1. ME 142 Engineering Computation I
Debugging Techniques

2. Key Concepts Types of Program Errors Debugging Tools & Techniques
Testing

3. Types of Programming Errors
Syntax Errors
Run-Time Errors
Logic Errors

4. Syntax Errors
Syntax errors occur when the rules of the programming language are violated.
Examples:
Misspelling a keyword or function name
Misplacement or omission of a parenthesis or quote mark
Omission of necessary keywords in a command.

5. 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.

6. 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

7. Debugging Tools & Techniques
MsgBox Function
Debug.Print Statement
Debug Mode

8. 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)

9. 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

10. Debug.Print Statement

11. 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

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

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

14. 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:

15. 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

16. 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

17. Testing 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.

18. 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