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Language: Visual Basic
Expertise: Beginner
Feb 3, 1999

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# Compute Credit Card Check Digits

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 i As Integer
Dim iEven As Integer
Dim iOdd As Integer
Dim iTotal As Integer
Dim strOneChar As String
Dim iTemp As Integer

' Add digits in even ordinal positions
' starting from rightmost
For i = Len(strNum) - 1 To 2 Step -2
strOneChar = Mid\$(strNum, i, 1)
If IsNumeric(strOneChar) Then
iEven = iEven + CInt(strOneChar)
End If
Next i

' Process digits in odd ordinal positions
' starting from rightmost
For i = Len(strNum) To 1 Step -2
strOneChar = Mid\$(strNum, i, 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 i

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.
Arnel J.

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