Debugging: Syntax and Logical Errors Handling in Programming

1. CIS 338: Debugging Dr. Ralph D. Westfall May, 2011 Change font back to Tahoma from AjaxSurrealFreak (see Come On, I Thought I Knew That! for rationale)

2. Error Handling/Debugging • syntax errors • logical errors • good coding practices to minimize errors • testing your code • debugging methods • "bullet proofing"

3. Syntax Errors • spelling or punctuation errors "select * where Sitty="Pomona" 'in SQL MsgBox "Hello" 'use MsgBox("Hello") • missing parts of control structures • For but no Next 'on tests • If but no End If ' " " • extra parts for control structures • extra End Sub at end

4. Syntax Errors - 2 • using a method as a property MsgBox = "Welcome" • don't use = with methods • using a property as a method • lblName.Caption("Name") • properties are values, so need to use =

5. Syntax Errors - 3 • writing to a "read only" property • changed values could destroy data or make it inaccessible • lstSpecialty.Items.Count() = 10 • not using New to create an object objPeople = Persons() 'should be objPeople = New Persons()

6. Syntax Errors - 4 • type mismatch • e.g. can't do arithmetic with text • 5 / "hello" MsgBox(5/txtName.Text) 'text or empty • or with an undefined value MsgBox(5/CInt(txtNameText))'missing dot • won't show as an error right away if not using Option Strict On)

7. Logical Errors • compiler usually) catches division by literal value of zero as a syntax error Dim z As Integer = 3 / 0 • division by a variable that becomes zero is much harder to spot Dim x as Integer = 3 Dim y as Integer = 3 y = x \ (x - y) 'integerdivision 'divide decimals by 0 = infinity

8. Logical Errors - 2 • code that doesn't do what you want it to do • example: trying to swap values of nA, nB • nA = nB 'destroys value of nA • nB = nA • 'nB gets own previous value back

9. Logical Errors - 3 • code that will never run (“dead code”) • example: comparison If nAge < 3 Then Print "less than 3" ElseIf nAge >= 3 Then Print "3 or older" ElseIf nAge >= 55 Then Print "55 or older" 'will never run End If

10. Logical Errors - 4 • changing value of the counter in a loop • students make this mistake often on tests • final exam tip: memorize loop syntax For i = 0 to 9 lstName.Item.Add(names(i)) i += 1 'NO!!!! Next

11. Good Coding Practices • indent code inside loops and conditionals • "pretty printing" • For nI = 1 to 2 • nJ = nI^2 'exponentiation • Next nI 'show (optional) counter • use comments • but only where they are needed

12. Good Coding Practices - 2 • use Option Strict at start of code for forms and other modules • forces you to declare all variables • helps spot spelling mistakes • stops implicit data type conversions • should you use Option Strict when the code goes into "production?" • why or why not? • turn off debugging when ASP goes live

13. Good Coding Practices - 3 • consistent objectnaming convention • frmDataEntry 'a Windows form • lstName 'a ListBox • VB capitalizes function names for you • e.g. string function names • sName.ToUpper() 'to Upper Case • sAddress.TrimStart() • 'removes left side spaces

14. Good Coding Practices - 4 • type checking: is data what you expect? • If IsNumeric(txtQuantity.Text) Then • nTot = nTot + CInt(txtQuantity.Text) • End If • 'can check IsDBNull( ) in databases • type conversion: change data so it is usable • nValue = CInt(dItem1 + dItem2) • 'convert to Integer

15. Good Coding Practices - 5 • use subprocedures, functions, queries • for anything more than 3-5 lines of same or similar code in more than one place • advantages: • shorter programs • easier to debug and maintain • may be able to reuse them elsewhere

16. VB.NET Debug Settings • Tools>Options>Projects>VB Defaults or project>Properties> Compile • Option Explicit On requires that all variables be declared • Option Strict On prevents implicit conversions e.g., • from a String to a numeric data type • from a larger numeric type to a "narrower" type e.g., Double to Integer

17. VB.NET Debug Settings -2 • Tools>Options>Text Editor>Basic • Auto list members shows/inserts object properties and procedures • can help avoid typing errors • Parameter information – shows function parameters (order, data type) • Options>Text Editor>Basic>VB Specific • Pretty listing (reformatting) code indents next line the same as previous one

18. After You Code Project • must be tested • alpha testing – in house • beta testing – outside users • how much (%) of budget for testing? • depends on intended use of project and consequences of a failure • minor inconvenience vs. someone dies

19. Regression Testing • maintenance accounts for up to 80% of system costs, so it's important to test for errors in code that used to work but has been modified • this is called "regression testing" • testing to see if the system has "gone backwards" from working as it should

20. Alpha Testing Approaches • manual • automated testing software • TestComplete • QA Wizard Pro 2009

21. Manually Test Your Code • test the different ways a user could go through the application • data entry example: Add, Change, Delete and navigation buttons • what happens if click a button when no data shows? • what happens if do something twice e.g., Delete, then Delete again without moving to different record?

22. Testing Your Code - 2 • test possible data values • example: zip codes • boundary values (included or not?) • 00000 and 99999 • values in between boundaries • 00001 to 99998 (random samples) • out-of-bounds values • negative #s? 4 or 6 digit #s? • text #s? ("three" has 5 characters!)

23. Exercise: Create Test Plan • data entry form • 4 TextBoxes • Add, Edit, and Delete buttons • 4 Navigation buttons (First, etc.) • write down list of steps that you think would be adequate to test this form • indicate reasons for each test • identify things that could go wrong

24. Debugging Statements • "trace" statements can help determine values before place where problem is MsgBox(sSQL) 'view SQL string • files with no user interface could use Console.Writeline instead to show values • if use Autos Window, can see variable values when crashes occur • discussed in later slides

25. Debugging Class Modules • create another VB project to just provide inputs to module and get outputs from it • set up code ("driver") in a form to call class module's procedure(s) with possible test values • similar to how you would test classes in Java

26. Other VB.NET Debugging • as you write code, real time error checking spots syntax errors • if running and execution is halted at a "break," can see data values in current procedure by moving mouse pointer over variable names • good tutorial on commonly used debugging capabilities

27. VB Debugging Environment • debug menu items • Debug toolbar has some of same items • breakpoints toggle on and off • click on editor's left border to stop (break) where want to start running line by line • e.g., call to DemoStep( )in notes • can also Clear All Breakpoints with a selection on menu

28. VB Debug Capabilities - 2 • running code one line at a time from breakpoint with Step icons • Step Over – only steps into lines in this procedure (doesn't step in called ones) • Step Into – also runs lines one by one in called procedures • Step Out - finishes current procedure or block, then stops on line after it started

29. Debugging Example • set a breakpoint in some VB code • e.g., in a loop or at a subroutine call • run program • use different step keys • to view path through code • view changes in values in when mouse goes over a variable in code that has already ran

30. Debugging Example - 2 • run program • open Debug>Windows>Locals • can expand items by clicking + • use different step keys • view changes in values in Locals window as step through code

31. Run to Cursor • click in code at statement to run to • right-click:>Run to Cursor, then Start the project • or can click/drag yellow arrow on Breakpoint to line to execute next, then click on Step Into icon • can skip executing lines with this • note "tool tip" when mouse over arrow

32. Other Debug Windows • while running, can use following • right-click variable> Add Watch – shows values for variables you identify • Debug>Windows>Locals (slide 30) • Debug>Windows>Autos – like Locals, but limited to current line and up to three lines before/after it (7 lines)

33. Debug Windows Example • open Debug>Windows>Immediate while code is running • can change the value of a variable • assign a value, calculate a new value, run a MsgBox, etc. • view effect in Locals window • type in a line of executable code e.g.,[declared variable] += 1

34. Debug Windows Example - 2 • open a code window and Start program • highlight and right-click variable name • Add Watch • Step Into, view changes in Watch window • how is a Watch removed? • right click variable in Watch window>Delete Watch

35. Debug Windows Example - 3 • Debug>Windows>Call Stack • can click when program is running • shows current subroutine, routine that called it, routine that called the previous routine, etc. 'see Notes • indicates place where each called from • Clear All Breakpoints on Debug menu • don't have to remember where you set individual breakpoints

36. "Bullet Proofing" Code • prevent all possible errors from happening • make it impossible for user to enter bad data or to trigger an invalid action • include code in project that will "trap" errors that can't be prevented and take appropriate corrective action

37. Controlling Program Use • example: state abbreviations • all have 2 characters e.g., CA, OR, WA • 3 characters could crash database • only around 50 valid combinations (states) • possible "design-time" solutions: • set TextBox MaxLength property = 2 • use a drop-down ListBox or ComboBox instead so user can't enter bad values

38. Controlling Program Use - 2 • example: key field in a database should not be changed or input because database creates key value • property TextBox.Enabled = False shows data but user can't do anything with it • property TextBox.ReadOnly = True shows data, allows user to copy (but not change) it

39. Controlling Program Use - 3 • can use code to prevent inappropriate events • example: repeating some buttons used to cause a crash e.g., Delete, Delete • when the first button is clicked, have code set btn[name].Enabled = False for the button that causes the crash • need to enable the button again (after user moves away from the deleted record) 'notes

40. Validating/Validated Events • code can prevent bad inputs into TextBoxes • Validating – when control loses Focus • for preventing bad inputs • Validated - after Validating has occurred • can use Validated to trigger different actions based on different input values, after bad inputs have been prevented

41. Validating Example Private Sub TextBox1_Validating(etc. 'runs when textbox loses focus '.Text property initially set to: "me" MsgBox("TextBox1 is validating") If TextBox1.Text <> "Me" Then MsgBox("Don't try to change Me!!!") TextBox1.Focus() 'returns to it e.Cancel = True 'event not Validated End If End Sub 'or can use regular expression conditions

42. Validated Example Private Sub TextBox1_Validated(etc. MsgBox("TextBox1 has been validated") MsgBox("Thanks 4 leaving Me alone :-)") End Sub

43. Exception Handling • catch (trap) exceptions (errors) before they crash the program, and then: • end the program with all normal clean up • or keep running but avoid code related to the error (limited functionality) • or give a message to user to fix problem • e.g., put a CD in the E:\ drive

44. Try … Catch • Try block "traps" exceptions • more efficient than writing subroutines to handle the same types of problems • Try block redirects ("throws") flow to Catch block(s) below, where programmer's code handles problem(s) • can also have a Finally block to deal with exceptions not handled in individual Catch block(s)

45. Types of Exceptions • VB.NET has Exception classes that handle specific types of errors • Exception classes can have more specific subclasses • ArithmeticException 'general • DivideByZeroException 'more specific • ArgumentException • ArgumentNullException

46. Exception Handling Structure Try [code that could cause a crash] [Catch [object] as [type of]Exception ] [code for general [or specific] problem] 'there can be 0, 1 or several Catch blocks [Finally] (optional) [catch other exceptions, or do something else e.g., Dispose of object resources] End Try

47. Exception Handling Example Option Strict On Private Sub etc. Dim intZ As Integer = 33 Try Dim intA As Integer = 3 intZ = 5 \ (intA - 3) 'integer division Catch ex As ArithmeticException MsgBox("Error: " & ex.ToString()) Finally 'not needed here, just for example MsgBox(CStr(intZ)) 'still original value End Try