Mega Code Archive
Check if an ISBN is valid
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Title: Check if an ISBN is valid
Question: Find out if a ISBN is valid...
Answer:
function ValidISBN(aISBN: string): boolean;
var hNumber,
hCheckDigit: string;
hCheckValue,
hCheckSum,
Err: integer;
i,Cnt: Word;
begin
Result := false;
hCheckDigit := Copy(aISBN, Length(aISBN), 1);
{Get rest of ISBN, minus check digit and its hyphen}
hNumber := Copy(aISBN, 1, Length(aISBN) - 2);
{Length of ISBN remainder must be 11 and check digit
between 9 and 9 or X}
if (Length(hNumber)=11)and(Pos(hCheckDigit,
'0123456789X') 0) then
begin
{Get numeric value for check digit}
if (hCheckDigit = 'X') then
hCheckSum := 10
else Val(hCheckDigit, hCheckSum, Err);
{Iterate through ISBN remainder, applying decode
algorithm}
Cnt := 1;
for i := 1 to 12 do
begin
{Act only if current character is between "0" and "9"
to exclude hyphens}
if (Pos(hNumber[i], '0123456789') 0) then
begin
Val(hNumber[i], hCheckValue, Err);
{Algorithm for each character in ISBN remainder, Cnt
is the nth character so processed}
hCheckSum := hCheckSum + hCheckValue * (11 - Cnt);
Inc(Cnt);
end;
end;
{Verify final value is evenly divisible by 11}
if (hCheckSum mod 11 = 0) then Result := true
else Result := false;
end;
end;
This is how it works.
A valid ISBN is: '0-672-31285-9'. Every digit is multiplied with the number of its position and than add up: 0*10 + 6*9 + 7*8 + 2* 7 etc. If the result is divisible trough 11 then the ISBN is valid!
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