Debugging

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Debugging

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• Preventing bugs

• Detecting/locating bugs

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Bug Location

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Use the process of elimination!

• Many students do not know where to start when looking for a bug

• Any bug you are looking for is probably not where you think it is (or you would have found it already!)

• To locate a bug, split your code into "regions"

• Try to show that the bug is not in each region

• If you can show this, you can eliminate that region from consideration

• Eliminate regions one by one, until you've located the problem

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Splitting Code Into Regions

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• If you are using a debugger:
  • Set breakpoints between regions

• If no debugger is available:
  • Add print statements between regions

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Examine Intermediate Values

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• If you are using a debugger:
  • While running the program, inspect the values of important intermediate results at each breakpoint

• If no debugger is available:
  • Print out the values of important intermediate results in the print statements between regions

• Never assume you know what is happening to a variable or value--always inspect its value. It is far too easy to "think" one thing is happening while the code is actually doing another!

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Use Intermediate Values To Narrow Scope

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• By checking intermediate values for appropriateness, you should be able to determine:
  1. If the program is operating properly on entry into the region
  2. If the program is operating properly on exit from the region

• If the program is operating properly on entry to and exit from the region, then the bug probably is not inside the region

• If the program is not operating properly on entry, then the bug must be before the region

• If the program is not operating properly on exit, then the bug must be before the end of the region

• If the program is operating properly on entry, but is not operating properly on exit, then the bug must be inside the region

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Once You've Found the Faulty Region

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• You may be able to see the bug by carefully inspecting the code, now that you know what series of statements the bug must be in

• Often, you still may not be able to spot the bug

• So, use the process of elimination:
  • Further divide the faulty region into subregions

  • Inspect intermediate values at the boundaries

  • Determine which subregion contains the fault

• Repeat as many times as necessary

• Don't feel bad--some bugs are difficult enough to see that you end up putting print statements or breakpoints between every statement