CODEWITHSUNDEEP
javaparsingstackpostfix-notation
Yunus Emre Güler
Yunus Emre Güler

Reputation: 387

Evaluate Postfix Using Stack

I am trying to create a program which evaluates postfix expression.For example "3500 43 12 * 47 2 / + -" .Here is my code

 public static int EvaluatePostfixExpression(String postfixExpr){
     Stack s = new Stack();
     int result = 0;
     String operand = null;


     for(int i = 0; i<postfixExpr.length();i++){
         if(Character.isDigit(postfixExpr.charAt(i)) == true){

             operand = operand + postfixExpr.charAt(i);
             if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
                 s.push(operand);
                 operand = null;

                 }

         }
         if(postfixExpr.charAt(i) == '+'){
            result = result + Integer.parseInt((String) s.pop()) + Integer.parseInt((String) s.pop()) ;
         }
         if(postfixExpr.charAt(i) == '-'){
                result = result + Integer.parseInt((String) s.pop()) - Integer.parseInt((String) s.pop()) ;
             }
         if(postfixExpr.charAt(i) == '*'){
                result = result + Integer.parseInt((String) s.pop()) * Integer.parseInt((String) s.pop()) ;
             }
         if(postfixExpr.charAt(i) == '/'){
                result = result + Integer.parseInt((String) s.pop()) / Integer.parseInt((String) s.pop()) ;
             }

     }

    return result;
  } //end-EvaluatePostfixExpression

When I try to run it, there occurs an error.

Exception in thread "main" java.lang.NumberFormatException: For input string: "null12"
    at java.lang.NumberFormatException.forInputString(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)
    at java.lang.Integer.parseInt(Unknown Source)

I can't find a solution.It would be great if anyone could help.

Edit: I handled the errors and my code now works;

 public static int EvaluatePostfixExpression(String postfixExpr){
     Stack s = new Stack();
     int result = 0;
     String operand = "";


     for(int i = 0; i<postfixExpr.length();i++){
         if(Character.isDigit(postfixExpr.charAt(i)) == true){

             operand = operand + postfixExpr.charAt(i);
             if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
                 s.push(operand);
                 operand = "";

                 }

         }
         if(postfixExpr.charAt(i) == '+'){
                int x = Integer.parseInt((String) s.pop()) + Integer.parseInt((String) s.pop());
                result = result + x ;
                s.push(String.valueOf(x));
         }
         if(postfixExpr.charAt(i) == '-'){
                int x = Integer.parseInt((String) s.pop()) - Integer.parseInt((String) s.pop());    
                result = result + x ;
                s.push(String.valueOf(x));
             }
         if(postfixExpr.charAt(i) == '*'){
                int x = Integer.parseInt("" + s.pop()) * Integer.parseInt("" + s.pop());    
                result = result + x ;
                s.push(String.valueOf(x));
             }
         if(postfixExpr.charAt(i) == '/'){
                int x = Integer.parseInt((String) s.pop()) / Integer.parseInt((String) s.pop());    
                result = result + x ;
                s.push(String.valueOf(x));
             }

     }

    return result;
  }

but now the result is wrong.It should be 2961 but I am getting -1952.

Upvotes: 2

Views: 5684

Answers (2)

Phylogenesis
Phylogenesis

Reputation: 7880

A slight rewrite:

public static int evaluatePostfixExpression(String postfixExpr) {
    Stack<Integer> s = new Stack<Integer>();
    String[] items   = postfixExpr.split(" ");

    for (String item : items) {
        try {
            s.push(Integer.valueOf(item));
        } catch (NumberFormatException e) {
            Integer value1 = s.pop();
            Integer value2 = s.pop();

            switch (item) {
                case "+":
                    s.push(value2 + value1);
                    break;
                case "-":
                    s.push(value2 - value1);
                    break;
                case "*":
                    s.push(value2 * value1);
                    break;
                case "/":
                    s.push(value2 / value1);
                    break;
            }
        }
    }

    return s.pop();
}

Improvements to make to this would be:

  • Assumes that each item is either a valid Integer or a valid operand
  • Assumes that the Stack contains 2 items when receiving an operand
  • Should check that the stack only has 1 item before returning

Upvotes: 1

Paco Abato
Paco Abato

Reputation: 4065

Try this one:

 String operand = "";

Change it also when 

if(Character.isDigit(postfixExpr.charAt(i+1)) == false){
    s.push(operand);
    operand = "";
}

Upvotes: 0

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