Excel's Formula - sisman

Chapter 23: Function Procedure Basics 625
To force the code in a VBA module to be checked for syntax errors, choose Debug➜
Compile xxx (where xxx is the name of your project). Executing this command highlights
the first syntax error, if any exists. Correct the error and issue the command again
until you find all of the errors.
An error in code is sometimes called a bug. The process of locating and correcting such an error
is debugging.
When you test a Function procedure by using a formula in a worksheet, runtime errors can be
difficult to locate because (unlike syntax errors) they don’t appear in a pop-up error box. If a runtime
error occurs, the formula that uses the function simply returns an error value (#VALUE!).
This section describes several approaches to debugging custom functions.
While you’re testing and debugging a custom function, it’s a good idea to use the
function in only one formula in the worksheet. If you use the function in more than one
formula, the code is executed for each formula, which will get annoying very quickly!
Using the VBA MsgBox statement
The MsgBox statement, when used in your VBA code, displays a pop-up dialog box. You can use
MsgBox statements at strategic locations within your code to monitor the value of specific variables.
The following example is a Function procedure that should reverse a text string passed as
its argument. For example, passing Hello as the argument should return olleH. If you try to use
this function in a formula, however, you will see that it does not work — it contains a logical error:
Function REVERSETEXT(text) As String
‘ Returns its argument, reversed
Dim TextLen As Long, i As Long
TextLen = Len(text)
For i = TextLen To 1 Step -1
REVERSETEXT = Mid(text, i, 1) & REVERSETEXT
Next i
End Function
You can insert a temporary MsgBox statement to help you figure out the source of the problem.
Here’s the function again, with the MsgBox statement inserted within the loop:
Function REVERSETEXT(text) As String
‘ Returns its argument, reversed
Dim TextLen As Long, i As Long
TextLen = Len(text)
For i = TextLen To 1 Step -1
REVERSETEXT = Mid(text, i, 1) & REVERSETEXT
MsgBox REVERSETEXT
Next i
End Function