1. Debugging and Troubleshooting
INFO/CSE 100, Spring 2006
Fluency in Information Technology
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2. Readings and References
Fluency with Information Technology
Chapter 7, To Err is Human
“To err is human, but it takes a computer to really foul things up”
References
World Wide Web Consortium
Jedit (java-based editor)
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3. Advanced HTML <hr /> a horizontal rule
______________________________
<p>This paragraph has space around it.</p>
This is a line<br /> break carried onto the next line
P tag has to have an open and a closing tag.
The horizontal rule and line break open and close at the same time.
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4. HTML Tables <table border="1"> <tr>
<th>Animal</th>
<th>Fav food</th>
</tr>
<td>monkeys</td>
<td>bananas</td>
<tr><td colspan="2"><hr /></td></tr>
</table>
Animal
Fav food
monkeys
bananas
___________________
Tables are a really useful way to align items on a page. See CNN website for example.
The FIT100 website repositions the items on the screen as the window resizes.
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5. List Items List item 1 Another list 1. first item 2. second item
<ul>
<li>List item 1</li>
<li>
<ol>
<li>Another list</li>
<li>first item</li>
<li>second item</li>
</ol><li>
</ul>
List item 1
Another list 1. first item 2. second item
List items can be nested.
Each web browser will handle errors differently.
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6. Character Entities Some characters are special in HTML
<, >, &, ", '
They are interpreted by the web browser
To get them to display properly we have to encode them specially
< = <
> = >
& = &
" = "
&apos; = ' (note: Not in IE)
  = non-breaking space
Show the example from Lab 3
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7. HTML Comments
<p>If you want to leave yourself some comments that won't appear in the web browswer window use an HTML comment.</p>
<!-- Any text that is between the opening and closing tag will be ignored by the HTML interpreter -->
<!-- COMMENT OPEN
--> COMMENT CLOSE
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8. Using Computers...
In IT, stuff goes wrong … debugging is the process of finding the error
Term coined by Grace Murray Hopper
Best solution … make no mistakes!
Be accurate … get it right the 1st time
Follow a process that makes it easier to get it right
Computers can’t make "common sense" decisions about what we really meant. They do what we say, not what we mean.
1945, GMH found a moth inside of Mark II Relay Harvard, she reported that they had "debugged" the computer.
Use a colored editor!
Indent your text.
Use an HTML validator.
Don't forget that path names need to be CASE SENSITIVE
Comment, comment, comment!
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9. Cost of Debugging
Debugging may consume 60-70% of your development time
80% of overruns may be due to debugging
Keep this in mind when you are budgeting time for your projects!
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10. Common Bug Types Compilation/Syntax errors Logic errors
Program doesn’t won’t run due to problems with what you typed in
HTML tags must be entered precisely
Required attributes must be present
Logic errors
Program runs, but output/behavior is wrong
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11. When You Debug... There are guidelines for debugging…
Debugging is not algorithmic: no guaranteed process
There are guidelines for debugging…
Rather than trying things aimlessly and becoming frustrated, think of yourself as solving a mystery
Be objective: What are my clues? What is my hypothesis? Do I need more data?
Consciously ‘watch’ yourself debug -- its an out-of-body experience
When stumped, don’t become frustrated, but ask, “What am I misunderstanding?”
Become Sherlock Holmes
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12. Debugging Guidelines Verify that the error is reproducible
Determine exactly what the actual failure is
Eliminate the “obvious” causes by checking
Divide process into working/faulty parts
On reaching a dead end, reassess the information you have, trying to identify the mistake you are making
Work through process making predictions and checking they’re fulfilled
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13. Getting Out and Getting Back In
Reproducibility
First step: verify the error is reproducible
You can't find something that you can't reproduce
Get out and get back in. Does it still happen?
Restart the application.
Try a different application
Reboot the operating system. Sometimes this is appropriate, especially for errors involving peripheral devices (printers, modems)
Open page in both IE and Mozilla!
Getting Out and Getting Back In
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14. This is not a printer problem!
Determine the Problem
Second step: figure out what’s wrong
Often there is a sequence of steps following an error and propagating it … work backwards looking to see where the error first occurred
Empty Database
Mailing Label Pgm
Mailing Label File
No Labels Printing
This is not a printer problem!
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15. My fan is broken, it won't turn on!
Eliminate the Obvious
Third step: eliminate obvious causes
“If the cause were obvious, the problem would have been fixed!” - Yeah, right.
There are standard things to check:
Inputs
Connections
“Permissions”
Physical connectivity
Requirements
My fan is broken, it won't turn on!
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16. Isolate the Problem
Try to “partition” the situation into working and non-working parts
Form a hypothesis of what’s wrong
Make as few assumptions as possible
Take nothing for granted
The goal is to eliminate as many things from consideration as possible
In the case of HTML, is the problem in the head or the body?
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17. At a Dead End, Reassess
When everything seems to check out, don’t get frustrated
Instead, ask yourself “What am I overlooking or misunderstanding?”
Your goal is to see the situation as it is, not as you think it should be
Am I assuming too much?
Am I misreading the clues?
What can I eliminate or simplify?
Explain the situation to a friend
Our brains do a good job of filtering out the small errors. After staring at the same piece of code for a week, its practically impossible to spot small errors!
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18. Make Predication/Check
Beginning with the isolated part, step through the process, predicting the outcome and verifying it
A prediction that is not fulfilled shows…
A possible bug
A possible misunderstanding
A chance to narrow the search
‘Sleeping on it’ often helps!
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19. Summary
Debugging is not algorithmic, but there are guidelines to follow
Stay calm - high blood pressure clouds your brain
Be organized as you investigate and fix things
Recognize that you may feel a little embarrassed when you finally figure out the problem.
If we were perfect, we would never make mistakes ...
A little humility is a good thing for all of us
Watch yourself debug -- assess how you are doing, what you need to know
Only try to fix one bug at a time!
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