the information school of the university of washington oct 9fit100-06-debug1 debugging and...

Oct 9 fit100-06-debug 1

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Debugging and Troubleshooting

INFO/CSE 100, Fall 2006

Fluency in Information Technology

http://courses.washington.edu/info100/


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Readings and References

• Reading» 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

• http://www.w3schools.com/html/html_reference.asp

• http://validator.w3.org

» Jedit (java-based editor)• http://www.jedit.org


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Anchors

• The link starts with an a tag…<a href=" ... ">

• The value of the href attribute is a URL. <a href="http://www.cs.washington.edu">

• The link encloses some text, and ends with… <a href=" ... ">CSE home page</a>


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Types of URLS

• http://www.site.com/dir/file.html

• http://www.site.com/dir/

• ftp://ftp.site.come/pub/

• news:soc.culture.religion

• mailto:[email protected]

• file:///hardisk/path/file.html


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HTML Tables<table border="1"> <tr> <th>Animal</th> <th>Fav food</th>

</tr> <tr> <td>monkeys</td> <td>bananas</td> </tr> <tr><td colspan="2"><hr /></td></tr></table>

Animal Fav food

monkeys bananas

___________________


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Simple HTML

• What we’ve seen here is very simple HTML

• HTML is changing» Each document should start with a “DOCTYPE” comment telling which

version of HTML it follows• http://www.w3.org/QA/2002/04/valid-dtd-list.html

• We used HTML 4.01 Transitional

» There is a validator service that will check your page• http://validator.w3.org/

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">


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Editing byHand vs. a Tool

• WYSIWYG type tools (what you see is what you get)» Microsoft FrontPage - $$$» Macromedia DreamWeaver - $$$» Amaya - Free

• http://www.w3.org/Amaya/

» And MANY MANY others!

• Text Editors» Jedit - Free

• http://www.jedit.org/

• Pick the right tool for you!


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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» < = <» > = >» & = &» " = "» ' = '» = non-breaking space


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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.


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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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Common Bug Types

• Compilation/Syntax errors» Program 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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When You Debug...

• 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

Debugging is not algorithmic: no guaranteed process


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Debugging Guidelines Overview

» 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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• 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)

Getting Out and Getting Back In

Reproducibility


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tonDetermine 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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tonEliminate 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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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


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tonAt 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


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tonMake 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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Questions

• Why learn HTML at all if authoring tools will do the work for you?

• Give other examples of where you are expected to learn something when there are tools available that will do the work.


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Questions

• Why do we need to end a tag?

• Why were the characters < and > chosen to denote tags?


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Questions

• How does a web browser know where to get an image from to display on a web page?


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Questions

• You walk into a room and flip the light switch. Nothing happens. Describe the debugging process you use to solve the problem.


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Questions

• What advantages does an organized debugging approach have over a trial and error approach?


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Project 1• Make a website of “misinformation”

» Project 1a• Planning, image collection, basic web site creation

» Project 1b• Website design, image manipulation

• Project 1a due Friday, October 13, before 10:00 pm.» Use online turn in page to submit your files» We will not accept late projects - turn in what you have

even if you are not done (some points are better than no points)

• http://courses.washington.edu/info100/classwork/assignments/files/project1.html


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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!

the information school of the university of washington oct 9fit100-06-debug1 debugging and...

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