ISBN Book Number Codes - Validation

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/***********************************************
Libby is the librarian at OHS.  She must type ISBN book numbers
into her database, and she occasionally makes mistakes.  She asked
programmer Pete if he could write a program to check the ISBN
numbers and reject obvious mistakes, like too few digits or
a mismatched check digit.  Pete read the following web-page
http://isbn-information.com/10-digit-isbn.html
and then wrote the following test program.  It expects the user to type
10 digits (the last might be X), without any dashes or spaces.
This is only a first attempt - it won't really solve Libby's problem,
as this needs to be incorporated into Libby's database program.
************************************************/

public class ISBN
{
  public ISBN()
  {
    String code = input("Type an ISBN numbers - no dashes or spaces");
    code = code.toUpperCase();
    System.out.println(code);
    if(code.length()!=10)                                 // check length = 10
    { System.out.println("Not valid - need exactly 10 characters");
      System.exit(0);
    }
    int sum = 0;
    for(int p = 0; p < 10; p = p+1)                // count through characters
    {
      char c = code.charAt(p);
      if( c < '0' || (c > '9' && c != 'X') )       // check if not digit or X
      { System.out.println("Invalid character : " + c); 
        System.exit(0);
      }
      int val;                                     // value of the character
      if(c == 'X') { val = 10; }
      else { val = (int)c - 48;}                   // convert ASCII code to value
      sum = sum + val * (10-p);
    }
    if(sum % 11 != 0)                         // check-sum must be divisible by 11
    {
      System.out.println("Check-sum is not divisible by 11, so incorrect");
      System.exit(0);
    }
    System.out.println("Valid");
  }
  
  public static void main(String[] args)
  {  new ISBN();  }
  
  public String input(String prompt)
  { return javax.swing.JOptionPane.showInputDialog(null,prompt); }
  
  public void output(String message)
  {  javax.swing.JOptionPane.showMessageDialog(null,message);  }
}

Characters and Check-sums

Java can input and store Strings, which are sequences of keyboard characters -
letters and numbers and punctuation.  Each character in the String is stored in two Bytes as a number. 
That's right, the letters are not actually stored in the memory,
but rather a number is stored that represents the letter (or digit or punctuation mark).

A Little History

Around 1960-1965, there were not very many computers in the world. 
But some companies did have computers and they wanted to exchange data with other companies
or other offices in the same company.  That was relatively simple with NUMBERS that were stored
in binary (base two).  However, text data was stored in a variety of different ways, with no standard. 
IBM created a system called EBCDIC (Extended Binary Coded Decimal Interchange Code),
but this only  worked on the hardware (card punch machines and printers) that IBM built. 
Around the same time, a group of companies formed a committee of engineers to standardize
the numeric codes used to represent text data. 
The result was 7-bit ASCII - the American Standard Code for Information Interchange. 
    For example :     'A' = 65    'B' = 66    '*' = 42    'd' = 100
The 7-bit code could store 128 different numbers, representing 128 different characters. 
That was enough for all English letters, digits and punctuation, plus extra codes for control signals
like #7 to ring the bell on a teletype machine (very important in The Andromeda Strain!)

After the President of the United States mandated in 1968 that every computer used by the US government
must support the ASCII standard, this became  a de facto standard for all computer systems and it is still in use. 
You might wish to read more about ASCII here.

In subsequent years various small changes and additions were made to ASCII, and ASCII became an 8-bit system.
That was very convenient as 1 character occupied 1 Byte, and allowed 256 characters to be defined,
including a few extra characters for German, French, Spanish and a couple other languages,
plus special symbols for mathematics (everyone's favorite subject!)  Around 1990,
the need for more international characters for other languages prompted the expansion to 16-bit Unicode. 
This 2 Byte code supports 65536 different characters, which is enough for virtually every language on the planet.

Characters and Codes

Java uses 16-bit UniCode (actually UTF-16, that's almost the same) to store each character. 
If you need to change ASCII codes (or Unicodes) to characters, or vice versa, use the following commands:

        char C = '';
    int A = (int)C;
    println(A);        ==> 227

    int A = 88;
    char C = (char)A;
    println(C);        ==> X

Check-Sums

A check-sum for a String is calculated by converting the characters to int codes, then doing a calculation
with those codes to produce a result.  Now the check-sum should be transmitted along with the String. 
Then the String can be checked against the check-sum.  If it does not match correctly, we know the String
has been corrupted during transmission.  Then the receiver should ask the sender to "retransmit".

The check-sum for an ISBN code is calculated as follows:

ISBN Calculation

The program above checks whether an ISBN code is valid by calculating the check-sum. 
The for loop goes through each character, converts it to the corresponding code, multiplies by 10, then 9, ... 
and adds up all the results.  After the loop finishes, if checks divisibility by 11:

       if(sum % 11 != 0)

The % sign calculates : sum mod 11 .  That is the remainder when sum is divided by 11.  For example:

       79 % 11 = 79 mod 11 ==>  79/11 = 7 remainder 2 ==>  result is 2

If the result of the mod operation is 0 (zero), then the number is evenly divisible by 11.

Practice Questions

Programming Practice