Editing an 8 Character Hexadecimal Field in a Form

[kimmer]

I am designing a form for a data entry function where two fields that are entered are hex (8characters). I want to edit this field for valid hex numbers. How do I do this?

[Stewart Ross]

Hi. There are no built-in facilities for entering and validating hexadecimal values into textboxes. You will need to validate what the user entered yourself using the BeforeUpdate events of the controls concerned to apply validation.

If you are entering each value as a four-character hex value you will need to:

1. ensure that four characters have been entered
2. ensure that the characters are either digits 0..9 or letters A..F

Here's an example of validating a four-character Hex string to get you going, using the BeforeUpdate event of a textbox called Text0 in this case:

Private Sub Text0_BeforeUpdate(Cancel As Integer)
    If fInvalidHex(Me.Text0) Then
        Cancel = True
        MsgBox "Please re-enter as four-character hex address"
    End If
End Sub


Public Function fInvalidHex(ByRef ControlValue As Variant) As Boolean
    Dim blResult As Boolean
    Dim lngChrCount As Long
    Dim lngCounter As Long
    Dim strCh As String
    blResult = False
    If IsNull(ControlValue) Then
        fInvalidHex = True
    ElseIf Len(ControlValue) <> 4 Then
        fInvalidHex = True
    Else
        For lngCounter = 1 To 4
            strCh = UCase(Mid(ControlValue, lngCounter, 1))
            Select Case strCh
                Case "0" To "9", "A" To "F"
                   'do nothing
                Case Else
                    blResult = True
            End Select
        Next
        fInvalidHex = blResult
    End If
End Function

I've no doubt there are many different ways to do this, but at least this gives you a start.

-Stewart

[ADezii]

I took a totally different approach on this Thread. Since you indicated that this Text Box will be used for Data Entry, then each and every Character entered into the Text Box should be Validated to ensure that it is in the Range (0 to 9, A - F). If the Character is outside this Range, it is then simply not allowed to be entered, and is technically stripped while leaving the remainder of the Valid String in tact. Fortunately, Access has an Event which can be used specifically for this type of functionality, the Change() Event. The following Code will allow only the Characters 0,1,2,3,4,5,6,7 ,8,9,A,B,C,D,E, F to be entered into the Text Box, while stripping invalid ones. The Arrow Keys, and Backspace will not effect the outcome, but you may wish to check other Keystrokes on your own.

Private Sub txtHex_Change()
Dim tHex As TextBox
Dim intCtr As Integer

Set tHex = Me![txtHex]

If Len(tHex.Text) = 0 Then
  Exit Sub
Else
  For intCtr = 1 To Len(tHex.Text)
    Select Case Asc(Mid$(tHex.Text, intCtr, 1))
      Case 48 To 57         'Valid, 0 to 9
      Case 65 To 70         'Valid, A to F
      Case Else             'Invalid, strip Character then move
                            'Cursor to end of string
        tHex.Text = Left$(tHex.Text, Len(tHex.Text) - 1)
        tHex.SelStart = Len(tHex.Text)
    End Select
  Next
End If
End Sub

P.S. - The Spacebar seems to cause a problem, so negate it. I'm assuming that there should be no spaces in the Hex String. This can be done because the KeyDown() Event is fired before the Change() Event.

Private Sub txtHex_KeyPress(KeyAscii As Integer)
  If KeyAscii = 32 Then KeyAscii = 0
End Sub

[kimmer]

Thanks for the help. It works great.