This document discusses debugging techniques. It begins by noting that debugging was discovered by early programmers who found that programs were not as easy to get right as initially thought. It then notes that writing bugs is easy for everyone. Effective debugging techniques discussed include running code in your head, reproducing the issue in isolation to tighten the debugging loop, asking why at each step, doubting your assumptions, fixing the bug without introducing new ones, and looking for clustered bugs when one is found. The key tools for debugging are identified as the brain, unit tests, and print statements.

1. Debugging
John Graham-Cumming
September 2013

2. Ada Lovelace (1840s)

3. Maurice Wilkes (1950s)
 As soon as we started programming,
we found to our surprise that it wasn't
as easy to get programs right as we
had thought. Debugging had to be
discovered. I can remember the
exact instant when I realized that a
large part of my life from then on
was going to be spent in finding
mistakes in my own programs.

4. Writing code is hard
 But writing bugs is easy!
 Everyone does it all the time without
even thinking about it.
 Estimates in GOOD code there are 2
to 4 defects per 1,000 lines
 You’re not that good
 You will spend many hours debugging

5. Programmer
 If a person who debugs code is a
debugger than a person who writes
the bugs must be a... bugger?

6. Always have a fighter pilot in
your presentation

7. Colonel John Boyd

8. OODA Loop

9. THE GREATEST
DEBUGGER YOU WILL
EVER USE IS IN YOUR
HEAD
Ignore your tools!

10. ESSENTIAL SKILL: RUN
CODE IN YOUR HEAD

11. RISE
 Reproduce
 Isolate
 Target
 Excise

12. Reproduce

13. Isolate
 Tighten the OODA loop
◦ The smallest
◦ Fastest
◦ Most easily repeated
 And automate it
 Then turn it into a test case

14. Target

15. ESSENTIAL SKILL:
ALWAYS BE ASKING
WHY

16. Debugging Christmas Lights

17. ESSENTIAL SKILL:
DOUBTING

18. Excise
 Fix the bug
 Don’t introduce new bugs!
 You can check this with the test case
that you made earlier

19. Talk to the teddy

20. Look Around Debugging
 Where there’s smoke...
 ... research shows that bugs tend to
cluster
 When you fix a bug look for more in
the same spot.

21. 5 bugs in a single line
dir.permissions = 666; // Make the file writable
 Bug 1: updates permissions without
asking
 Bug 2: doesn’t make the file writable
 Bug 3: creates security hole
 Bug 4: used decimal not octal
 Bug 5: comment is incorrect

22. Remember This One Thing!
 The most effective debugging tools
are: your brain, a unit test, and the
print statement.