Check if element in stack is a number or an operator?

Question

I am attempting to create a RPN calculator using a Scanner. I am gathering numbers and operators from the user and concat the inputs onto a string called results. Once they quit (by saying "Q" or "q"), I would like to pass results to the method evaluateResults, which would perform the operations they inputed. For example, if the string passed to evaluateResults is '45+', I would like to add 4 and 5. However, if the the input is '456+x' I would like to perform '(4 + 5) x 6' , meaning I am reading the string until a character is an operator, then I perform that operator with the first two numeric characters of the string... I am attempting to do this with the following program:

import java.util.*;

/**
  This calculator uses the reverse Polish notation.
*/
public class P15_7 {

  public static void evaluateResult(String inp){
    int output = 0;
    Stack<Integer> results = new Stack<>();
    for (int i = 0; i < inp.length(); i++){
      char c = inp.charAt(i);   
      if (Character.isDigit(c)){
        results.push(Character.getNumericValue(c));
      }else{
        Iterator<Integer> itr = results.iterator();
        while(itr.hasNext()){
          if (c == '+'){
            int f = itr.next();       // Getting the First value of the Stack
            itr.remove();             // Removing it (first val)
            int s = itr.next();       // Getting the Second value of the Stack
            itr.remove();             // Removing it (second val)
            output = output + (f + s);// calculate
            itr = results.iterator(); // Starting the iterator back at Index 0
            itr.add(output);          // Adding the calculated value at the start : Index 0
          }else if (c == '-'){
            int f = itr.next();
            itr.remove();
            int s = itr.next();
            itr.remove();
            output = output + (f - s);
            itr = results.iterator(); 
            itr.add(output); 
          }else if (c == '*' || c == 'x'){
            int f = itr.next();
            itr.remove();
            int s = itr.next();
            itr.remove();
            output = output + (f * s);
            itr = results.iterator();
            itr.add(output);
          }else if (c == '/'){
            int f = itr.next();
            itr.remove();
            int s = itr.next();
            itr.remove();
            output = output + (f / s);
            itr = results.iterator();
            itr.add(output);
          }
        }
      }
    }
    System.out.println("You answer is: " + output);
  }
  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    String results = "";
    System.out.println("Enter one number or operator per line, Q to quit. ");
    boolean done = false;

    while(!done){
      String input = in.nextLine();
      if(input.equals("Q") || input.equals("q")){
        evaluateResult(results);
        done = true;
      }else{
        results = results.concat(input);
        System.out.println(results);
      }
    }
  }
}

The problem I am running into is that I can not get the int output to get added at the start of the stack with the use of the iterator. How can I modify this code to perform the method how I have described it above? Any modifications will be helpful and please let me know if I am unclear about anything and I will clarify.

Your conception of what `456+x` means is incorrect. In RPN, this evaluates first to `4 11 x`, then to `44`. — Tim Biegeleisen, Oct 31 '17 at 01:42
The stack iterator does not have an `add` method. Your code will not compile in its current form. — jrook, Oct 31 '17 at 01:46
You cannot use the iterator to add an element to the underlying collection. Even the use of `remove()` is sometimes frowned upon [See here](https://stackoverflow.com/questions/11196561/why-there-is-no-add-method-in-iterator-interface) — jrook, Oct 31 '17 at 01:50

score 0 · Answer 1 · answered Oct 31 '17 at 02:07

Here's the basic process for interpreting RPN, described in pseudocode:

while (more input available) 
{
    get next input synbol S
    if (S is a number)
        push S on the Stack
    else // S must be an operator
    {
        pop the top item from Stack into op2
        pop the (new) top item from Stack into op1
        compute the result R by performing the operation named by S
           on op1 and op2 
        push R on the stack
    }
}
pop the final result from the Stack

Check if element in stack is a number or an operator?

1 Answers1