1. Make sure that you understand postfix evaluation.
   
2. Łukasiewicz was a Polish logician, so his notation for parentheses-free expressions is often called Reverse Polish Notation. To get your brain in gear, convert the following expressions to RPN! What are the values of the expressions?
   
   1. 1 * 2 + 3
   2. 1 + 2 * 3
   3. 1 + 2 - 3 ^ 4
   4. 1 ^ 2 - 3 * 4
   5. 1 + 2 * 3 - 4 ^ 5 + 6
   6. ( 1 + 2 ) * 3 + ( 4 ^ ( 5 - 6 ) )
   7. 1 + 2 + 3 / 4 + 5 + 6 * ( 7 + 8 )
   8. 9 - 1 - 2 - 3 * 2 - 1
       
3. For the infix expression a + b ^ c * d ^ e ^ f - g - h / ( i + j ), do the following:
   
1. Show how to generate the corresponding postfix expression.
2. Show how to evaluate the resulting postfix expression.

 
4. Explain, in general terms, how unary operators can be incorporated into the expression evaluators. Assume that the unary operators precede their operands and have high precedence.

Lab exercises

Read through all of the exercises before starting! Oh dear, this is a lot of work. I guess we can't play one-person-types-while-the-other-looks-on this week.... I would strongly suggest that one person get exercise 1 to work while the other one starts exercise 2. Then you exchange code, and voilà, it works! Now you can get back together to do the third exercise. The bored are, of course, done in half an hour, so they go on to do other interesting things.

1. Implement a class Stack.java as discussed in the lecture, using a linked list of objects that you implement yourself! Don't use the Stack or LinkedList that is available by default in Java. Try and type it in yourself, not just copy the handout. How will you test this? Your class should include both an exception on stack underflow as well as stack overflow. Will you really need both exceptions? Why or why not? Override the toString() method to provide a useful way of printing a stack. Now make it generic, so it can take values of any type. Coordinate your interface with your partner.
   
   
2. Implement a class Postfix.java that has a method
   public int evaluate (String pfx){...}
   that takes a String representing a postfix expression and determines the value represented by that expression. You will need to access the individual characters of the string and store them in a stack. This is necessary for the evaluation, luckily your partner is currently in the process of making one. Build a test class and check the postfix expressions you did in the finger exercises. If there is a difference between the value computed and the value expected, either you were wrong, or the implementation is wrong or both.
   
   Do not go on before you are sure that this is working right!
   
   
3. Now add another method to the Postfix.java class
   public String infixToPostfix (String ifx){...}
   that converts an infix expression which is presented as a String to a String representing a postfix expression! Throw an exception if your input is not well-formed.


4. Now add another method that reads a string from the console, evaluates the result and prints the result to the console.
   
   
5. (For the bored) Once this works for digits, go on and parse multidigit Integers out of the String. Can you do it for double values as well? If you are still bored, parse mixed expressions (doubles and ints in the same expression).
   
   
6. (For the really bored) How can you convert prefix to postfix? Find an algorithm and implement it. Can you handle unary operators like - or ! as well?

Work in groups of two, each of you submitting the same report, detailing who did what part of the work. Your reports are due by 23.00 next week.