ISBN13 check digit: Difference between revisions

A couple of additional checks were made to verify a correct length,   and also that the ISBN-13 code is all numerics   (with optional minus signs).

<lang rexx>/*REXX pgm validates the check digit of an ISBN─13 code (it may have embedded minuses).*/

parse arg $ /*obtain optional arguments from the CL*/

if $='' | if $="," then $= '978-1734314502 978-1734314509 978-1788399081 978-1788399083'

@ISBN= "ISBN─13 code isn't" /*a literal used when displaying msgs. */

/* [↓] remove all minuses from X code.*/

do j=1 for words($); y= word($,j) /*obtain an ISBN─13 code from $ list.*/

x= space( translate(y, , '-'), 0) /*remove all minus signs from the code.*/

L= length(x) /*obtain the length of the ISBN-13 code*/

if L \== 13 then do; say @ISBN '13 characters: ' x; exit 13; end

if verify(x, 9876543210)\==0 then do; say @ISBN 'numeric: ' x; exit 10; end

sum= 0

do k=1 for L; #= substr(x, k, 1) /*get a decimal digit from the X code. */

if \(k//2) then #= # * 3 /*multiply every other digit by three. */

sum= sum + # /*add the digit (or product) to the SUM*/

end /*k*/

if right(sum, 1)==0 then say ' ISBN-13 code ' x " is valid."

else say ' ISBN-13 code ' x " isn't valid."

end /*j*/ /*stick a fork in it, we're all done. */</lang>

{{out|output|text=&nbsp; when using the four default inputs:}}

<pre>

ISBN-13 code 9781734314502 is valid.

ISBN-13 code 9781734314509 isn't valid.

ISBN-13 code 9781788399081 is valid.

ISBN-13 code 9781788399083 isn't valid.

</pre>