Postfix Stack Calculator

Question

I created a stack calculator for my Java class to solve equations like

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

We intend to implement { } [ ] in our code. I did this with parentheses. It works 100% only with ( ) . When I try to add { } [ ] , these are 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

I assume that (), {} and [] all have the same weight in terms of the order of operations, and you just need to change your code to allow all three interchangeably.

If so, I would simply use the matcher class with a simple regex check to see if the current char you are looking at is either a bracket, a curly bracket, or a 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 opening wildcards (just replace (, {and [for),} and] in the regular expression to find the closing characters). This can be used in your isParenthesis () method.

Answer 2

You can use a method like this to make checking easier if '(', '[' or '{' matches or not.

 static char getExpected(char val){ char expected=' '; switch (val) { case '(': expected=')'; break; case '[': expected=']'; break; case '{': expected='}'; break; } return expected; }