ISBN13 check digit: Difference between revisions

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=={{header|Red}}==

<lang Red>check_valid_isbn13: function [str] [

is_digit: charset [#"0" - #"9"]

remove-each i str [not pick is_digit i] ; remove non-digits

either 13 = length? str [ ; reject strings of incorrect length

sum: 0

repeat i 13 [

mul: either even? i [3] [1] ; multiplier for odd/even digits

sum: sum + (mul * to integer! to string! pick str i)

]

zero? (sum % 10) ; check if remainder mod 10 is zero

] [

false

]

]

; check given examples

foreach [str] ["978-1734314502" "978-1734314509" "978-1788399081" "978-1788399083"] [

prin str

prin " - "

print check_valid_isbn13 str

]

</lang>

{{out}}

<pre>

978-1734314502 - true

978-1734314509 - false

978-1788399081 - true

978-1788399083 - false