Debugging

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Debugging
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Motivation

• To err is human to really foul things up
  requires a computer
• While we are becoming master VB programmers,
  sometimes well err.
• We want to recover as quickly as possible!

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Types of Errors

• Syntax
• VB makes it easy to identify these!
• Run-time
• You get a Run-time error 28 message box
• End or Debug
• Logic
• Hardest to find fix
• Presumes we test (almost?) all paths/data

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Break Mode

• Not design mode
• Not run mode
• Hybrid of both
• We can evaluate expressions
• We can see results values of variables
• We can change properties
• We can change the code itself

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Entering Break Mode

• Breakpoints in code
• Stop statements in code
• Break button clicked
• Run-time error generated DEBUG clicked
• Debug.Assert statement evaluates to FALSE

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Breakpoints

• Marker in your code
• Tells VB to halt enter Debug mode
• Not persistent (not saved when you leave VBs
  IDE)
• F9 toggles on/off

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Stop Statement

• Like Breakpoint, except its persistent (saved w/
  code)
• When compiled (made into an EXE), Stop becomes
  End

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Debug.Assert

• Conditionally enters debug mode
• Debug.Assert booleanExpression
• Debug.Assert intDenominator ltgt 0
• If conditional is NOT true, enter debug
• When compiled, assertion expressions are ignored
• Persistent saved w/ project

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Debug Toolbar

• Start/Continue, Break, End
• Toggle Breakpoint (set current lines breakpoint)
• Step into (trace into procedure call)
• Step over (do procedure call, wait at next line)
• Step out (finish procedure, wait at parents next
  line)
• Windows
• Local (scoped variables of procedure)
• Immediate (evaluation)
• Watch (selected expressions)
• Quick Watch (show result of current expression)
• Call Stack (display the activation stack)

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Watch Expression

• Useful to keep an eye on variables, properties,
  expressions
• Limit scope as possible
• Runtime will be faster!
• Can break on TRUE or on CHANGE

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Immediate Window

• Useful for seeing immediate results (thus the
  name!)
• Evaluate procedures
• Test expressions
• Display variable property values

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Print

• ? Print
• Debug.Print expression
• Print in immediate window
• Displays info in the immediate window

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Debug.Print

• Displays information to the immediate window
• Is persistent and saved with project
• Is removed when compiled
• But parameter remains, so whatever happened
  before WILL still happen, just no printing to
  immediate window
• Execution of program isnt interupted

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Locals Window

• Displays identifier, value, and type of variables
• Useful to make sure what you think should happen
  is happening!
• You can edit the data/value and see how the
  program responds

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Call Stack

• Shows from whence youve come
• Most recent at top
• First invoked procedure at bottom
• Double clicking on item jumps to invocation point

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Summary

• Debugging is an acquired skill
• VB helps by providing useful tools
• Immediate window shows information and evaluates
  expressions
• Call Stack useful to show from where youve come
• Locals window shows scoped variables
• Watch expressions and assert can conditionally
  enter debug mode
• Breakpoints are a good way to force entry into
  debug mode

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