ISBN13 check digit: Difference between revisions

</pre>

=={{header|Forth}}==

<lang>: is-digit [char] 0 - 10 u< ;

: isbn? ( a n -- f)

1- 2dup chars + c@ [char] 0 - >r 2>r 0 1 2r> bounds ?do

dup i c@ dup is-digit

if [char] 0 - * rot + swap 3 * 8 mod else drop drop then

loop drop r> + 10 mod 0=

;</lang>

{{out}}

In Forth, a "true" value is indicated by "-1".

<pre>

s" 978-1734314502" isbn? . -1 ok

s" 978-1734314509" isbn? . 0 ok

s" 978-1788399081" isbn? . -1 ok

s" 978-1788399083" isbn? . 0 ok