Interesting Integer Overflow

  • Hi All,

    Found an interesting integer overflow condition today, thought I would share in case anybody else runs into same issue.

    The condition arises when you use a literal int as the test expression in a Select Case statement:

    When the Selection.Rows.Count is larger than maximum 16-bit integer value 32,767, the "Case Selection.Rows.Count" line throws an overflow error.

    I don't know how the compilation of Select Case constructs to p-code is implemented, but obviously each statement is evaluated in the (assumed) data type of the test expression.

    The fix is easy: use a 32-bit integer (Long) constant as the test expression

    Sub DoesNotOverFlow()
        Const lVALID_REGION_COUNT As Long = 2
        Select Case lVALID_REGION_COUNT
            Case Selection.Rows.Count
  • Re: Interesting Integer Overflow

    Alternatives would be

    Select Case CLng(2)

    Select Case 2&

    or store the value in an appropriately typed variable.

    Interesting potentional error though.


  • Re: Interesting Integer Overflow

    Hi shg
    why does this code need to have the CDbl() around the variable to stop an error when I have all ready stored the variable as a double ??

    or what coding error have i made?

  • Re: Interesting Integer Overflow

    This might be a little late - but the problem is related to the way VBA and (VB, plus I believe .NET as well) handles numeric variables.

    For example:

    Public Sub Test
    [INDENT]Dim x as Integer
    Dim y as Integer
    x = 200
    y = 300
    MsgBox (x*y)   <- Throws an error
    [/INDENT]end sub

    The reason for the error is that VBA creates a 'shadow' variable with a precision equal to the highest declared variable. In this case, Integer. It then uses that variable to store the results of the calculation before passing it back to the call to Msgbox. However, the variable is an integer so it overflows.

    Even if you declare 'Z as Long' and try to assign Z = (X*Y) to it, the error will occur as VBA still creates an Integer shadow.

    The only way around this is to declare one of the input variables as Long, then the shadow will be a long.

    The same is happening here. AS Fencliff pointed out, the Case statements are assumed to have the same type as the Case comparison value (And 2 is an Integer) so it overflows when rows get to be greater than 32767 (or whatever...)

    This problem has existed in Visual Basic since version 1. 0 and for all I know in the DOS QuickBasic as well...


Participate now!

Don’t have an account yet? Register yourself now and be a part of our community!