Using the vi editor, write a C program on the Sun cluster that obeys the following specification.

An International Standard Book Number (ISBN) is a code of 10 characters. Each book published has its own unique ISBN. The last (tenth) character of an ISBN is a check character that is used to validate an ISBN. The check character is computed from the rest of the ISBN as follows: First, compute the sum of the first digit plus two times the second digit plus three times the third digit, ..., plus nine times the ninth digit. The last (tenth) character is the remainder when this sum is divided by 11. If the remainder is 10, the last character is X. For example, the ISBN 0806509597 is valid because:

and 249 mod 11 is 7, which is the last digit of the ISBN.

Write a function with one parameter that is an array of characters. The function returns true (1) if the array represents a valid ISBN and false (0) otherwise. Write a main program that prompts the user for an ISBN, passes the ISBN to the function, and reports whether or not the ISBN is valid.

More detailed specifications include:

• The program may assume that the user always enters a string that is exactly 10 characters long, that the first 9 characters of the string are digits from '0' to '9', and that the tenth character is either a digit or 'X'. That is, the input is in an acceptable ISBN format.
• The file:

  ~taw2/pub/isbn.c

  contains some code that will help you get started, including the correct prototype for function check (the function you need to write), and the definition of function toint, which converts a character such as '9' to the integer 9. For example, if isbn is an array of characters and the value of isbn at index i is the character '3', then the expression:

    toint(isbn[i])

  evaluates to the integer 3.
• To hand in:
  • Either turn in a print out of your source code, or email me (wahls@psu.edu) the file containing your source code. If you email me the program, I will email you your score and any comments on your program sometime after the due date. I will also save a copy of your program to prevent later disputes.
  • Your code will be graded on correctness, programming style, readability, the clarity of messages printed for the user and appropriate use of comments.

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