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numeric validation

Posted on 2011-02-28
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I have a textbox that receives inches as input.  I only want to allow the user to enter a whole number or a number with a .5 to denote a half inch.  Does anybody have an example of how to validate this?
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Question by:oak29
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LVL 75

Expert Comment

ID: 35002104

``````Dim value As String = Me.textBox1.Text
Dim converted As Decimal

If Decimal.TryParse(value, converted) AndAlso (converted - Int(converted) = 0.5) Then
MsgBox("Yay!")
End If
``````
0

LVL 75

Accepted Solution

käµfm³d   👽 earned 500 total points
ID: 35002119
Correction:
``````Dim value As String = Me.textBox1.Text
Dim converted As Decimal

If Decimal.TryParse(value, converted) AndAlso ((converted - Int(converted) = 0.5) OrElse (converted - Int(converted) = 0.0)) Then
MsgBox("Yay!")
End If
``````
0

LVL 13

Expert Comment

ID: 35002200
Hi,

You can do a couple of things, first use a NUMERICAUPDOWN control instead of a textbox, second set the DECIMALSPLACE property to 1, and finally you can put this code in the Validating Event

``````Dim intPart As Int32
intPart = Int32.Parse(NumericUpDown1.Value.ToString.Substring(0, NumericUpDown1.Value.ToString.IndexOf(".")))
If NumericUpDown1.Value > intPart And NumericUpDown1.Value <> (intPart + 0.5) Then
MsgBox("Only Whole Numbers or Half", MsgBoxStyle.Critical)
e.Cancel = True
End If
``````
0

LVL 8

Expert Comment

ID: 35003349
Try the below code...
``````Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
Dim KeyAscii As Short = Asc(e.KeyChar)
' Check if the typed character is a letter.
If Char.IsDigit(e.KeyChar) = False And _
Char.IsControl(e.KeyChar) = False Then
e.Handled = True
End If
' Check if the typed character is a period.
If KeyAscii = 46 Then
e.Handled = False
End If
End Sub
``````
0

LVL 5

Expert Comment

ID: 35005585
1.) Using code like this can be dangerous as other countries use different formatting for numeric values. For example the German standard is a comma instead of a point. If you parse (4711.815) you get 47110815, because the dot is interpreted as thousand delimiter => not good for a finance application ;-)

2.) With this code you cant the sure, if the user enters two dots. There are some conditions you cannot check with the KeyPress. For example the Text "4711." is not valid, but the text "4711.5" is valid. So typing the dot will create a temporary invalid number, even if you do it the right way.

3.) You can use a masked textbox for helping the user to enter the correct format, but you have to check the data, when parsing it.

4.) Be sure to not check for =, when using decimal values. Because of the limited amount of binary digits a value like 0.1 will be stored as 0.1000000000000001 or 0.099999999999. For 0.5 values you do not have this problem directly, but after the first calculation.

5.) I would use code like this for the check method. Call this code when parsing the TextBox (Or MaskedTextBox). If you want, you can also call it in the Validating event after the user leaves the TextBox

Dim DoubleValue As DoubleValue
If Double.TryParse(TXT_YourTextBox.Text,DoubleValue Then
Dim DoubleDoubleValue =DoubleValue*2
If Math.Abs(DoubleDoubleValue - Math.Round(DoubleDoubleValue,0)) < 0.000001 Then
MsgBox("Your Value is: " & DoubleValue)
Else
MsgBox("Only enter x.0 or x.5")
EndIf
Else
MsgBox("Invalid Format")
EndIf
0

LVL 8

Expert Comment

ID: 35005704
@andr qin,

Nice, tips...

I am just a little curios why need this line of code?
Dim DoubleDoubleValue =DoubleValue*2
0

LVL 5

Expert Comment

ID: 35005792
Well you want the user to enter only full or halfs, so I double it. Then it must be in integer value. So I compare the rounded value with the original value and if the difference is less then 10^-6 I accept it.

If it is not exactly 0.5 you can also write it this way:

Private Const LittleDouble as Double=0.000001

Dim DoubleValue As DoubleValue
If Double.TryParse(TXT_YourTextBox.Text,DoubleValue Then
Dim Difference = Math.Abs(DoubleValue - Math.Round(DoubleValue,0))
If Differnce < LittleDouble OrElse (Difference >= 0.5-LittleDouble  AndAlso Difference <= 0.5+LittleDouble) Then
MsgBox("Your Value is: " & DoubleValue)
Else
MsgBox("Only enter x.0 or x.5")
EndIf
Else
MsgBox("Invalid Format")
EndIf
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