check digit ?


I need to check wether an Isin code specified by a user is valid or not..
I found this article exaplaining an algoryhtm to compute a "check digit" :
But what do I have to do after calculating this check digit ?

Maybe if you have some experience in security, you understand what this is about ? (I have none)

Thank you.
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Chris BottomleySoftware Quality Lead EngineerCommented:
Check digits are often used to support validation of a data sequence which is how it appears to b eused in this instance.

i.e. if you enter the ISIN but make a typo the check digit will likely indicate the fact and you can prompt for a re-entry.  It means when entering an ISIN for validation you should calculate the correct check digit and ensure the data as entered validates against it.

matthew016Author Commented:
I am not sure I understand.
If user makes a typo, I will compute a certain digit. How can I detect from this digit that he made a typo ?
Chris BottomleySoftware Quality Lead EngineerCommented:
Error checking is a speciality in it's own right ... i'm not saying i'm an expert but as far as I can see the referenced syntax will indicate an error exists but it is not error correcting and will not indicate which character, (or characters are wrong).

i.e. when requesting or receiving the data you should calculate the check digit for each datum and ensure it is correct before proceeding and where any individual datum is wrong reject it and move on.

matthew016Author Commented:
Well I need to know how I detect an error exists.
I do not want to correct the error.

But now I am maybe confusing
* error detection in some kind of I/O operations, etc. and
* validation to check if the data entered is a valid sequence
matthew016Author Commented:
nevermind ...

I see that I can check the digit computed with the last number of the sequence number.
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