Idealists, those who think design, analysis, complexity theory, and the like are more fundamental than debugging, are not working programmers. The working programmer does not live in an ideal world. Even if you are perfect, you are surrounded by and must interact with code written by major software companies, organizations like GNU, and your colleagues. Most of this code is imperfect and imperfectly documented. Without the ability to gain visibility into the execution of this code, the slightest bump will throw you permanently. Often this visibility can be gained only by experimentation: that is, debugging.

The common ways of looking into the ‘innards’ of an executing program can be categorized as:

- Using a debugging tool,

- Printlining - Making a temporary modification to the program, typically adding lines that print information out, and

- Logging - Creating a permanent window into the programs execution in the form of a log.

Debugging tools are wonderful when they are stable and available, but the printlining and logging are even more important. Debugging tools often lag behind language development, so at any point in time they may not be available. In addition, because the debugging tool may subtly change the way the program executes it may not always be practical. Finally, there are some kinds of debugging, such as checking an assertion against a large data structure, that require writing code and changing the execution of the program. It is good to know how to use debugging tools when they are stable, but it is critical to be able to employ the other two methods.

Next [How to Debug by Splitting the Problem Space](02-How to Debug by Splitting the Problem Space.md)

Debugging is fun, because it begins with a mystery. You think it should do something, but instead it does something else. It is not always quite so simple---any examples I can give will be contrived compared to what sometimes happens in practice. Debugging requires creativity and ingenuity. If there is a single key to debugging it is to use the divide and conquer technique on the mystery.

Next [How to Remove an Error](03-How to Remove an Error.md)

I've intentionally separated the act of examining a program's execution from the act of fixing an error. But of course, debugging does also mean removing the bug. Ideally you will have perfect understanding of the code and will reach an ‘A-Ha!’ moment where you perfectly see the error and how to fix it. But since your program will often use insufficiently documented systems into which you have no visibility, this is not always possible. In other cases the code is so complicated that your understanding cannot be perfect.

In fixing a bug, you want to make the smallest change that fixes the bug. You may see other things that need improvement; but don't fix those at the same time. Attempt to employ the scientific method of changing one thing and only one thing at a time. The best process for this is to be able to easily reproduce the bug, then put your fix in place, and then rerun the program and observe that the bug no longer exists. Of course, sometimes more than one line must be changed, but you should still conceptually apply a single atomic change to fix the bug.

Next [How to Debug Using a Log](04-How to Debug Using a Log.md)