检验ISBN是否合法

Tags: /超级猛料/Alogrith.算法和数据结构/校验算法/ Date Created:

标题：检验ISBN是否合法

function IsISBN(ISBN: String): Boolean;
var
  Number, CheckDigit: String;
  CheckValue, CheckSum, Err: Integer;
  i, Cnt: Word;
begin
  { 取得检验数字 }
  CheckDigit := Copy(ISBN, Length(ISBN), 1);
  { Get rest of ISBN, minus check digit and its hyphen }
  Number := Copy(ISBN, 1, Length(ISBN) - 2);
  { ISBN长度必须为11，并且组成数字必须介于9和9或X }
  if (Length(Number) = 11) and (Pos(CheckDigit, '0123456789X') > 0) then
  begin
    { Get numeric value for check digit }
    if (CheckDigit = 'X') then
      CheckSum := 10
    else
      Val(CheckDigit, CheckSum, Err);
    { Iterate through ISBN remainder, applying decode algorithm }
    Cnt := 1;
    for i := 1 to 12 do
    begin
      { 如果当前字符介于0~9，则不包含连字符 }
      if (Pos(Number[i], '0123456789') > 0) then
      begin
        Val(Number[i], CheckValue, Err);
        { Algorithm for each character in ISBN remainder, Cnt is the nth
          character so processed }
        CheckSum := CheckSum + CheckValue * (11 - Cnt);
        Inc(Cnt);
      end;
    end;
    { 校验最后的值能否被11整除 }
    if (CheckSum MOD 11 = 0) then
      IsISBN := True
    else
      IsISBN := False;
  end
  else
    IsISBN := False;
end;

----------------------------------

PRODUCT : Delphi NUMBER : 2988

VERSION : All

OS : Windows/Win32

DATE : June 1, 1996

TITLE : How to Validate ISBNs

ISBNs (or International Standard Book Numbers) are mystical code numbers

that uniquely identify books. The purpose of this article is to remove the

mystery surrounding the structure of the ISBN, allowing applications to

perform data validation on entered candidate ISBNs.

ISBNs are composed of thirteen characters, limited to the number

characters "0" through "9", the hyphen, and the letter "X". This thirteen-

character code is divided into four parts, each separated by hyphens:

group identifier, publisher identifier, book identification for the

publisher, and the check digit. The first part (group identifier) is used

to identify countries, geographical regions, languages, etc. The second

part (publisher identifier) uniquely identifies the publisher. The third

part (book identifier) uniquely identifies a given book within a

publisher's collection. The fourth and final part (check digit) is used

with the other digits in the code in an algorithm to derive a verifiable

ISBN. The number of digits in the first three parts of an ISBN may contain

a variable number of digits, but the check digit will always consist of a

single character (between "0" and "9", or "X" for a value of 10) and the

ISBN as a whole will always consists of thirteen characters (ten numbers

plus the three hyphens dividing the four parts of the ISBN).

The ISBN 3-88053-002-5 breaks down into the parts:

Group: 3

Publisher: 88053

Book: 002

Check Digit: 5

An ISBN can be verified to be a valid code using a simple mathematical

algorithm. This algorithm takes each of the nine single digits from the

first three parts if the ISBN (sans the non-numeric hyphens), multiplies

each single digit by a number that is less than eleven the number of

positions from the left each digit that is in the ISBN, adds together the

result of each multiplication plus the check digit, and then divides that

number by eleven. If that division by eleven results in no remainder

(i.e., the number is modulo 11), the candidate ISBN is a valid ISBN. For

example, using the previous sample ISBN 3-88053-002-5:

ISBN: 3 8 8 0 5 3 0 0 2 5

Digit Multiplier: 10 9 8 7 6 5 4 3 2 1

Product: 30+72+64+00+30+15+00+00+04+05 = 220

Since 220 is evenly divisible by eleven, this candidate IDBN is a valid

ISBN code.

This verification algorithm is easily translated into Pascal/Delphi code.

String manipulation functions and procedures are used to extract the

check digit and the remainder of the ISBN from the String type value

passed to a validation function. The check digit is converted to Integer

type, which forms the start value of the aggregate variable onto which the

multiplication of each digit in the remainder of the ISBN (the single

digits that comprise the first three parts of the ISBN) will be added. A

For loop is used to sequentially process each digit in the remainder,

ignoring the hyphens, multiplying each digit times its position in the

ISBN remainder relative to the other digits in the remainder. The final

value of this aggregate variable is then checked to see whether it is

evenly divisible by eleven (indicating a valid ISBN) or not (indicating an

invalid candidate ISBN).

Here is an example of this methodology applied in a Delphi function:

function IsISBN(ISBN: String): Boolean;
var
 Number, CheckDigit: String;
 CheckValue, CheckSum, Err: Integer;
 i, Cnt: Word;
begin
 {Get check digit}
 CheckDigit := Copy(ISBN, Length(ISBN), 1);
 {Get rest of ISBN, minus check digit and its hyphen}
 Number := Copy(ISBN, 1, Length(ISBN) - 2);
 {Length of ISBN remainder must be 11 and check digit between 9 and 9 or
  X}
 if (Length(Number) = 11) and (Pos(CheckDigit, '0123456789X') > 0) then
   begin
   {Get numeric value for check digit}
   if (CheckDigit = 'X') then
     CheckSum := 10
   else
     Val(CheckDigit, CheckSum, 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(Number[i], '0123456789') > 0) then begin
       Val(Number[i], CheckValue, Err);
       {Algorithm for each character in ISBN remainder, Cnt is the nth
       character so processed}
       CheckSum := CheckSum + CheckValue * (11 - Cnt);
       Inc(Cnt);
     end;
   end;
   {Verify final value is evenly divisible by 11}
   if (CheckSum MOD 11 = 0) then
     IsISBN := True
   else
     IsISBN := False;
 end
 else
   IsISBN := False;
end;

This is a simplified example, kept simple to best demonstrate the

algorithm to decode ISBNs. There are a number of additional features that

would be desirable to add for use in a real-world application. For

instance, this example function requires the candidate ISBN be passed as a

Pascal String type value, with the hyphens dividing the four parts of the

ISBN. Added functionality might accommodate evaluating candidate ISBNs

entered without the hyphens. Another feature that might be added is

checking that ensures three hyphens are properly included, as opposed to

just thirteen number characters.

DISCLAIMER: You have the right to use this technical information subject to the terms of

the No-Nonsense License Statement that you received with the Borland product to which

this information pertains.