Question

I have problem concerning evaluating postfix expression using stack

for(int i=0;i<postfix.length;i++){
  System.out.println("iteration:"+(i+1));
  if(postfix[i].equalsIgnoreCase("+") || postfix[i].equalsIgnoreCase("-") 
      || postfix[i].equalsIgnoreCase("*") || postfix[i].equalsIgnoreCase("/") ) {
    num1 = Float.parseFloat((String)out.pop());
    System.out.println("pop:"+num1);
    num2 = Float.parseFloat((String)out.pop());
    System.out.println("pop:"+num2);
    if(postfix[i].equalsIgnoreCase("+")) { 
      temp = num2+num1; 
      out.push(temp);
      System.out.println("push:"+temp);
      }
    else if(postfix[i].equalsIgnoreCase("-")) {   
      temp = num2-num1;   
      out.push((""+temp));
      System.out.println("push:"+temp);
      }
    else if(postfix[i].equalsIgnoreCase("*")) {   
      temp = num2*num1;   
      out.push((""+temp));
      System.out.println("push:"+temp);
      }
    else if(postfix[i].equalsIgnoreCase("/")) {
      temp = num2/num1;
      out.push((""+temp));
      System.out.println("push:"+temp);
      }
  }
  else{
    System.out.println("push:"+postfix[i]);
    out.push(postfix[i]);
  }
}

the code works well if i only do operation among two numbers e.g “2 4 +”
but when it becomes like “2 4 + 5 +” the error suddenly appears.. i even put markers.. the prints there to check the flow of my code ..thanks
any help?

Answer 1

On one line out.push(temp); which is pushing a Float instead fo a String.

I suggest you use

Stack<Double> out = new Stack<Double>();

with

double num1 = out.pop();

and

out.push(num2 + num1);

and

out.push(Double.parseDouble(postfix[i]));

As Stack is a legacy class, a better choice would be a Deque, but I assume you don’t need to worry about changing it for this project.