
/**
 * This program evaluates postfix expressions entered by the
 * user.  The expressions can use non-negative real numbers and
 * the operators +, -, *, /, and ^ (where ^ represents exponentiation).
 * Numbers must begin with a digit.  That is, you have to say
 * 0.7 rather than just .7.  The value of an expression might
 * be given as infinity or NaN.
 * 
 * A stack is used to evaluate the expression.  The stack is represented
 * by an object of type StackOfDouble.  This program depends on the non-standard
 * classes StackOfDouble and TextIO.
 * 
 * For demonstration purposes, the stack operations are reported as
 * they are performed.
 */
public class PostfixEval {

    public static void main(String[] args) {

        System.out.println("This program can evaluate postfix expressions such as\n");
        System.out.println("        2 2 +");
        System.out.println("or");
        System.out.println("        7.3 89.2 + 9 1.83 * 2 + /\n");
        System.out.println("The operators +, -, *, /, and ^ can be used.");

        while (true) {
                // Get and process one line of input from the user.
            System.out.println("\n\n\nEnter a postfix expression or press return to end:\n");
            System.out.print("?  ");
            TextIO.skipBlanks();
            if (TextIO.peek() == '\n') {
                    // If the line is empty (except for spaces), we are done.
                break;
            }
            readAndEvaluate();  // Process the input.
            TextIO.getln();  // Discard the end-of-line.
        }

        System.out.println("\n\nExiting program.");

    } // end main();



    /**
     *  Read one line of input and process it as a postfix expression.
     *  If the input is not a legal postfix expression, then an error
     *  message is displayed.  Otherwise, the value of the expression
     *  is displayed.  It is assumed that the first character on
     *  the input line is a non-blank.  (This is checked in the
     *  main() routine.)
     */
    private static void readAndEvaluate() {

        StackOfDouble stack;  // For evaluating the expression.

        stack = new StackOfDouble();  // Make a new, empty stack.

        System.out.println();

        while (TextIO.peek() != '\n') {

            if ( Character.isDigit(TextIO.peek()) ) {
                    // The next item in input is a number.  Read it and
                    // save it on the stack.
                double num = TextIO.getDouble();
                stack.push(num);
                System.out.println("   Pushed constant " + num);
            }
            else {
                    // Since the next item is not a number, the only thing
                    // it can legally be is an operator.  Get the operator
                    // and perform the operation.
                char op;  // The operator, which must be +, -, *, /, or ^.
                double x,y;     // The operands, from the stack, for the operation.
                double answer;  // The result, to be pushed onto the stack.
                op = TextIO.getChar();
                if (op != '+' && op != '-' && op != '*' && op != '/' && op != '^') {
                        // The character is not one of the acceptable operations.
                    System.out.println("\nIllegal operator found in input: " + op);
                    return;
                }
                if (stack.isEmpty()) {
                    System.out.println("   Stack is empty while trying to evaluate " + op);
                    System.out.println("\nNot enough numbers in expression!");
                    return;
                }
                y = stack.pop();
                if (stack.isEmpty()) {
                    System.out.println("   Stack is empty while trying to evaluate " + op);
                    System.out.println("\nNot enough numbers in expression!");
                    return;
                }
                x = stack.pop();
                switch (op) {
                case '+':  
                    answer = x + y; 
                    break;
                case '-':  
                    answer = x - y;  
                    break;
                case '*':  
                    answer = x * y;  
                    break;
                case '/':  
                    answer = x / y;  
                    break;
                default:   
                    answer = Math.pow(x,y);  // (op must be '^'.)
                }
                stack.push(answer);
                System.out.println("   Evaluated " + op + " and pushed " + answer);
            }

            TextIO.skipBlanks();

        }  // end while

        // If we get to this point, the input has been read successfully.
        // If the expression was legal, then the value of the expression is
        // on the stack, and it is the only thing on the stack.

        if (stack.isEmpty()) {  // Impossible if the input is really non-empty.
            System.out.println("No expression provided.");
            return;
        }

        double value = stack.pop();  // Value of the expression.
        System.out.println("   Popped " + value + " at end of expression.");

        if (stack.isEmpty() == false) {
            System.out.println("   Stack is not empty.");
            System.out.println("\nNot enough operators for all the numbers!");
            return;
        }

        System.out.println("\nValue = " + value);


    } // end readAndEvaluate()


} // end class PostfixEval

