Announcements

	Project 1a is due Friday … see turn-in instructions
	Midterm 1 is next Monday

Debugging & Troubleshooting

	“To err is human, but it takes a computer to really foul things up”

Debugging

	Debugging is the process of finding the error in a faulty (IT) system
		The term was coined by Grace Hopper

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?”

Debugging Guidelines

		Verify that the error is reproducible
		Determine exactly what the problem is
		Eliminate the “obvious” causes
		Partition the process, separating out the parts that work from the part that doesn’t work
		When you reach a dead end, reassess the information you have, trying to identify the mistake you are making

Reproducibility

	First step: verify the error is reproducible
		Transient errors are very rare, but they do happen … try again
		Rebooting the operating system is advisable, especially for errors involving peripheral devices (printers, modems)

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

Eliminate the Obvious

	Third step: eliminate obvious causes
		“If the cause were obvious, the problem would have been fixed!”
		There are standard things to check:
			Inputs
			Connections
			“Permissions”
			Physical connectivity

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

On Reaching a Dead End

	When everything seems to check out, don’t get frustrated ... ask, “What am I misunderstanding?”
		Your goal is to see the situation as it is, not as you think it should be
		Step through the process predicting what will happen, looking for places where your prediction is wrong

A Debugging Example

A Buggy Web Page

Slide 13

Illustration

Summary

	Debugging is not algorithmic, but there are guidelines to follow
		It probably pays to memorize them so they come to mind while debugging
		Watch yourself debug -- assess how you are doing, what you need to know
		Being accurate -- avoiding textual mistakes at all -- saves frustration