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Excel UDF Tutorial

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Microsoft Excel supports macros but it also supports library functions, known as User Defined Functions (UDF). Library functions are less risky than macros because they must return a value to a cell. This is a quick tutorial, mostly for my students, but as usual for anybody who’s interested.

Microsoft Excel User Defined Functions (UDFs) are different than standard VBA macros. They’re behavior is restricted. You can’t access other cells in a workbook, and may only return a value (also known as an expression) to the cell that uses the formula. That having been said, they can dramatically hide the complexity of mega-formulas and remove them from the editing control of users.

Here are the steps to configure your Excel 2007 environment to work with Visual Basic for Applications (VBA) and UDFs. They show you how to display the developer ribbon, open a module for editing, save VBA code into a library (*.xlam file), and add the library file as an “Add-in” library to your Excel 2007 installation. The name of the library will be the same as the Workbook where you created it.

  1. Displaying Excel’s Developer Tab

There are four steps to make this visible. They are:

  1. Choose the Office Button. You’ll see the following:

  1. Click the Excel Options button.

  1. Click the Popular tab if not highlighted (it’s the default). Inside the right side pane, click the Show Developer tab in the Ribbon check box to enable it, and click the OK button to set it.

  1. Click the Developer ribbon tab. Click the left most icon to launch the Visual Basic Editor screen.

  1. Open a Module in the Visual Basic Editor by clicking the Insert menu item and choosing the Module element.

  1. Copy the following function definition into the open Module. The name of the UDF will be the case-sensistive name of the function in the VBA module. That means you’ll now have a newSerialDate function in your list of functions when you click the insert function button.

I struggled to come up with a simple function to illustrate this and how you debug UDFs. A word to the wise, you can’t use a numeric variable inside a MsgBox by itself. If you attempt it, Excel will return a #Value! error in the cell where you call the UDF. You must include a numeric variable as an argument (also known as a call parameter) to the CStr() function. An example is: Cstr(myVariable). You call this sample function by entering:


When you pass a date (actually an integer in the spreadsheet), the function handles it as a string. While no data type is assigned the dateIn variable in the example, variables without an explicit data type are always of the Variant data type. The Variant data type is a master data type and can hold any other type, which makes it like the Object data type in Java.

This program parses the string, then uses the DateSerial() function to return it as a number. It’s critical to note that the last line returns the value in the newSerialDate variable, and that variable must always be the function name. Place a single quote mark before all MsgBox() function calls after verifying that the function works.

Public Function newSerialDate(dateIn)
  ' Define local variables.
  Dim day As String
  Dim month As String
  Dim year As String
  Dim startPosition As Integer
  Dim endPosition As Integer
  Dim length As Integer
  ' Initialize local variables and parse the month from the left.
  startPosition = 1
  endPosition = InStr(startPosition, dateIn, "/")
  month = Left(dateIn, endPosition - 1)
  ' This is a debugging tool to track variables during execution.
  MsgBox ("Month [" + month + "]")
  ' Shift the start position and parse the day from the middle.
  startPosition = endPosition + 1
  endPosition = InStr(startPosition, dateIn, "/")
  day = Mid(dateIn, startPosition, endPosition - startPosition)
  ' This is a debugging tool to track variables during execution.
  MsgBox ("Day [" + day + "]")
  ' Get the remainder of the string.
  startPosition = endPosition
  length = Len(dateIn) - startPosition
  year = Right(dateIn, length)
  ' This is a debugging tool to track variables during execution.
  MsgBox ("Year [" + year + "]")
  ' Return a value from a User Defined Function (UDF) by using
  ' the function name as the return variable.
  newSerialDate = DateSerial(year, month, day)
End Function

Aside from the fact that all this parsing isn’t really necessary because the problem is much simpler and cleaner. At least, it becomes so when you understand the breadth of built-in functions in VBA. You can solve the problem by designating the formal parameter as a Double like the example below.

Function newSerialDate(dateIn As Double)
  newSerialDate = dateIn
End Function

Alternatively, you can accept a Variant, which maps to a String. Then, you convert it to a Date like this:

Function newSerialDate(dateIn)
  newSerialDate = DateValue(dateIn)
End Function
  1. Create the following spreadsheet, the formula values are noted below the screen shot. You should be able to copy and past them into the spreadsheet. After you’ve created the spreadsheet, entering a new date in cell A1 causes the UDF to run. When the UDF runs “as-is”, you’ll see message dialog boxes that show values set during runtime.

Spreadsheet Functions
Cell Formula
A2 =newSerialDate(A1)
C1 =VLOOKUP(TYPE(A1),$E$3:$F$7,MATCH("Meaning",$E$2:$F$2,FALSE))
C2 =VLOOKUP(TYPE(A2),$E$3:$F$7,MATCH("Meaning",$E$2:$F$2,FALSE))

  1. You can now save this as an Add In library but comment out those debug MsgBox() function calls. Click the Office Button and click SaveAs in the menu, then accept Excel Workbook initially. When you get to the SaveAs dialog, choose Excel Add-In as the file type. Below is a screen capture of the drop down selection box.

  1. Open a new Excel Workbook. Click the Excel Options button. Click the Add-Ins tab. Inside the right side pane, make sure the Manage drop down says Excel Add-ins before you click the Go button.

  1. Check the Samplefunction check box as shown below. Samplefunction is the name of the Workbook that contains the module, and it is saved as an *.xlam file. Click the OK button to add the library. You’ve now created, and added an Add-In library to your new spreadsheet. It will create a SampleFunction.xlam file in the users directory.

The file is found by default in:


Ranjit asked how you could call a UDF from inside a module. In the answer noted below, I show how to do it with the Unary function:

Function Unary(number As Integer)
  Unary = number + 1
End Function

Then, enter the following ordinary Increment macro:

Sub Increment()
  ActiveCell.FormulaR1C1 = Unary(ActiveCell.Value())
End Sub

Enter a number in a cell, then navigate to Tools -> Macro -> Macros… and choose the increment macro, which increments the value previously in the cell by calling the Unary function.

As always, I hope this is helpful to a few folks.

Written by maclochlainn

May 26th, 2010 at 1:03 am