Ensure Accurate Credit Card Numbers
The last digit in your credit card number is a check digit derived from the other digits using the Luhn Formula as described in ISO/IEC 7812-1:1993. Its primary purpose is to ensure accurate entries of the credit card number during transactions. You can apply the same technique to other applications such as employee numbers or patient numbers. Using check digits for these numbers also ensures more accurate data entries:
Public Function CheckDigit(strNum As String) As Integer
Dim Counter As Integer
Dim iEven As Integer
Dim iOdd As Integer
Dim iTotal As Integer
Dim iTemp As Integer
Dim strOneChar As String
' Add digits in even ordinal positions
' starting from rightmost
For Counter = Len(strNum) - 1 To 2 Step -2
strOneChar = Mid$(strNum, Counter, 1)
If IsNumeric(strOneChar) Then
iEven = iEven + CInt(strOneChar)
End If
Next Counter
' Process digits in odd ordinal positions
' starting from rightmost
For Counter = Len(strNum) To 1 Step -2
strOneChar = Mid$(strNum, Counter, 1)
If IsNumeric(strOneChar) Then
' Double it
iTemp = CInt(strOneChar) * 2
If iTemp > 9 Then
' Break the digits (e.g., 19 becomes 1+9)
iOdd = iOdd + (iTemp \ 10) + (iTemp - 10)
Else
iOdd = iOdd + iTemp
End If
End If
Next Counter
' Add even and odd
iTotal = iEven + iOdd
' Return the 10's complement
CheckDigit = 10 - (iTotal Mod 10)
End Function
To test, pass your credit card number, excluding the last digit, as a string parameter. The result should be the last digit of your credit card number.
