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.<br><br>I think the question is far from clear, and I doubt students would make much of it in a real exam.<br>
<br>- 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.<br>- 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.<br>
- It does not tell us that the stack pointer in updated <b>before</b> a push/pop instead of being changed afterwards, as often happens. Again, this changes everything.<br>- 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.<br>
<br>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.<br><br>Grrr.<br>
<br>
<div class="gmail_quote">On 21 April 2011 14:45, Mike Brookes <span dir="ltr"><<a href="mailto:mikebr@tpg.com.au">mikebr@tpg.com.au</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div text="#000000" bgcolor="#ffffff">
Hi Mark et Al<br>
<br>
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. <br>
<br>
Below is the completed table and the stack at the start and after
each step:<br>
<table border="0" cellpadding="0" cellspacing="0" width="817">
<colgroup><col style="width:26pt" width="35"> <col style="width:68pt" width="91"> <col style="width:23pt" width="30"> <col style="width:93pt" width="124"> <col style="width:22pt" width="29"> <col style="width:23pt" span="2" width="30"> <col style="width:48pt" span="7" width="64"> </colgroup><tbody>
<tr style="min-height:15.75pt" height="21">
<td style="min-height:15.75pt;width:26pt" width="35" height="21"><br>
</td>
<td style="width:68pt" width="91"><br>
</td>
<td style="width:23pt" width="30"><br>
</td>
<td style="width:93pt" width="124"><br>
</td>
<td style="width:22pt" width="29"><br>
</td>
<td style="width:23pt" width="30"><br>
</td>
<td style="width:23pt" width="30"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
<td style="width:48pt" width="64"><br>
</td>
</tr>
<tr style="min-height:15.75pt" height="21">
<td style="min-height:15.75pt" height="21"><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td colspan="7" style="border-right:1pt solid black">Stack at start and after each step</td>
</tr>
<tr style="min-height:15.75pt" height="21">
<td style="min-height:15.75pt" height="21">Step</td>
<td style="border-left:medium none">Function</td>
<td style="border-left:medium none">Top</td>
<td style="border-left:medium none">Output</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td style="border-top:medium none">Start</td>
<td style="border-top:medium none;border-left:medium none">Step 1</td>
<td style="border-top:medium none;border-left:medium none">Step 2</td>
<td style="border-top:medium none;border-left:medium none">Step 3</td>
<td style="border-top:medium none;border-left:medium none">Step 4</td>
<td style="border-top:medium none;border-left:medium none">Step 5</td>
<td style="border-top:medium none;border-left:medium none">Step 6</td>
</tr>
<tr style="min-height:15pt" height="20">
<td style="min-height:15pt;border-top:medium none" height="20">1</td>
<td style="border-top:medium none;border-left:medium none">Push (23)</td>
<td style="border-top:medium none;border-left:medium none">2</td>
<td style="border-top:medium none;border-left:medium none">Item added (23)</td>
<td><br>
</td>
<td rowspan="5" style="border-bottom:1pt solid black">Stack position</td>
<td style="border-left:medium none" align="right">4</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
<td style="border-top:medium none;border-left:medium none" align="right">92</td>
</tr>
<tr style="min-height:15pt" height="20">
<td style="min-height:15pt;border-top:medium none" height="20">2</td>
<td style="border-top:medium none;border-left:medium none">Push (18)</td>
<td style="border-top:medium none;border-left:medium none"> </td>
<td style="border-top:medium none;border-left:medium none">Item added (18)</td>
<td><br>
</td>
<td style="border-top:medium none;border-left:medium none" align="right">3</td>
<td style="border-top:medium none;border-left:medium none" align="right">75</td>
<td style="border-top:medium none;border-left:medium none" align="right">75</td>
<td style="border-top:medium none;border-left:medium none" align="right">18</td>
<td style="border-top:medium none;border-left:medium none" align="right">18</td>
<td style="border-top:medium none;border-left:medium none" align="right">75</td>
<td style="border-top:medium none;border-left:medium none" align="right">75</td>
<td style="border-top:medium none;border-left:medium none" align="right">75</td>
</tr>
<tr style="min-height:15pt" height="20">
<td style="min-height:15pt;border-top:medium none" height="20">3</td>
<td style="border-top:medium none;border-left:medium none">Pop</td>
<td style="border-top:medium none;border-left:medium none">2</td>
<td style="border-top:medium none;border-left:medium none">Item removed (18)</td>
<td><br>
</td>
<td style="border-top:medium none;border-left:medium none" align="right">2</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
<td style="border-top:medium none;border-left:medium none" align="right">23</td>
</tr>
<tr style="min-height:15pt" height="20">
<td style="min-height:15pt;border-top:medium none" height="20">4</td>
<td style="border-top:medium none;border-left:medium none">Push (75)</td>
<td style="border-top:medium none;border-left:medium none"> </td>
<td style="border-top:medium none;border-left:medium none">Item added (75)</td>
<td><br>
</td>
<td style="border-top:medium none;border-left:medium none" align="right">1</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
<td style="border-top:medium none;border-left:medium none" align="right">83</td>
</tr>
<tr style="min-height:15.75pt" height="21">
<td style="min-height:15.75pt;border-top:medium none" height="21">5</td>
<td style="border-top:medium none;border-left:medium none">Push (92)</td>
<td style="border-top:medium none;border-left:medium none">4</td>
<td style="border-top:medium none;border-left:medium none">Item added (92)</td>
<td><br>
</td>
<td style="border-top:medium none;border-left:medium none" align="right">0</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
<td style="border-top:medium none;border-left:medium none" align="right">52</td>
</tr>
<tr style="min-height:15.75pt" height="21">
<td style="min-height:15.75pt;border-top:medium none" height="21">6</td>
<td style="border-top:medium none;border-left:medium none">Push (47)</td>
<td style="border-top:medium none;border-left:medium none"> </td>
<td style="border-top:medium none;border-left:medium none">Stack full</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
</tr>
<tr style="min-height:15.75pt" height="21">
<td style="min-height:15.75pt" height="21"><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td><br>
</td>
<td>Top</td>
<td style="border-left:medium none" align="right">1</td>
<td style="border-left:medium none" align="right">2</td>
<td style="border-left:medium none" align="right">3</td>
<td style="border-left:medium none" align="right">2</td>
<td style="border-left:medium none" align="right">3</td>
<td style="border-left:medium none" align="right">4</td>
<td style="border-left:medium none" align="right">4</td>
</tr>
</tbody>
</table>
<br><font color="#888888">
Mike Brookes <br>
Semi retired gentleman from Copperfield College</font><div class="im"><br>
<br>
On 21/04/2011 1:35 PM, Mark KELLY wrote:
<blockquote type="cite">
<p class="MsoNormal">Would someone be kind enough to throw me a
bone about the
stack question - Q3?<br>
<br>
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. <br>
</p>
<p class="MsoNormal"><br>
</p>
<p class="MsoNormal">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...<br>
</p>
<p class="MsoNormal"><br>
</p>
<p class="MsoNormal">I’d be delighted if anyone could walk me
through this...</p>
<p class="MsoNormal"><br>
</p>
<p class="MsoNormal">Regards</p>
<p class="MsoNormal">Mark<br>
</p>
<br>
</blockquote>
<br>
</div></div>
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