> > It is needed for calculating the check digit, in the special case that
> > > the steps 1..3 produce 10 (which is, if the $totalsums mod 10 equal 0).
> > 10 is not allowed as digit. Doing modulo 10, will give 0.
>
> Ahh - thanks for the clarification.
Note you only need to do that if calculating the check digit, (when it
matters that you get 0 not 10) if you are just checking that a given
checksum is right then you don't need that complication as using 10 or 0
is the same in a (..) mod 10 = 0 test.
If you a_re_ calculating the digit, rather than write
(10 - (foo mod 10)) mod 10
it's probably more natural just to use
-(-foo mod 10)
David
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