Mod with Doubles

Am I doing something wrong or does the VBA Mod operator actually not work with floating point values like Doubles?

So I've always sort of assumed that the VBA Mod operator would work with Doubles based on the VB documentation, but in trying to figure out why my rounding function doesn't work, I found some unexpected Mod behavior.

Here is my code:

Public Function RoundUp(num As Double, Optional nearest As Double = 1)
    RoundUp = ((num \ nearest) - ((num Mod nearest) > 0)) * nearest
End Function

RoundUp(12.34) returns 12 instead of 13 so I dug a little deeper and found that:

12.5 Mod 1 returns 0 with the return type of Long, whereas I had expected 0.5 with a type of Double.

Conclusion

As @ckuhn203 points out in his answer, according to the VBA specification,

The modulus, or remainder, operator divides number1 by number2 (rounding floating-point numbers to integers) and returns only the remainder as result.

And

Usually, the data type of result is a Byte, Byte variant, Integer, Integer variant, Long, or Variant containing a Long, regardless of whether or not result is a whole number. Any fractional portion is truncated.

For my purposes, I need a floating point modulo and so I have decided to use the following:

Public Function FMod(a As Double, b As Double) As Double
    FMod = a - Fix(a / b) * b

    'http://en.wikipedia.org/wiki/Machine_epsilon
    'Unfortunately, this function can only be accurate when `a / b` is outside [-2.22E-16,+2.22E-16]
    'Without this correction, FMod(.66, .06) = 5.55111512312578E-17 when it should be 0
    If FMod >= -2 ^ -52 And FMod <= 2 ^ -52 Then '+/- 2.22E-16
        FMod = 0
    End If
End Function

Here are some examples:

FMod(12.5, 1) = 0.5 FMod(5.3, 2) = 1.3 FMod(18.5, 4.2) = 1.7

Using this in my rounding function solves my particular issue.

Solution

According to the VB6/VBA documentation

The modulus, or remainder, operator divides number1 by number2 (rounding floating-point numbers to integers) and returns only the remainder as result. For example, in the following expression, A (result) equals 5. A = 19 Mod 6.7 Usually, the data type of result is a Byte, Byte variant, Integer, Integer variant, Long, or Variant containing a Long, regardless of whether or not result is a whole number. Any fractional portion is truncated. However, if any expression is Null, result is Null. Any expression that is Empty is treated as 0.

Remember, mod returns the remainder of the division. Any integer mod 1 = 0.

debug.print 12 mod 1 
'12/1 = 12 r 0

The real culprit here though is that vba truncates (rounds down) the double to an integer before performing the modulo.

?13 mod 10
 '==>3 
?12.5 mod 10
 '==>2 

debug.print 12.5 mod 1
'vba truncates 12.5 to 12
debug.print 12 mod 1
'==> 0