[Year 12 SofDev] Sample Exam Questions

Sun May 1 16:56:26 EST 2011

G'Day again

Whilst 'tis true that this question on the end of year exam would unduly 
stress the little dears, to play devil's advocate, it does have a useful 
function as a sample to provoke discussion about what assumptions must 
be made. i.e. for the question to have a non trivial answer, what 
assumptions are necessary?

Firstly the memory locations used for the stack do not have to be 
initialised to 0 or any other value when the stack is created. The 
pointer is the only thing that must be initialised.

If you assume the stack starts at 1 instead of 0, then the stack will 
not be full after step 6 reducing the value of the question.

I also assumed that the bottom two values were in bold for a purpose, 
the most obvious (at least to me) being that they were valid existing 
members of the stack.
At the start of the question the stack is not full, the TOS pointer is 
1, the stack would be full when it reaches 4, so the non-bold values 
were from previous uses of the stack. NB. a pop does not delete the 
value from the stack, it just changes the TOS pointer.

It may well be that a different set of assumptions will produce a 
meaningful answer, these were the first consistent set I thought of.

If you assume an empty stack at the start then, given that the pointer 
is 1 at the start of the question, the TOS pointer must point to the 
next available location (closer to convention) and the numbering must 
start at 1 instead of 0.  In this case the stack is not full after step 
6 so another push step would be needed in the question.

Stack at start and after each step

Step

Function

Top

Output

Start

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

1

Push (23)

2

Item added (23)

Stack position

5

92

92

92

92

92

92

92

2

Push (18)

  3

Item added (18)

4

75

75

18

18

75

75

47

3

Pop

2

Item removed (18)

3

23

23

23

23

23

92

92

4

Push (75)

  3

Item added (75)

2

83

83

18

18

75

75

75

5

Push (92)

4

Item added (92)

1

52

23

23

23

23

23

23

6

Push (47)

5

Item added (47)

Top

1

2

3

2

3

4

5

One would hope that a "real" question would at least have numbered the 
stack positions and provided a key to explain what the bold signified. A 
question to sort sheep from goats????????????

Mike

1

On 1/05/2011 12:12 AM, Kevork Krozian wrote:
>
> Hi Mark and Mike,
>
>   I am a little late on the scene but have to support Mark here.
>
> The question on stacks is more a puzzle than a reasonable problem for 
> a student to solve in limited time.
>
> The question begins with a picture of a stack to consider....... which 
> suggests the starting position of the problem. Apparently it is in a 
> full state .... and that can be determined from ?????
>
> Then, a push (23) is introduced. Hang on, I thought we had a stack to 
> consider with 5 elements. Are we adding another 23 or picking up half 
> way through the introduced stack ?
>
> Where is it suggested that the introduced stack is in a "stack full" 
> state ?
>
> The push and pop operations are not the problem, just the starting 
> position of the problem. When is the beginning is what got me first. 
>  The bold items were no help as I didn't consider them to be significant.
>
> Also, the item Top (1) might mean top of stack to the writer, but only 
> adds to the confusion as it is not obvious it means the top of the 
> stack when we start with a stack with 5 elements.
>
> I have gone through my 2^nd year Data Structures textbooks and have a 
> strong recollection of the 1982 exams I did ( why wouldn't I ?? ) on 
> stacks and data structures and there was always a pointer or arrow 
> (with a value in a variable ) showing the top of a stack at the 
> beginning of a question and the requirement was to draw the stack 
> through the various states after push and pop functions. Apart from 
> that we either have empty or full states.
>
> I can't see how  this question would have helped in any way to show 
> student's understanding of a stack and its functions.
>
> Kind Regards
>
> Kevork Krozian
>
> Edulists Creator Administrator
>
> www.edulists.com.au
>
> tel: 0419 356 034
>
> *From:*sofdev-bounces at edulists.com.au 
> [mailto:sofdev-bounces at edulists.com.au] *On Behalf Of *Mark KELLY
> *Sent:* Wednesday, 27 April 2011 2:36 PM
> *To:* Year 12 Software Development Teachers' Mailing List
> *Subject:* Re: [Year 12 SofDev] Sample Exam Questions
>
> Thanks for the bone, Mike.  Woof!  It finally let me work out the many 
> assumptions underlying the question: and after all that, I'm not 
> wagging my tail.
>
> I think the question is far from clear, and I doubt students would 
> make much of it in a real exam.
>
> - It does not, for example, say that the stack is zero-based.  If one 
> assumes (as I did) that the stack indexing starts at 1, the whole 
> thing changes and nothing makes sense.
> - It does not indicate what the mysterious bolding of the bottom 2 
> items is meant to represent.  If the examiners invent their own 
> conventions, they should explain them to us.
> - It does not tell us that the stack pointer in updated *before* a 
> push/pop instead of being changed afterwards, as often happens.  
> Again, this changes everything.
> - One has to realise that the stack is expanding from the bottom and 
> not from the top. Stacks are often represented growing in either 
> direction.
>
> When it comes to real-world stack implementation, there are so many 
> different options such as these that I found the question 
> mind-boggling while I went through the permutations of possible readings.
>
> Grrr.
>
> On 21 April 2011 14:45, Mike Brookes <mikebr at tpg.com.au 
> <mailto:mikebr at tpg.com.au>> wrote:
>
> Hi Mark et Al
>
> For the question to make sense one has to make the assumption that the 
> stack positions start at the bottom with position zero and go up to 
> position 4 which is the top of the stack. At the start the bottom two 
> (in bold on the PDF) are valid, hence the top of stack pointer showing 1.
>
> Below is the completed table and the stack at the start and after each 
> step:
>
>
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
>
> 	
> 	
> 	
> 	
> 	
> 	
> 	
>
> Stack at start and after each step
>
> Step
>
> 	
>
> Function
>
> 	
>
> Top
>
> 	
>
> Output
>
> 	
> 	
> 	
> 	
>
> Start
>
> 	
>
> Step 1
>
> 	
>
> Step 2
>
> 	
>
> Step 3
>
> 	
>
> Step 4
>
> 	
>
> Step 5
>
> 	
>
> Step 6
>
> 1
>
> 	
>
> Push (23)
>
> 	
>
> 2
>
> 	
>
> Item added (23)
>
> 	
> 	
>
> Stack position
>
> 	
>
> 4
>
> 	
>
> 92
>
> 	
>
> 92
>
> 	
>
> 92
>
> 	
>
> 92
>
> 	
>
> 92
>
> 	
>
> 92
>
> 	
>
> 92
>
> 2
>
> 	
>
> Push (18)
>
> 	
>
> 	
>
> Item added (18)
>
> 	
> 	
>
> 3
>
> 	
>
> 75
>
> 	
>
> 75
>
> 	
>
> 18
>
> 	
>
> 18
>
> 	
>
> 75
>
> 	
>
> 75
>
> 	
>
> 75
>
> 3
>
> 	
>
> Pop
>
> 	
>
> 2
>
> 	
>
> Item removed (18)
>
> 	
> 	
>
> 2
>
> 	
>
> 23
>
> 	
>
> 23
>
> 	
>
> 23
>
> 	
>
> 23
>
> 	
>
> 23
>
> 	
>
> 23
>
> 	
>
> 23
>
> 4
>
> 	
>
> Push (75)
>
> 	
>
> 	
>
> Item added (75)
>
> 	
> 	
>
> 1
>
> 	
>
> 83
>
> 	
>
> 83
>
> 	
>
> 83
>
> 	
>
> 83
>
> 	
>
> 83
>
> 	
>
> 83
>
> 	
>
> 83
>
> 5
>
> 	
>
> Push (92)
>
> 	
>
> 4
>
> 	
>
> Item added (92)
>
> 	
> 	
>
> 0
>
> 	
>
> 52
>
> 	
>
> 52
>
> 	
>
> 52
>
> 	
>
> 52
>
> 	
>
> 52
>
> 	
>
> 52
>
> 	
>
> 52
>
> 6
>
> 	
>
> Push (47)
>
> 	
>
> 	
>
> Stack full
>
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
> 	
>
> 	
> 	
> 	
> 	
> 	
> 	
>
> Top
>
> 	
>
> 1
>
> 	
>
> 2
>
> 	
>
> 3
>
> 	
>
> 2
>
> 	
>
> 3
>
> 	
>
> 4
>
> 	
>
> 4
>
>
> Mike Brookes
> Semi retired gentleman from Copperfield College
>
>
>
> On 21/04/2011 1:35 PM, Mark KELLY wrote:
>
> Would someone be kind enough to throw me a bone about the stack 
> question - Q3?
>
> I thought I knew stack implementation from my assembly language days 
> with Z80... this question feels like walking into someone's 
> half-finished thought process.  I'm really confused about this.After 
> half an hour of conjecture, I've tried four times to ask a series of 
> sensible questions to make sense of it, and have failed every time.
>
> The only apparent answer involves time travel, reversed arrays, 
> irrelevant stack pointers and bogus bolding of stack items.  The 
> relationship between the stack and the table is only the beginning of 
> the problem...
>
> I'd be delighted if anyone could walk me through this...
>
> Regards
>
> Mark
>
>
> _______________________________________________
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>
>
>
>
> -- 
> Mark Kelly
> Manager of ICT, Reporting, IT Learning Area
> McKinnon Secondary College
> McKinnon Rd McKinnon 3204, Victoria, Australia
> Direct line / Voicemail: +613 8520 9085, Fax +613 9578 9253
> kel at mckinnonsc.vic.edu.au <mailto:kel at mckinnonsc.vic.edu.au>
> VCE IT Lecture Notes: http://vceit.com
> Moderator: IT Applications Edulist
>
> All generalisations are false, except this one.
>
>
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> IT Software Development Mailing List kindly supported by
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