Comparing doubles returns false - vba

Question

I have three numbers from my database and want to compare them in an if statement.
I have a simple conevert function that returns only doubles.
Public Function RetDbl(ByVal obj As Variant) As Double
On Error Resume Next
RetDbl = val(Replace(Nz(obj, 0), ",", "."))
End Function
The statement is
If RetDbl(rs.value("NumA")) + RetDbl(rs.value("NumB")) <> (RetDbl(rs.value("NumC")) * 1000) Then
'[... do some code ...]
End If
With RetDbl(rs.value("NumA")) = 0.33, RetDbl(rs.value("NumB") = 0.5 and RetDbl(rs.value("NumC")) = 0.00083
This always returns false
I also tried:
In the direct field (STRG + G): ?cdbl(0.33) + cdbl(0.50) = cdbl(0.83) returns false. When i leave out the last part it returns 0.83
How can i compare these numbers?

Answer 1

Comparing floating numbers is hard. Only yesterday, I've posted this question
My solution:
Public Function DblSafeCompare(ByVal Value1 As Variant, ByVal Value2 As Variant) As Boolean
'Compares two variants, dates and floats are compared at high accuracy
Const AccuracyLevel As Double = 0.00000001
'We accept an error of 0.000001% of the value
Const AccuracyLevelSingle As Single = 0.0001
'We accept an error of 0.0001 on singles
If VarType(Value1) <> VarType(Value2) Then Exit Function
Select Case VarType(Value1)
Case vbSingle
DblSafeCompare = Abs(Value1 - Value2) <= (AccuracyLevelSingle * Abs(Value1))
Case vbDouble
DblSafeCompare = Abs(Value1 - Value2) <= (AccuracyLevel * Abs(Value1))
Case vbDate
DblSafeCompare = Abs(CDbl(Value1) - CDbl(Value2)) <= (AccuracyLevel * Abs(CDbl(Value1)))
Case vbNull
DblSafeCompare = True
Case Else
DblSafeCompare = Value1 = Value2
End Select
End Function
Note that the AccuracyLevel (epsilon) could be set to a smaller value, and I'm using the same value for singles and doubles, but it did well for my purposes.
I'm using a relative epsilon, but multiplying it with the first, and not the largest value, since if there's a significant difference the comparison will fail anyway.
Note that I'm using <= and not < since else DblSafeCompare(cdbl(0) ,cdbl(0)) would fail
Note that this function checks for type equality, so comparing integers to longs, doubles to singles, etc. all fails. Comparing Null to Null passes, however.
Implement it:
?DblSafeCompare(cdbl(0.33) + cdbl(0.50) ,cdbl(0.83))
?DblSafeCompare(cdbl(0.331) + cdbl(0.50) ,cdbl(0.83))

Answer 2

Comparing floating point numbers is really an issue, if you try to do it without understanding of the nature of the floating numbers.
Here is a nice article about it - https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html and How dangerous is it to compare floating point values?
In general, this problem is so big, that some languages like C# have developed a specific class called Decimal which makes comparing run as it would be expected by a non-programmer. Decimal info. In VBA, a similar class is Currency. Thus
CCur(0.33) + CCur(0.50) = CCur(0.83)
Returns True. VBA supports the function CDec, which converts a double to a decimal number, but it does not support the class Decimal. Thus:
CDec(0.33) + CDec(0.50) = CDec(0.83)
would also return True. And is with some better accuracy than Currency. CDec documentation.