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Info_Mac IV CD-ROM (Pacific HiTech Inc.)(August 1994).iso
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1994-03-21
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This FileMaker Pro database demonstrates how to validate the integrity
of an ISBN using calculated fields. I wrote these for use in my
Personal Librarian template, but I figured someone else might find
them useful, or at least interesting. If you find anything wrong
with the way I've implemented these calculations, or know of an
easier way to do it, please let me know.
Ken
kkirksey@world.std.com
------
A Short Discourse on ISBNs
ISBNs are of the form X-YYY-ZZZZZ-C where X is the country or language
code, YYY is the publisher number, ZZZZZ is the book number, and C
is the check digit. Larger publishers will have fewer digits in their
publisher number, and more in their book number. Smaller publishers
are the opposite.
Given an ISBN number, for instance 0-938151-31-2, the algorithm for
calculating validity works like
so:
10 * 0 = 0
9 * 9 = 81
8 * 3 = 24
7 * 8 = 56
6 * 1 = 6
5 * 5 = 25
4 * 1 = 4
3 * 3 = 9
2 * 1 = 2
1 * 2 = 2
--------------
209
209 Mod 11 = 0
If the sum of the products Mod 11 equals zero, then the number is good.
Those of you with a mathematical bent will notice that this is simply a
10D dot product with numbers coprime to 11. Since the 10 is coprime to 11,
and it's not a single digit, the ISBN uses 'X' in it's place. 'X' will
only appear as the last digit, the check digit.