Incrementing textbox value up and down in vba does not cancel evenly - ms-access

Question

I can't figure out why my code isn't maintaining significant digits when incrementing a textbox value.
I have a spin control (textbox plus two small command buttons to move textbox value up or down in value) on a form.
The textbox default value is zero.
The up arrow command button should increment the textbox value by + 0.1. Here is the code:
Private Sub cmdIndexSpinUp_Click()
If Me!txtIndexSpin >= 1.5 Then
MsgBox "The maximum Index adjustment has been reached."
Exit Sub
Else
Me!txtIndexSpin = Me!txtIndexSpin + 0.1
End If
End Sub
The down arrow command button should increment the textbox value by -0.1. Here is the code:
Private Sub cmdIndexSpinDown_Click()
If Me!txtIndexSpin <= -1.5 Then
MsgBox "The minimum Index adjustment has been reached."
Exit Sub
Else
Me!txtIndexSpin = Me!txtIndexSpin - 0.1
End If
End Sub
So I would expect that from the default value of 0, if I spin up once and down once, I should return to 0. That works fine. If I spin up twice and then down twice, my textbox value suddenly becomes 2.77555756156289E-17 instead of 0.
After more testing, it does not consistently happen based on the number of clicks, but it may be related to the time between clicks. The more rapid, the more prone to this error it seems.
How could this be happening?
I am going to code around it, since I see nothing wrong, but am curious what I am missing.

Answer 1

As #HansUp says floating points aren't precise. Source. An easy solution that you could use is to round the number before putting it in the text box.
Example :
Private Sub cmdIndexSpinDown_Click()
Dim value As Double
value = Me!txtIndexSpin
If value <= -1.5 Then
MsgBox "The minimum Index adjustment has been reached."
Exit Sub
Else
value = Round(value - 0.1, 1)
Me!txtIndexSpin = value
End If
End Sub
Private Sub cmdIndexSpinUp_Click()
Dim value As Double
value = Me!txtIndexSpin
If value >= 1.5 Then
MsgBox "The maximum Index adjustment has been reached."
Exit Sub
Else
value = Round(value + 0.1,1)
Me!txtIndexSpin = value
End If
End Sub

Answer 2

Round() is a reasonable solution for your floating point precision problem. And it may well be exactly what you want. However, be aware that you will be using "banker's rounding", sometimes called "round to even". So you might not get what you expect from rounding to 1 decimal place when the second decimal place is 5:
? Round(1.15, 1)
1.2
? Round(1.25, 1)
1.2
If that is not what you want, you could use a different rounding strategy. Or you could switch to integer math instead of floating point math ... and then the floating point precision challenge goes away. That might sound challenging, but it's actually simple to implement. Add a hidden text box to your form and use it like this ...
Private Sub cmdIndexSpinDown_Click()
With Me!txtHidden
If .Value <= -15 Then
MsgBox "The minimum Index adjustment has been reached."
Else
.Value = .Value - 1
Me!txtIndexSpin.Value = .Value / 10
End If
End With
End Sub
If txtIndexSpin is bound to a field in the form's Record Source, you can load txtHidden from the form's Current event:
Me!txtHidden.Value = Me!txtIndexSpin.Value * 10
And if you also allow the users to edit txtIndexSpin directly (not just via those command buttons), do that again from its After Update event.