Errors Those Evil Dudes

1
Errors - Those Evil Dudes

• Lecture 12
• 10/30/00

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Agenda

• Errors and Definitions
• Issues associated with errors
• Detection
• Prevention
• Manifestation

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Definitions

• Error - The physical mistake you make
• A plus instead of a minus, dereferencing a
  pointer instead of assigning, variable mis-spell
• You selected an O(nn) sort instead of an
  O(nlogn), because you didnt realize E-bay would
  grow as large as it did.
• Y2K

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Fault

• Fault - The manifestation of the error
• I meant to write, X AB
• I wrote, XAB,
• The manifestation is the value of X when the
  expression is evaluated.
• The sort choice is not a fault

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Definitions

• Failure - The software fails to meet its intended
  operational requirements.
• X is new balance, A is current balance, and B is
  Deposit or Withdrawal. XAB causes the answer to
  the calculation to result in a balance change
  that is significantly different than was
  expected.
• The sort takes too long.

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So what does all of that mean?

• You test or somehow try to find faults
• You try to prevent errors
• You sometimes can only test for failures or
  choose to only test for failures
• Debugging is the process of isolating and
  repairing errors. It is not the same as testing.

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Issues associated with errors

• Try to prevent them in the first place
• You cant find all of them
• Find them and fix them early (incremental
  development)
• You cant find all of them
• Know what you are looking for when you are
  testing
• You cant find all of them

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How do you prevent them?

• Be careful
• Use static analysis techniques
• Use automated tools
• Be careful
• Watch the assumption list
• There are two basic types of errors

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The two basic types of errors

• Errors of commission (you knew what to do, but
  somehow didnt do it)
• Errors of omission (you didnt know what to do)
• Those translate into issues of
• Correctness (Did I build the thing right?)
• Validity (Did I build the right thing?)

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Back to E/F/F

• What are the issues of correctness?
• Errors, Faults or Failures?
• What are the issues of validity?
• Errors, Faults or Failures?
• You need to know what you are looking for, or
  trying to prevent, and why.