Introduction to User Defined Functions

Excel’s built-in functions are powerful, but sometimes you need something more specific to your needs. User Defined Functions (UDFs) allow you to create custom functions using VBA that can be used just like Excel’s native functions. Whether you’re a scientist, engineer, statistician, or accountant, UDFs can help automate complex calculations and save time.

Functions vs Procedures

Functions and procedures both perform tasks, but there’s a key difference:

• Functions: Return a value (like Excel’s SUM function)
• Procedures: Perform actions but don’t return values

Function Limitations

Function Syntax and Structure

[AccessModifier] Function FunctionName([parameters]) As DataType
    [Code]
End Function

Access Modifiers Explained

Public Access Modifier

Public: The function can be accessed from any module in the current workbook. This is the default if no modifier is specified. Public functions appear in Excel’s function list and AutoComplete.

Private Access Modifier

Private: The function can only be accessed from procedures within the same module. Private functions won’t appear in AutoComplete or the Insert Function dialog.

Static Access Modifier

Static: Variables retain their values between function calls, unlike normal functions where variables are cleared after execution.

Function Naming Rules

Creating Your First UDF – Step by Step

Let’s create a simple function that calculates the cube of a number.

1. Create a New Workbook
   Open Excel and create a new workbook for your UDF development.
2. Open VBA Editor
   Press Alt + F11 to open the Visual Basic Editor. You should see the Project window with your workbook name and worksheets.
3. Insert a Module
   Click Insert > Module to create a new code module.
4. Write Your Function
   Type the following code in the module:
   Public Function CUBENUMBER(value) As Long ‘Returns the cubed value of a number CUBENUMBER = value * value * value End Function
5. Return to Excel
   Press Alt + F11 or click the Excel button to return to the worksheet.

Testing Your Function

1. Use AutoComplete
   In cell A1, type =cuben. Excel’s AutoComplete will show your function. Press Tab to select it, then type 5) and press Enter. Result: 125
2. Reference Other Cells
   Put the number 6 in cell D2. In cell B4, type =CUBENUMBER(D2) and press Enter. Result: 216

Using the Insert Function Dialog

1. Click in cell B6
2. Go to Formulas tab > Insert Function
3. Select “User Defined” from the category dropdown
4. Select CUBENUMBER from the function list
5. Click OK and enter your parameters

Enhancing Your UDF with Documentation

Adding Function Descriptions

You can add descriptions to your functions using the MacroOptions method:

Categorizing Your Functions

You can organize your functions into Excel’s built-in categories or create custom ones:

Function Category Reference

Saving and Distributing Your UDF

Save as Excel Add-in

1. Save as Add-in
   Go to File > Save As, and select “Excel Add-in (*.xlam)” as the file type. Excel will automatically save it to the Add-ins folder.
2. Install the Add-in
   Go to File > Options > Add-ins. Select your add-in and click “Go” to install it.
3. Share with Others
   Distribute the .xlam file to other users who want to use your functions.

Best Practices for UDF Development