ME 142 Engineering Computation I Debugging Techniques

Key Concepts Types of Program Errors Debugging Tools & Techniques Testing

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

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.

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.

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

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

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)

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

Debug.Print Statement

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

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

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

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:

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

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

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.

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