Notes

Outline

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