<%@ Page Language="VB" %>
Function ValidateCard(cardNumber As String) As Boolean
Dim reverseNumber As String
Dim ch As Char
For Each ch in cardNumber
reverseNumber = ch & reverseNumber
Next
Dim doubledAlternates As String
Dim thisDigit, index As Integer
For index = 1 to reverseNumber.Length
thisDigit = CInt(reverseNumber.SubString(index - 1, 1))
If index mod 2 = 0 Then
thisDigit = thisDigit * 2
End If
doubledAlternates = doubledAlternates & thisDigit
Next
Dim sum As Integer
For index = 1 to doubledAlternates.Length
sum = sum + CInt(doubledAlternates.SubString(index - 1, 1))
Next
ValidateCard = sum <> 0 AND sum mod 10 = 0
End Function
Sub Button1_Click(sender As Object, e As EventArgs)
If ValidateCard(TextBox1.Text) Then
Label1.Text = "That'll do nicely sir!"
Else
Label1.Text = "There's nothing for you here!"
End If
End Sub
Enter the number: