Postfix stack calculator

Question

I've created a stack calculator for my Java class to solve equations such as

2 + ( 2 * ( 10 – 4 ) / ( ( 4 * 2 / ( 3 + 4) ) + 2 ) – 9 )
2 + { 2 * ( 10 – 4 ) / [ { 4 * 2 / ( 3 + 4) } + 2 ] – 9 }

We are suppose to implement { } [ ] into our code. I did it with just parentheses. It works 100% with just ( ). When I try to add { } [ ], it goes bananas.

This is what I have so far:

package stackscalc;

import java.util.Scanner;
import java.util.Stack;
import java.util.EmptyStackException;


class Arithmetic {
    int length;
    Stack stk;
    String exp, postfix;

    Arithmetic(String s) {
        stk = new Stack();
        exp = s;
        postfix = "";
        length = exp.length();

    }
    boolean isBalance() {
        boolean fail = false;
        int index = 0;

        try {
            while (index < length) {
                char ch = exp.charAt(index);

                switch(ch) {
                case ')':
                    stk.pop();
                    break;

                case '(':
                    stk.push(new Character(ch));
                    break;

                default:
                    break;
                }
                index++;
            }
        } catch (EmptyStackException e) {
            fail = true;
        }
        return stk.empty() && !fail;
    }
    void postfixExpression() {
        String token = "";
        Scanner scan = new Scanner(exp);
        stk.clear();

        while(scan.hasNext()) {
            token = scan.next();
            char current = token.charAt(0);

            if (isNumber(token)) {
                postfix = postfix + token + " ";
            } else if(isParentheses(current)) {
                if (current == '(') {
                    stk.push(current);
                } else {
                    Character ch = (Character) stk.peek();
                    char nextToken = ch.charValue();

                    while(nextToken != '(') {
                        postfix = postfix + stk.pop() + " ";

                        ch = (Character) stk.peek();

                        nextToken = ch.charValue();
                    }
                    stk.pop();
                }
            } else {
                if (stk.empty()) {
                    stk.push(current);
                } else {
                    Character ch = (Character) stk.peek();
                    char top = ch.charValue();

                    if (hasHigherPrecedence(top, current)) {
                        stk.push(current);
                    } else {
                        ch = (Character) stk.pop();

                        top = ch.charValue();

                        stk.push(current);

                        stk.push(top);
                    }
                }
            }
        }
        try {
            Character ch = (Character) stk.peek();
            char nextToken = ch.charValue();

            while (isOperator(nextToken)) {
                postfix = postfix + stk.pop() + " ";
                ch = (Character) stk.peek();
                nextToken = ch.charValue();
            }
        } catch (EmptyStackException e) {}
    }
    boolean isNumber(String s) {
        try {
            int Num = Integer.parseInt(s);
        } catch(NumberFormatException e) {
            return false;
        }
        return true;
    }
    void evaluateRPN() {
        Scanner scan = new Scanner(postfix);
        String token = "";
        stk.clear();

        while(scan.hasNext()) {
            try {
                token = scan.next();
                if (isNumber(token)) {
                    stk.push(token);
                } else {
                    char current = token.charAt(0);
                    double t1 = Double.parseDouble(stk.pop().toString());
                    double t2 = Double.parseDouble(stk.pop().toString());
                    double t3 = 0;

                    switch (current) {
                    case '+': {
                        t3 = t2 + t1;
                        stk.push(t3);
                        break;
                    }
                    case '-': {
                        t3 = t2 - t1;
                        stk.push(t3);
                        break;
                    }
                    case '*': {
                        t3 = t2 * t1;
                        stk.push(t3);
                        break;
                    }
                    case '/': {
                        t3 = t2 / t1;
                        stk.push(t3);
                        break;
                    }
                    default: {
                        System.out.println("Reverse Polish Notation was unable to be preformed.");
                    }
                }
            }

        } catch (EmptyStackException e) {}
    }
}
String getResult() {
    return stk.toString();
}

int stackSize() {
    return stk.size();
}

boolean isParentheses(char current) {
    if ((current == '(') || (current == ')')) {
        return true;
    } else {
        return false;
    }
}

boolean isOperator(char ch) {
    if ((ch == '-')) {
        return true;
    } else if ((ch == '+')) {
        return true;
    }
    else if ((ch == '*')) {
        return true;
    }
    else if((ch == '/')) {
        return true;
    } else {

    }
    return false;
}

boolean hasHigherPrecedence(char top, char current) {
    boolean HigherPre = false;

    switch (current) {
    case '*':
        HigherPre = true;
    break;

    case '/':
        HigherPre = true;
    break;

    case '+':

        if ((top == '*') || (top == '/') || (top == '-')) {
             HigherPre = false;
        } else {
             HigherPre = true;
        }

        break;

    case '-':
        if ((top == '*') || (top == '/') || (top == '-')) {
            HigherPre = false;
        } else {
            HigherPre = true;
        }
        break;

    default:
        System.out.println("Higher Precedence Unsuccessful was unable to be preformed.");
        break;
    }

    return HigherPre;


   }

    String getPostfix() {
        return postfix;
    }
}

Answer 1

What I am assuming is that (), {}, and [] all have the same weight in terms of order of operations, and you just need to modify your code in order to allow for all three interchangeably.

If that is the case, I would just use the matcher class with a simple regex check to see if the current char that you are looking at is either a parenthesis, curly brace, or bracket.

    //convert char to string
    String temp += currentChar;
    //this will check for (, [, and { (need escapes because of how regex works in java)
    Pattern bracePattern = Pattern.compile("[\(\{\[]");
    Matcher matcher = numPatt.matcher(temp);
    if(matcher.find()){
    //you know you have a grouping character
    }

This code should allow you to find all the opening grouping characters (just substitute (,{, and [ for ),} and ] in the regex to find closing characters). This can be used in your isParenthesis() method.