Debugging !!! 

3. Debugging !!!  • Believe it or not, errors happen • The most frustrating part of programming • Hunting down an error can be harder and more time consuming than the solution to the original problem • Debugging tools and methodologies are still in their infancy • It’s still an art form: must be done manually • Can’t tell you how to be Da Vinci, but can certainly give you a brush and some paint

4. Just use a debugger! • Unfortunately, we don’t live in that perfect world just yet • Several problems with debuggers: • May not be available for several reasons • Don’t work for complex systems like distributed computing • It’s just plain confusing sometimes • Can be thought of more as reverse engineering than a straight forward process

5. Methodology • Use your brain… not your fingers • Keep it simple initially • Look for simple solutions first • The most likely culprit will usually be it • Move on to more complex solutions • For the most part, try and assume the computer is doing its job. It’s not wrong • There are different kinds of bugs to look for

6. Kinds of Errors • Compile time errors • Usually pointed out to you, easy to fix • Syntactical errors • Run time errors • “System Errors”: Out of bounds, class cast exception, null pointer exception • “Logic Errors”: No visible errors, you just get the wrong answer • Causes of Errors • Flawed Logic (semantics) • Typos (syntactical)

7. Thought Process • Its not working… now what? • Make sure it is actually a bug • Was your solution model flawed? • Are you printing what you want to print? • Did you forget to save / correct version? • Libraries are up to date? • Did you forget to end a comment? • Was there a crucial input missing?

8. Easy Bugs: Lots of Clues • How to look for clues? • Tip: Avoid writing too much before running • Did a recent change break the code? • Have you seen this bug before? What did you do in the past to fix it? • Think first, write down on paper what is going on • A second set of eyes might be able to see what you are missing. You have been staring at it so long you might miss something simple.

9. More complex problems • Might be system specific, check manuals • Narrow down inputs which reproduce the bug. If you know which inputs make it fail you can pinpoint where the error may be. • Are you close to data type limits? • Look for suspicious values: Powers of 2, weird ASCII (more applicable in other languages) • Are your data structures valid? • Does taking out bits of code make it work? • Inserting print statements…

10. Foreign/Old Code • Get a copy of the design / problem statement • Work it out by hand first • Group their functions into small bits of readable English: “Formats text”, “Convert hex to binary” • Know what it all does before attempting to fix it • Sadly: Complete rewrites often faster than debugging other people’s code • Look for examples of ‘hard coding’: declared strings, integers, counters which are defined instead of read as input

11. The Art of Print Statements • System.out.println(“UUUUUUUUUUUUUGH”); • System.out.println(“YESSSSSSSSSSSSSSSS”); • Usually best way to find errors in small systems, especially number-based • Things to print • Counter values • Min/Max values • Array indices • Data structure values • Where to print • Before / after declarations/initializations • Before / after function calls • Before / after entering loops / recursion • Before “breaking” from a loop

12. Java Specific • Always give your objects a .toString() or a .toDebug() method • Make use of Arrays.toString() etc • print(Index + “ : “ + value) in loops • Use iterators where possible • Instead of setting your own parameters for a prebuilt class, override it • If building GUI/JS, use System.err.println()

13. Examples • NullPointerExceptions • Print statement after initialization or immediately before passing into functions • ArrayOutOfBounds • Lucky if you are using java that they are thrown • Print index counters, array values • Infinite Loops • Do searches for index counter names/updates • “Wrong answers” • Go over it again by hand, your solution probably is not correct in the first place. Put the code on hold

14. Example: Mathematics • Sine and Cosine mixed up? • Negative values? • Values close to zero / rounding errors? • Adding small values to large values • Reversing min / max values? • Pass by reference / value? • Loops correct?

15. Examples (ctd) • Loops / Conditionals • Are your counters in the right order? • ++counter vs. counter++ • Multi dimensional – do it by hand first • Make sure bracketing matches with indenting • Copy / Paste changes

16. Life Lessons… • Format of a file is not standard ascii • Format of a file is little endian instead of big endian • Naming conventions actually matter in fortran • Programming in C for the first time – arrays do not initialize

17. Reverse Engineering • Start from the error and work backwards • Make sure all your parts work before testing the system as a whole • Surround the affected area with output information / line breaks • Edit lines of ‘fluff’ code out to test the ‘meat’ • Experiment with different input values • Fix the code line by line

18. Debugger • Built into Eclipse for java • GUI / Text based, same idea • Only used as a last resort, very time consuming • Allow for insertion of break lines to halt code execution at any point • Allow you to see the contents of any object / array / primitive at any time by query • Amount of data shown often overwhelming • Doesn’t provide insight into logical error • Still a manual process to find errors

19. How to avoid • Don’t write 1000 lines before testing • Variable names can have meaning • Don’t hard code ANYTHING. If you must, define all of them in the same place where they are easily seen / changed • Comment everywhere it isn’t obvious • If its more than a few lines… put it in a function • Always use { }

20. Personal Horror Stories • if (conditional which ran longer than the width of my screen); break; • while (index < end) go from the end of a string to the front… index = index -1; • for (int i=0; i<x; i++) for (int j=0; j<y; i++) • Forgot about a hard coded n*n matrix read • Drew background last in graphics class • Spent 4 hours debugging because C didn’t flush a buffer • Tried to do anything in fortran or prolog