Debugging !!! ?

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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
• Its still an art form must be done manually
• Cant tell you how to be Da Vinci, but can
  certainly give you a brush and some paint

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Just use a debugger!

• Unfortunately, we dont live in that perfect
  world just yet
• Several problems with debuggers
• May not be available for several reasons
• Dont work for complex systems like distributed
  computing
• Its just plain confusing sometimes
• Can be thought of more as reverse engineering
  than a straight forward process

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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. Its not wrong
• There are different kinds of bugs to look for

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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)

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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?

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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.

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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

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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 peoples code
• Look for examples of hard coding declared
  strings, integers, counters which are defined
  instead of read as input

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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

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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()

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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

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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?

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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

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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

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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

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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
• Doesnt provide insight into logical error
• Still a manual process to find errors

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How to avoid

• Dont write 1000 lines before testing
• Variable names can have meaning
• Dont hard code ANYTHING. If you must, define all
  of them in the same place where they are easily
  seen / changed
• Comment everywhere it isnt obvious
• If its more than a few lines put it in a
  function
• Always use

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Personal Horror Stories

• if (conditional which ran longer than the width
  of my screen)
• break
• while (index lt end)
• go from the end of a string to the front
• index index -1
• for (int i0 iltx i) for (int j0 jlty i)
• Forgot about a hard coded nn matrix read
• Drew background last in graphics class
• Spent 4 hours debugging because C didnt flush a
  buffer
• Tried to do anything in fortran or prolog