1. ME 142 Engineering Computation I Debugging Techniques

2. Key Concepts  Types of Program Errors  Debugging Tools & Techniques  Testing  VBA Help Facility

3. Types of Programming Errors

4.  Syntax Errors  Run-Time Errors  Logic Errors

5. Syntax Errors  Syntax errors occur when the rules of the programming language are violated.  Examples: Misspelling a keyword or function name Omission of necessary keywords in a command.

6. Run-Time Errors  Runtime errors are those errors that occur when a program free of syntax errors, runs and generates an error.  Examples: Attempting an invalid operation such as division by zero Providing the wrong data type to a function.

7. Logic Errors  Logic errors are due to incorrect program logic.  The program may run without errors  The results it returns are inaccurate.  Often the most difficult to locate

8. Debugging Tools & Techniques

9.  MsgBox Function  Debug.Print Statement  Debug Mode

10. MsgBox Statement  Place MsgBox statement in your code as desired  Causes the value of the variables to be immediately displayed in a dialog box and pauses execution of program  Example: MsgBox (“A = “ & A)

11. Debug.Print Statement  Place Debug.Print statement in your code as you would MsgBox function  Causes the value of the variables to be printed in the Immediate window  Example: Debug.Print A,B,C

12. Debug.Print Statement

13. Debug Mode  Allows you step through your program and examine the value of variables at specific points  May step through line-by-line or set specific breakpoints

14. Debug Mode  When a run-time error occurs may enter Debug Mode by selecting button on dialog box that appears:

15. Debug Mode  Once in Debug Mode may mouse-over a variable to find its current value:

16. Debug Mode  May also hit the F8 key to single step through the program  Each time the F8 key is hit, one line of code is executed:

17. Debug Mode  May also set a breakpoint by clicking in the gray column on the left side of the line of code you want to program to pause on  Click on run (from menu above) to cause the program to resume

18. Debug Mode  Another powerful feature to use in conjunction with the previous techniques is the Locals Window  It displays the current value of all local variables in your program

19. Testing

20.  Deadly Misconception of Beginning Programmers: If a program runs without errors, the results must be correct  Very important that before you use your program to make engineering decisions (or turn it in to get a grade in class) that you perform a series of test to make certain that the output is correct.

21. Testing Suggestions  Compare your results to that of a simple, known problem by using a calculator or Excel  Test a typical range of inputs  Test unusual but valid data  Test extreme values

22. More Suggestions to Help Eradicate Program Bugs  Use VBA’s debugger tool  Structure your program with indentation for loops and if statements  Keep your programs simple and small. Break up large programs into smaller programs where appropriate.  Use the Option Explicit statement at the beginning of your modules to force you to define every variable used in your program  Use the macro recorder to help identify unfamiliar properties and methods.

23. VBA Help Facility

24. In Class Activity  Use the debugger to debug the program provided  Your result should be:

25. Sub cylvol() ' Calculates the volume of a cylinder ' Input: diameter and height ' Output cylinder volume ' Input the diameter and height h = Range("B2").Value d = Range("C2").Value ' Calculate the volume vol = Pi * (d ^ 2 / 2) * h 'Round and output the volume vol = Round(vol, 2) Range("D4").Value = volume End Sub

26. Review Questions

27. Review Question Programming Errors Which of the following is an example of a runtime error: A.Misspelled keyword B.Division by zero C.Inaccurate results D.All of the above

28. Review Question Debugging Tools & Techniques Which debugging tool/technique gives you the greatest amount of power/flexibility in finding program bugs: A.MsgBox Function B.Debug.Print Statement C.Debug Mode

29. Homework Help ‘n Hints