Debugging by Thinking

3.4 Review 47

3.4

Review

An alibi has three components: time, place, and corroboration. A program

that has side effects has no alibi for the time a problem occurred or the

place it occurred. The debugging equivalent of an alibi is the assertion that

a given system component can't be the cause of a problem.

Exercising curiosity is one way we leverage the labor of finding the cause

of a defect. We use this curiosity when we investigate potential related prob-

lems, even before they're reported.

Facts aren't complete without qualification. Use the reporter's ques-

tions who, what, where, when, how often~to qualify the facts of a defect

report to make it useful.

If you believe that a phenomenon observable by the human senses is

both true and false, you're going to have a hard time with the scientific

method. All classical reasoning methods depend on this principle.

The trick to enumerating possibilities is knowing when you're done. You

can enumerate possibilities by using the principle of noncontradiction and

a binary tree.

There are lots of good ways to organize a search for the explanation of a

defect, but few programmers use any system. Some possible approaches

include the black box, the scientific method, the reporter's method, and the

tester's method.

In many disciplines, we're told, don't make assumptions. Unfortunately,

this is easier said than done. You can follow a method that minimizes the

effects that assumptions have on your search.

If you're working with a large amount of data, display it visually so that

your brain can recognize larger patterns you might not recognize from look-

ing at numerical values. Color-coding items in a listing is an effective way

to make your brain pick out patterns.

Use one of several diagram types to show how a defect could occur.

I Chapter 3

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