Talk:Luhn algorithm
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[edit] First Comment (Untitled)
When converting every other digits (e.g. 8 -> 16 -> 7), the doubling step helps check for people who accidentally swapped adjacent digits (e.g. 1234->1243). But what's rationale behind converting 16 to 7?
- My guess is that it keeps the overall numbers smaller, keeping things down to single check digits at each iteration.
[edit] The Pseudocode Is Wrong
The given pseudocode is incorrect. The for loop must iterate from 0 to (nDigits - 1), not to nDigits. Iterating to the length of a zero-based array will exceed the upper bound of the array. For example, an array Arr[] of only one element has a length of 1, but the only valid entry is Arr[0]. The loop presented would attempt to check both Arr[0] and Arr[1], exceeding the upper bound of the array. In memory-managed environments this will generate a runtime error; in C, it's interesting pointer arithmetic resulting in the inclusion of a more-or-less random memory location as an unintended digit of the number being checked. A good compiler may catch the error.
Also, the general purpose of pseudocode is to be illustrative. As such, it would be more informative to use modulus 2 in place of AND 1. AND 1 is a programming optimization which exploits a bitwise AND to perform the same logical operation as modulus 2. AND 1 is used in programming solely because it generally compiles to faster executable code than does modulus 2. However, speed optimization is rarely the point of pseudocode. The use of modulus 2 would relate the pseudocode more directly to the informal discussion in the article, and make the pseudocode easier to follow for a more general audience.
- You are absolutely right, and indeed argue with too much force; both seem clearly good changes to me. I also changed XOR to ≠. The result seems a lot simpler. In the future I encourage you to be bold and make changes yourself, as well as leave a note on the talk page, if you wish. Deco 06:26, 7 Jan 2005 (UTC)
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- Woah now, isn't the pseudo code supposed to go from nDigits - 1 down to 0? This is the first I've read the algorithm, and the words don't match the code. If I was sure which was correct I'd fix it. --Nofxjunkee 17:21, 20 September 2006 (UTC)
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- You are correct. According to Vital (Now TSYS) the wording is correct and the pseudo-code is wrong. I have recommended that it be changed. Have to be careful though in how you word the for loop cause not all languages implement for the same. While loops may be safer and more language independant. --dmprantz 17 October 2006 (UTC)
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[edit] Created/Adopted When?
The article states that the formula was adopted by credit card companies "shortly after its creation in the late 1960s". But Luhn died in 1964, so wasn't creating much in the late 1960s. I suspect it's just a dangling modifier, and it should probably say that it was adopted in the late 1960s, shortly (or perhaps "not long", or "a few years") after its creation. However, without knowing the specifics of when it was created and when it was adopted, I'm not sure how to re-write. Darguz Parsilvan 6 July 2005 12:12 (UTC)
[edit] Strengths/Weaknesses
The Luhn formula will catch any single-digit error. It will also catch almost any transposition of adjacent digits (e.g., 18 <-> 81). There is one adjacent-digit transposition that Luhn will not catch: 09 <-> 90. It would be a prudent step for applications using Luhn to refrain from issuing numbers with 09 and/or 90 in them, so that this particular transposition cannot result in another valid number. The Verhoeff check digit formula (not currently described in Wikipedia) might be a better choice, since it catches all adjacent-digit transposition errors. Richwales 21:17, 14 April 2006 (UTC)
- The Verhoeff algorithm is described now. Richwales 00:06, 20 April 2006 (UTC)
[edit] No longer valid?
Is this algorithm no longer valid? There are now VISA cards in circulation that fail this test but are legitimate cards.
[edit] Computing the check digit
The algorithm stated checks if a number is valid according to the Luhn algorithm, but does anyone have an algorithm that computes a Luhn check digit, i. e. modifies an existing number by appending a digit so that the result is Luhn valid? I mean, something smarter than trying the ten possible digits until the matching one is found. - wr 10-oct-2006
[edit] Comments on Changes
This article was just plain wrong in the way it was worded. It talked about "calculating" a checksum digit, but explained how to VERIFY a checksum digit. Two slightly different tasks. I changed the text to match the algorithm since that is the most commonly used purpose. I tried to remove everything that didn't make sense. If some one really wants to add back in an area for calculating the checksum digit, by all means do so, but please be precise and clear.
Responses to various Talk Topics:
Computing the check digit is a simple excercise of mathematics, and a well written Luhn Algorithm will be able to do it with little stress. Implementation is left as an excersize.
The Mod 10 (Luhn) algorithm is very much valid. If you know of any Visa numbers that do not pass it, please elaborate, but my business depends on it, and there is no notice of which I am aware which depricates this algorithm.
The current state of the pseudocode is still wrong IMHO, and at least one other person agrees. The algorithm, as it is stated, goes from the end to the beginning, so make the pseudo-code loop go from the end to the beginning. Don't ever bother checking for this parity value that only stands to confuse the reader....if the pseudo-code goes from right to left with if statements, restate the algorithm to do the same. I have seen far too many implementations that take this crazy turn:( Also, ADD SOME CURLY BRACES!!!! Allow me to paraphrase Larry Wall: "Even if you aren't in doubt, consider the mental welfare of the person who has to read the code after you, and who will probably put braces in the wrong place."
Before any one tells me to be bold, please don't. Mod 10 is part of my business. As much as I would like to help the world understand the algorithm, I cannot share my knowledge without giving away my business' IP. I am glad to remove inaccuracies and to point others in the right direction, but creation of any specific code (pseudo or real) can get me into trouble. Sorry. :( --dmprantz 17 October 2006 (UTC)
[edit] Fixed Algorithm Section
I rewrote the algorithm and included a complementary one. I use a boolean flag instead of a mod 2 or and 1 to make the code simpler and more similar to the text. Feedback?
