st: summing up digits and comparing to a check digit

[Lia Orfei <lorfei@sgul.ac.uk>]

Dear all,
Here is a highly advanced programming challenge for you (!)

I have a variable that contains 7-digit numbers; the first digit is the 
remainder of the sum of the last six digits divided by 10 (check digit)

example: 1030332 => 0+3+0+3+3+2=11/10=1+1 => check digit=1
             12050457 => 0+5+0+4+5+7=21/10=2+1 => check digit=1
and so on.

Now I want to assess whther the check digit is correct for each entry.

How do I calculate the partial sum of the contents of this variable?

Extra challenge: some of the numbers sum up to 10, 20 etc so the check 
digit is actually 0 and it does not appear in the data for some reason 
(example 50456 is correct becasue 5+0+4+5+6=20 so the remainder is 0).

Any ideas?? I was thinking about using the string functions.

Thanks a lot

Lia

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