CS 241 Data Structures- Lab 3, Due 2/5/13 and 2/7/13 (no credit for late programs)

Expression Evaluation

Your task is to write a program that will: 1) read in a series of infix expressions from a file (one per line), 2) evaluate each expression and display it and its value in a TextArea. For this lab infix expressions contain only ints, the 4 binary arithmetic operators (/*-+), spaces, and parentheses. E.g.:

• 1 evaluates to 1
• 1 + 2 evaluates to 3
• 1 + 2 /3 evaluates to 1
• (1 + (2-3)/4)*5 evaluates to 5
• 1 + 1 + 4 / 2 evaluates to 4
• ( 1 + ( 1 + 4 ) / 2 ) * ( 5 + 2 ) evaluates to 21

Phase 1: Implement a postfix expression evaluator (Demo 2/5/13)

Details:

• No partners for this one! Everyone write their own code! Okay to collaborate on design.
• Input and display each line from file of postfix expressions; pass the path to the MyReader so you don't have to select the file each time you run
• Follow the algorithm presented in class (and reproduced below) to implement a postfix expression evaluator. To become familiar with the Singleton pattern, write a PostFixEvaluator class that has a static int evaluate(String inputLine) method. Thus, the code in your Frame might simply be:

  while (mr.hasMoreData()) {
      String nextLine = mr.giveMeTheNextLine();
      emit(nextLine + "=" + PostFixEvaluator.evaluate(nextLine) + "\n");
  }
              

• Test before you leap! Use a StringTokenizer to separate the input lines into tokens (StringTokenizer st = new StringTokenizer(s, "+-*/ ", true); will work well) -- or, if you prefer learn to use a Scanner to do it. Test your code to be sure it is generating the correct stream of tokens.
• What will you store in your Stack? You might choose to store Strings, or Integers, or Operands (you would have to write the Operand class), or something else; think through the design, and make a decision on that basis (what will be on the Stack in Phase 2? Or will you have two different kinds of Stack?).

Here is the algorithm presented in class:

  while (more input) {
     nextToken = get next token
     if (nextToken is an operand)
         push
     else {  // must be an operator
         x = pop();
         y = pop();
         push(y nextToken x);   // need to implement the operation as a method
     } // else
  }
  output(pop());   // the answer will be on top of the stack
    

Phase 2: Implement an infix expression evaluator without parentheses and then, with. Demo on or before 2/7/13

Details:

• Convert each infix expression to postfix with a String InfixToPostFix.convert(String) method following the algorithm presented in lecture, then...
• Let the Phase 1 code do the rest!

Getting credit