Illegal start of expression java operators

Question

I am trying to write a program that evaluates a postfix expression. My idea is to stack the operands and then pop out the necessary operands when an operator occurs. I am trying to check if the input is an operand is an operator or operand, but the only "if" statement of this beginning code is returning "error: illegal start of expression" at the first instance of ||. Am I missing escape characters or something else? Thanks in advance.

public class Evaluate{
char input[] = new char[50];
Stack operands = new Stack();

    public string Evaulate(string input){
        for (int k = 0; input[k] != null; k++){
            char symb = input.charAt[k];
            if (symb != (+ || - || * || / || $){
                operands.push(symb);
            }
        }
    }
}

Answer 1

For a start, conditions don't quite work that way:

if (symb != (+ || - || * || / || $) ...

What you should be doing is first comparing them with characters rather than "naked" Java tokens (or $ ).

You should also be doing full comparisons between each || or, since you're using negative logic here to detect non-operators, && :

if ((symb != '+') && (symb != '-') && ...

---

There are a few other problems in your code which I haven't mentioned but that should fix your immediate problem.

Answer 2

The expression

symb != (+ || - || * || / || $

is not valid in Java. Even after closing the parentheses and quoting the symbols, like this:

symb != ('+' || '-' || '*' || '/' || '$')

it is still not valid. You may want it to mean something that it does not, based on the way we can use "or" in English (ie, "symbol is not a plus or a minus or a star or a slash or a dollar sign"). But Java just doesn't do things that way.

Fortunately there is a better way to determine whether a character (in this case symb ) is one of a certain set of characters :

if ("+-*/$".indexOf(symb) >= 0) {

This computes the position of symb within the string "+-*/$" which, if symb is one of the desired characters, will be 0, 1, 2, 3, 4, or 5. If it is not, the expression returns -1. So the >=0 check serves as the membership check you are looking for.