ISBN13 check digit: Difference between revisions

=={{header|Modula-2}}==

<lang modula2>MODULE ISBN;

FROM InOut IMPORT WriteString, WriteLn;

FROM Strings IMPORT Length;

PROCEDURE validISBN(s: ARRAY OF CHAR): BOOLEAN;

VAR total, i, length: CARDINAL;

BEGIN

total := 0;

length := Length(s);

IF (length # 14) OR (s[3] # '-') THEN

RETURN FALSE;

END;

FOR i := 0 TO length-1 DO

IF i # 3 THEN

IF (s[i] < '0') OR (s[i] > '9') THEN

RETURN FALSE;

ELSIF (i < 3) AND (i MOD 2 = 1) OR (i > 3) AND (i MOD 2 = 0) THEN

total := total + 3 * (ORD(s[i]) - ORD('0'));

ELSE

total := total + ORD(s[i]) - ORD('0');

END;

END;

END;

RETURN total MOD 10 = 0;

END validISBN;

PROCEDURE check(s: ARRAY OF CHAR);

BEGIN

WriteString(s);

WriteString(': ');

IF validISBN(s) THEN

WriteString('good');

ELSE

WriteString('bad');

END;

WriteLn;

END check;

BEGIN

check('978-1734314502');

check('978-1734314509');

check('978-1788399081');

check('978-1788399083');

END ISBN.</lang>

{{out}}

<pre>978-1734314502: good

978-1734314509: bad

978-1788399081: good

978-1788399083: bad</pre>