XL97: Calculation Problems with Arrays in Custom Functions

When you use a custom function in a workbook, the function may return incorrect values. Recalculating the workbook may also take longer than it does in earlier versions of Microsoft Excel.

This problem occurs when the following conditions are true:

• The custom function references a range of cells. -and-
  
  
• You enter the custom function across multiple cells as an array. -and-
  
  
• You recalculate the custom function.

Microsoft provides programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes that you are familiar with the programming language being demonstrated and the tools used to create and debug procedures. Microsoft support professionals can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific needs.
If you have limited programming experience, you may want to contact a Microsoft Certified Partner or Microsoft Advisory Services. For more information, visit these Microsoft Web sites:

Microsoft Certified Partners - https://partner.microsoft.com/global/30000104 (https://partner.microsoft.com/global/30000104)

Microsoft Advisory Services - http://support.microsoft.com/gp/advisoryservice (http://support.microsoft.com/gp/advisoryservice)

For more information about the support options that are available and about how to contact Microsoft, visit the following Microsoft Web site:http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS (http://support.microsoft.com/default.aspx?scid=fh;en-us;cntactms) To work around this problem, follow these steps:

1. Add the Volatile method to your Visual Basic for Applications custom function. This line must immediately follow the Function statement. For example, the following function uses the Volatile function:

         Function MyFunction(InputRange) As Integer
            Application.Volatile
            MyFunction = InputRange * 10%
         End Function
   						

2. Reenter the formula that contains the custom function. To locate and select the number of cells that comprises the array formula, follow these steps:
   
   
   1. In the worksheet, click any one cell that contains the custom array function.
   2. On the Edit menu, click Go To. In the Go To dialog box, click Special. In the Go To Special dialog box, click Current Array.
      
      Every cell that is in the array is selected.
   3. Press F2 to edit the entire array. Press CTRL+SHIFT+ENTER to reenter the array formula.
3. Save the workbook.

The custom function is calculated correctly.

Microsoft has confirmed this to be a problem in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Excel for Windows, Service Release 1, and in Microsoft Excel 98 Macintosh Edition.

Arrays in Microsoft Excel 97 are recalculated <n> times, where <n> is the number of elements in the array. In earlier versions of Microsoft Excel, the array function is recalculated only once. Therefore, the number of elements in the array is directly proportional to the decrease in performance when the array is recalculated. Depending on the calculations Microsoft Excel performs in the custom function, incorrect values may be returned. For example, an incorrect value is returned if the function increments a counter and uses the counter value as part of a calculation.