Prefix expression to evaluate multiple expressions simultaneously

问题

private class InputListener implements ActionListener
    {
      public void actionPerformed(ActionEvent e)
      {
         Stack<Integer> operandStack = new Stack<Integer>();
         Stack<Character> operatorStack = new Stack<Character>();

         String input = inputTextField.getText();

         StringTokenizer strToken = new StringTokenizer(input, " ", false);


         while (strToken.hasMoreTokens())
         {
             String i = strToken.nextToken();
             int operand;
             char operator;

             try
             {
                 operand = Integer.parseInt(i);
                 operandStack.push(operand);
             }
             catch (NumberFormatException nfe)
             {
                 operator = i.charAt(0);
                 operatorStack.push(operator);
             }
          }
          int result = sum (operandStack, operatorStack);
          resultTextField.setText(Integer.toString(result));
       }

My prefix expression code will only evaluate one expression at a time (i.e. + 3 1). I want it to evaluate multiple expressions in one user-input expression (i.e. * + 16 4 + 3 1). How can I edit the code provided to make it evaluate multiple expressions? Thank you for your help.

回答1:

To simply make your program do a bit more you can use a loop to keep operating on the operandStack and pushing the result of the previous result to the stack. I left my println statements in so you can see what its doing. Also I modified your method so it can sit inside a standalone main method.

You should look into the Shunting-yard algorithm, its quite fun to implement and it is somewhat like what your doing here. http://en.wikipedia.org/wiki/Shunting-yard_algorithm

public static void main(String[] args) {
    Stack<Integer> operandStack = new Stack<Integer>();
    Stack<Character> operatorStack = new Stack<Character>();

    String input = "12 + 13 - 4";

    StringTokenizer strToken = new StringTokenizer(input, " ", false);

    while (strToken.hasMoreTokens()) {
        String i = strToken.nextToken();
        int operand;
        char operator;

        try {
            operand = Integer.parseInt(i);
            operandStack.push(operand);
        } catch (NumberFormatException nfe) {
            operator = i.charAt(0);
            operatorStack.push(operator);
        }
    }

    // loop until there is only 1 item left in the operandStack, this 1 item left is the result
    while(operandStack.size() > 1) {
        // some debugging println
        System.out.println("Operate\n\tbefore");
        System.out.println("\t"+operandStack);
        System.out.println("\t"+operatorStack);

        // perform the operations on the stack and push the result back onto the operandStack
        operandStack.push(operate(operandStack, operatorStack));

        System.out.println("\tafter");
        System.out.println("\t"+operandStack);
        System.out.println("\t"+operatorStack);
    }

    System.out.println("Result is: " + operandStack.peek());
}

/**
 * Performs math operations and returns the result. Pops 2 items off the operandStack and 1 off the operator stack. 
 * @param operandStack
 * @param operatorStack
 * @return
 */
private static int operate(Stack<Integer> operandStack, Stack<Character> operatorStack) {
    char op = operatorStack.pop();
    Integer a = operandStack.pop();
    Integer b = operandStack.pop();
    switch(op) {
        case '-':
            return b - a;
        case '+':
            return a + b;
        default:
            throw new IllegalStateException("Unknown operator '"+op+"'");
    }
}

I left the operate method (previously called sum) as close to what you had it as possible, however I think that your code could be improved by simply passing 2 integers and a operator to the function. Making the function alter your stacks isnt a great thing and could lead to confusing problems.

Consider making your method signature this instead:

private static int operate(Integer a, Integer b, char operator) {
    switch(operator) {
        case '-':
            return b - a;
        case '+':
            return a + b;
        default:
            throw new IllegalStateException("Unknown operator '"+operator+"'");
    }
}

and then popping from the stack and passing those to the method. Keeping your stack altering code all in one place.

operandStack.push(operate(operandStack.pop(), operandStack.pop(), operatorStack.pop()));

来源：https://stackoverflow.com/questions/29594598/prefix-expression-to-evaluate-multiple-expressions-simultaneously

标签

java

expression