[Year 12 SofDev] Sample Exam Questions

Sun May 1 19:46:00 EST 2011

Hi Andrew,

It is interesting you refer to the Cert IV as this is also what I am
teaching at present ( Cert IV in IT Networking ) ICA40405 as well as the
Diploma in IT (Networking) ICA50405. Is it as hard as the work on stacks ?
Since I am writing the material in both learning and assessment I have a
better handle on it than second guessing what the right height of the bar
should be .

Clearly the stack data structure at the depth we are expecting teachers and
students to interpret - if we wish to consider the various implementations -
lives in about 2nd year Computer Science. 

The follow up question is, if we are not implementing this to see it in
action and the benefits the stack data structure brings in solving real
problems, what precisely is the value of this puzzle ?

To answer, at what point are we satisfied that teachers are on the right
track , I am afraid we and many ( is it 500 ?? ) would not want to admit
they are seeking guidance and playing catch up.

We really need quality learning and teaching material that covers this sort
of thing and clearly articulates what is in the minds of the writers of
these exams. 

Sounds like a wish ......

Speak soon

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 Andrew Shortell
Sent: Sunday, 1 May 2011 7:02 PM
To: Year 12 Software Development Teachers' Mailing List
Subject: Re: [Year 12 SofDev] Sample Exam Questions

Hi Y'all

I really believe this indicated the teaching (and understanding of the
teaching) that goes on in classrooms.

At what point are we satisfied that teachers actually understand what they
are teaching, are on the right "track", are NOT GUESSING, and really spend
time to make sure their students understand really deep material that is
referenced in only one (or two) KK.

I would because I like the concepts of stacks - and the difference between
FIFO and LIFO, cells that have been previously populated with what is now
GIGO  etc etc.

But given that this is quite deep for year 12 level students (or was until
this study design) I would have moved on in the face of incomprehension.

Now we cannot.

So how do we raise the level of knowledge of teachers so that they do NOT
LET DOWN the trust of their students..?????

It is very obvious that we do not all have the same level of
understanding... and hello to the 500 "readers who are too scared to post or
do not feel confident in their opinions". One MIGHT presume that they are
not posting because they are following the discussion in the hope of
figuring out what it all means?

even the current Cert IV in IT does not include material like this that I
have found!  (NOTE my careful caveat!)

I am just glad I am not teaching SD this year given the level of confusion
in what a question like this might mean...!

Andrew

ps even in VB, stacks (lists etc) start at zero but we can safely ignore
that in a windoze environment

Andrew Shortell

Heidelberg Teaching Unit in 2011

On 01/05/2011, at 5:57 PM, Robert Hind wrote:

Hi all!

I have never taught SD. Stopped with CS way back. This has sent me way back
to old texts - 70s and 80s

How about we ignore the bold entries in the original question and read it to
mean that the stack is

92

75

23

83

52

and the Top-of-stack pointer = 1, ie the value 52.

This is what the question seems to say.

We then proceed through the steps as given in the question

Start    Push (23)    Push (18)    Pop    Push (75)    Push (92) Push (47)

92            92               92            92          92            92
*47

75            75               75            75          75            *92
92

23            23               *18           18         *75            75
75    

83            *23               23           *23         23             23
23

*52            52               52            52         52             52
52

Where * indicates the top-of-stack pointer

So where is the problem? Except perhaps in the way the question has been
written.

Robert Hind (Semi-retired) OOF, GOM
Ashwood and Traralgon
robert at yinnar.com

----- Original Message ----- 

From: Mike Brookes <mailto:mikebr at tpg.com.au>  

To: Year 12 Software Development Teachers' Mailing List
<mailto:sofdev at edulists.com.au>  

Sent: Sunday, May 01, 2011 4:56 PM

Subject: Re: [Year 12 SofDev] Sample Exam Questions

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 2nd 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 <http://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> 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
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Direct line / Voicemail: +613 8520 9085, Fax +613 9578 9253
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