Question

Hey Guys I’m having a problem when I run my program. In the PostfixEvaluate() Method is where it takes in a string and solves the postfix problem and returns it. Well when I go to run it, I’m getting a bunch of random numbers(some repeated), I’m going crazy because I don’t know what else to try and I’ve spent more time on this than it should normally take.

Heres the PostfixEvaluate Method:

   public int PostfixEvaluate(String e){
        //String Operator = "";
    int number1;
        int number2;
        int result=0;
        char c;
        //number1 = 0;
        //number2 = 0;

        for(int j = 0; j < e.length(); j++){
            c = e.charAt(j);
            if (c != '+'&& c!= '*' && c!= '-' && c!= '/') {
                //if (c == Integer.parseInt(e)) {   
                s.push(c); 
        }
            else {
                number1 = s.pop();
                number2 = s.pop();
                switch(c) {
                    case '+':
                    result = number1 + number2;
                    break;
                    case '-':
                    result = number1 - number2;
                    break;
                    case '*':
                    result = number1 * number2;
                    break;
                    case '/':
                    result = number1 / number2;
                    break;
                    } s.push(result);
                }
            System.out.println(result);
        } 
            return s.pop();
}

public static void main(String[] args) {
    Stacked st = new Stacked(100);
    String y = new String("(z * j)/(b * 8) ^2");
    String x = new String("2 3 + 9 *");
    TestingClass clas = new TestingClass(st);

    clas.test(y);


    clas.PostfixEvaluate(x);




 }
        }

This is the Stack Class:

 public class Stacked  {

    int top;
    char stack[];
    int maxLen;

    public Stacked(int max) {
        top = -1; 
        maxLen = max; 
        stack = new char[maxLen];

        }

    public void push(int result) {
            top++;
            stack[top] = (char)result;

        }

    public int pop() {
        int x;
        x = stack[top];
        //top = top - 1;
        top--;

        return x;

    }

    public boolean isStackEmpty() {
            if(top == -1) {
                System.out.println("Stack is empty " + "Equation Good");

                return true;
            } 
            else 
                System.out.println("Equation is No good");
                return false;
    }

    public void reset() {

        top = -1;
    }

    public void showStack() {
        System.out.println(" ");
        System.out.println("Stack Contents...");
        for(int j = top; j > -1; j--){
            System.out.println(stack[j]);
        }
        System.out.println(" ");
    }

    public void showStack0toTop() {
        System.out.println(" ");
        System.out.println("Stack Contents...");
        for(int j=0; j>=top; j++){
            System.out.println(stack[j]); 
        }
        System.out.println(" ");
        }
    }

Answer 1

You need to split the string into tokens first:

/* Splits the expression up into several Strings,
 * all of which are either a number or and operator,
 * none of which have spaces in them. */
String [] expressionAsTokens = e.split(" ");

Then you need to make sure you compare Strings, not chars:

//compare strings instead of chars
String token = expressionAsTokens[j];
if (!"+".equals(token) && !"*".equals(token) && !"-".equals(token) && !"/".equals(token)) {
    s.push(Integer.parseInt(token)); 
} else {
    //same code as you had before
}

Also, is there any reason you are storing everything as a char array in your Stacked class? Your pop() method returns and integer, yet everything is stored as a char.

For this application, everything should be stored as an integer:

public class Stacked {
    int stack[]; // array is of type integer
    int top;
    int maxLen;

    // constructor

    public void push() {/*...*/}
    public int pop() {/*...*/} //pop returns an int as before

    //...
}

One final note: Be careful what order you add and subtract the numbers in. I don’t remember if postfix operands are evaluated left first or right first, but make sure you get them in the right order. As you have it now, 2 3 - 4 * would evaluate as 4 * (3 - 2) and I think it should be (2 - 3) * 4. This won’t matter with adding and multiplying, but it will with subtracting and dividing.