Floating point comparison in VB6

Question

What's the best way to test two Singles for equality in VB6?

I want to test two Single values for equality to 7 significant figures.

This MSDN article recommends using something like

If Abs(a - b) <= Abs(a / 10 ^ 7) Then
    valuesEqual = True
End If

However, that can fail for certain values, e.g.

Public Sub Main()

    Dim a As Single
    Dim b As Single

    a = 0.50000005
    b = 0.50000014

    Debug.Print "a = " & a
    Debug.Print "b = " & b
    Debug.Print "a = b: " & (a = b)
    Debug.Print "SinglesAreEqual(a, b): " & SinglesAreEqual(a, b)

    // Output:
    // a = 0.5000001
    // b = 0.5000001
    // b = b: False
    // SinglesAreEqual(a, b): False

End Sub

Private Function SinglesAreEqual(a As Single, b As Single) As Boolean

    If Abs(a - b) <= Abs(a / 10 ^ 7) Then
        SinglesAreEqual = True
    Else
        SinglesAreEqual = False
    End If

End Function

The simplest way I've found of getting the result I need is to convert the values to strings, but seems horribly ugly:

Private Function SinglesAreEqual(a As Single, b As Single) As Boolean

    SinglesAreEqual = (Str$(a) = Str$(b))

End Function

Are there any better ways?

Answer 1

I maintain a CAD/CAM application and I have to deal with floating point numbers all the time. I have a function that I call fComp that I pass a floating point value when I need to test for equality. fComp call a rounding function set to a certain level of precision. For our system I round to 6 decimal places. Yours may need higher or get away with lower it depends on the application.

The fComp Function exists so I have one spot to change the rounding factor used in these calculations. This proved handy a couple of years back when we started manufacturing higher precision machines.

Public Function pRound(ByVal Value As Double, ByVal Power As Double) As Double
    Dim TempValue As Double
    Dim tSign As Double
    TempValue = Value
    tSign = TempValue
    TempValue = Abs(TempValue)
    TempValue = TempValue * 10 ^ (Power * -1)
    TempValue = Fix(TempValue + 0.5)
    TempValue = TempValue / 10 ^ (Power * -1)
    pRound = TempValue * Sign(tSign)
End Function

To round to the 6th decimal place you go

RoundedNumber = pRound(MyValue, -6)

Negative is to the right of the decimal place positive to the left.

Answer 2

Instead if rounding and testing for equality, you can take the difference of two numbers and compare that with a factor

If Abs(a - b) < 0.000001 Then

You can adjust the 0.000001 to whatever resolution you need

Answer 3

I don't believe you can use the single data type to that many significant figures. You would need to use double instead:

Dim a As Single
Dim s As String

s = "0.50000005"
a =  0.50000005

Debug.Print s & " " & a

The above outputs:

0.50000005
0.5000001