Lecture Notes on 31 Oct 2014

Algorithm to check the validity of ISBNs.

* Open the file isbn.txt for reading
* Open the file isbnOut.txt for writing
* Read the file isbn.txt line by line
* Strip the line
* Replace all hyphens with nothing
* Check if the length of isbn string is 10
* Check if the first 9 characters are all digits
* Check the tenth character and see if it is a digit or 'x' or 'X'
* Create an empty isbn list
* Go through the isbn string character by character, convert character
  to an int and append to the isbn list
* Create a list s1 containing the partial sum of the isbn list
* Create a list s2 containing the partial sum of the s1 list
* Examine the last number in the list s2. If it is divisible by 11
  then the isbn is valid. 
* Write out the result
* Close the file isbn.txt
* Close the file isbnOut.txt