Notes on Project 4

A few questions about the project have arisen. Here are some comments that I hope will help clarify things. There will be an opportunity in class on Monday after break to ask other questions.

The method for computing s is available on the course Web page. Click here and paste it into your class definition. Note that the method assumes you have instance variables named r, s and m (as described in the project description). (NOTE: There is also now a link to the method from the project description.)
For the gcd method, just copy the one in the Rational class into your class.
In the encode method you need to convert each two character block into an integer. There are several different ways you can do this. One way is to immediately convert each character to an integer and combine the two appropriately to get the correct integer. Another is to get a two character string then convert that string to an integer (you still must do it one character at a time).
Your encode method should work for strings with letters and digits and blank spaces but no other characters (no punctuation marks for example). The getNumericValue method in the Character class returns a unique number for each digit (the value of the digit itself - so 8 for the character '8') and for each letter (10 for 'a', 11 for 'b', and so on). It does not return a unique value for other characters so we will not encode them. However, we do need to encode a blank - for that use the number 36 ('z' is 35). Because the blank is different, you need to check to see if a character is blank before converting to a number - if it is use 36 otherwise use the number returned by getNumericValue.
If the message to be encoded has an odd number of characters then add a blank character to the end to make a 2 character block - so for example the message "cat" would be two blocks - the number value of the block ca would be 1210 (12 for c and 10 for a) and the number value of the block with t followed by a blank would be 2936 (29 for t and 36 for the blank added to the end).
Testing your program: You need to use fairly large prime numbers to see if the program works since your program encodes by dividing by n then taking the remainder. If n isn't large enough lots of blocks will have exactly the same code making it impossible to uniquely decode them. The product of your primes needs to be larger than 3636. One combination to try is p = 113, q = 199, and r = 5.
Converting a number back to a character: There is no single method that converts numbers returned by getNumericValue back to characters. You need to use an if and take care of 3 cases - if the number is 36 the character is the blank space; if the number is between 0 and 9 (inclusive) the forDigit method in the character class will convert it to a character; if the number is between 10 and 35 (inclusive), you need to use cast operators - say the number is in the variable num, then the corresponding character is
```
    (char) ( num - 10 + (int)'a')
```
Subtracting 10 shifts the number to the 0 - 25 range, then adding (int)'a' (which is the ASCII or UNICODE value of 'a') gives the Unicode value of the character. This will cause all letters to be lowercase. If you prefer uppercase use 'A' instead of 'a'.