Question

I have a problem i can’t find the wrong in this function it sometimes do well with some inputs but sometimes no for examle this Input “6 2 / 3 – 4 2 * +” can any one help.

public static double Evaluating_postfix_expressions(String postfix) throws Exception{
    StringTokenizer st = new StringTokenizer(postfix);
    int numOF_tokens = st.countTokens();
    for (int i = 1; i <= numOF_tokens; i++) {
        Object term = st.nextToken();
        try { // if it is an operand there is no problem
            float x =  Float.parseFloat((String)term);
            stack.push(x);
        } catch (Exception e) { // it is an operator
            float v1 = (float) stack.pop();
            float v2 = (float) stack.pop();
            switch ((String) term) {
            case "+":
                stack.push(v2 + v1);
                break;
            case "-":
                stack.push(v2 - v1);
                break;
            case "*":
                stack.push(v2 * v1);
                break;
            case "/":
                stack.push(v2 / v1);
                break;
            }
        }
    }
    return (float) stack.pop();
}

Answer 1

Note that you’ve given an incorrect operator as input:

6 2 / 3 – 4 2 * +

Note the difference with a version hand-typed right here:

6 2 / 3 - 4 2 * +

You’ve used a dash instead of a hyphen — or the other way around:

$ echo -n – | xxd
0000000: e280 93                                  ...
$ echo -n - | xxd
0000000: 2d                                       -
$ 

This is part of the problem with a catch-all exception that doesn’t report any problems. You should modify your switch to report invalid operators, which would have made catching this one significantly easier. (I just happened to think that it looked funny.)