Luhn' s Algorithm - CodeProject

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Introduction

This sample implements the functionality of generating checksum number using Luhn's algorithm.

I've created a class LuhnAlgorithm with a static function:

public static int GetLuhnAlgorithmNumber(string data);

Return value is the generated number.

Definition

The Luhn algorithm or Luhn formula, also known as the "modulus 10" or "mod 10" algorithm, was developed in the 1960s as a method of validating identification numbers. It is a simple checksum formula used to validate a variety of account numbers, such as credit card numbers and Canadian Social Insurance Numbers. Much of its notoriety comes from credit card companies' adoption of it shortly after its creation in the late 1960s by IBM scientist Hans Peter Luhn (1896–1964).

Explanation of the Luhn's algorithm can be found here.

License

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About the Author

wapnethr

Austria

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Actually, this method only needs one line changed to compute the correct Luhn check-digit (the "return" equation is wrong)

kirkaiya

18:42 6 Aug '09

This is an old article, so maybe nobody will read this, but -

To the people who posted that the method should start with the second-to-last digit - I don't think that is correct. The author seems (from the title) to have intended this as a function to generate the check-digit, not to validate.

That said, his "return" equation has an error. It should be (expressed in VB.Net, which I used):
Return CInt((Math.Truncate(sum / 10) + 1) * 10) - sum
[not (((sum / 10) + 1) * 10) - sum, which is what the source-code has]

In other words - the author's code only fails to truncate the result of sum/10, which leads to equation returning 10 [since, for example, if sum = 6, then (((sum/10)+1)*10 - sum = 10 ].

After changing the line indicated above, and typing in all but the last digit of four different credit-card numbers (eg., give it a string of 15 digits, don't include the last "check-digit"), the function correctly returned the final digit of all four card-numbers.

Here's the entire (corrected)_function in VB.Net for anybody who is too lazy to convert:

Public Shared Function GetChecksum(ByVal DecimalString As String) As Integer
    Dim sum As Integer = 0
    Dim odd As Boolean = True
    For i As Integer = DecimalString.Length - 1 To 0 Step -1
        If odd = True Then
            Dim tSum As Integer = Convert.ToInt32(DecimalString(i).ToString()) * 2
            If tSum >= 10 Then
                Dim tData As String = tSum.ToString()
                tSum = Convert.ToInt32(tData(0).ToString()) + Convert.ToInt32(tData(1).ToString())
            End If
            sum += tSum
        Else
            sum += Convert.ToInt32(DecimalString(i).ToString())
        End If
        odd = Not odd
    Next
 
    Return CInt((Math.Truncate(sum / 10) + 1) * 10) - sum
 
End Function

Incorrect Algorithm?

Peebles

2:36 3 Mar '05

This function starts the "doubling" process on the last digit of the card number. It should begin with the 'second to last' digit, and then every other digit from there on (working backwards). Change the "if(odd == true)" part of the function to read "if(odd == false)" and it will work correctly (valid card numbers should return 10). paul.b
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use this code...

Joel Thoms

11:47 18 Jan '05

I ended up using this code instead. Very clean and functional. http://www.codeproject.com/aspnet/creditcardvalidator.asp[^] Joel Thoms http://joel.net
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how to use

Joel Thoms

11:41 18 Jan '05

A section on how to use would be nice. I input 2 different valid credit card numbers and received values 3 and 9. When an invalid credit card number was input, I received a 4. I think there may be something wrong in the algorythm, I cannot use this. Joel Thoms http://joel.net
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wrote an implementation myslef some time ago

MadHatter ¢

7:02 9 Dec '04

I had to write a mod10 validator a while back. pretty simple IMO. /bb\|[^b]{2}/
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Explaining my vote

Jason De Arte

8:45 8 Dec '04

Yes, I rated this article low. But I think it's important to write _why_ I rated it low for the benefit of the author. This is NOT meant to be taken as an attack - but I must admit I haven't had any caffeine yet this morning and I may still be a bit cranky.
1) The article is terse and leaves me wondering why I would need to reference this code in the future. Maybe an example of _why_ this code is cool and for what it can be used for?
2) the "Definition" is a copy and paste of the "Explanation" that you link to.
3) You only provide a zipped source file whose source appears to have the same byte count as your article. Maybe if you had a sample? or highlighted some bits of the code in the article?
4) The list of compatible platforms is HUGE, I had no idea that a C# source file would work on PalmOS. Wink | ;)

It's a nice first start for an article, but you should read "What is a good article" http://www.codeproject.com/scrapbook/whatisagoodarticle.asp[^]

And maybe a few of the top ranked articles for their structure
http://www.codeproject.com/script/articles/top_articles.asp?st=2[^]

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