the few lines before the errors; you might have left out a semicolon or mismatched some braces. Sharing your code would help us help you more effectively. Just post the specific method where you're stuck!

DafnyDude88 -

Yeah, definitely share that piece of code. Just the PopMiddle method would suffice. Once we can see the logic you're working with, we should be able to spot the syntax issue more easily.

Answered By HelpWantedHero On

I had a similar issue recently, and I found that sometimes the error is more about the logic than syntax. So after checking your syntax as the first step, review your logic closely. Just focus on that PopMiddle method and ensure that your indexing aligns with how Dafny expects it. Also, confirm that the way you're accessing elements in the BoundedStack and BoundedDeque is correct!

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