Java Algorithm for Evaluating Expressions

Question

I'm trying to implement this pseudo-code in java, to evaluate an arithmetic expression. But I keep getting the last digit: ie if i do 1+2, i'd just get 2; I don't know what I'm doing incorrectly, so If anyone has any idea, please let me know!!

Algorithm here

import java.util.Stack;

public class Main {

    static Stack<String> ops = new Stack<String>();
    static Stack<Double> vals = new Stack<Double>();

    private static void doOP() {
        double x = vals.pop();
        double y = vals.pop();
        String op = ops.pop();

        double v = 0;
        if (op.equals("+")) {
            v = y + x;
        } else {
            if (op.equals("-")) {
                v = y - x;
            } else {
                if (op.equals("*")) {
                    v = y * x;

                } else {
                    if (op.equals("/")) {
                        v = y / x;
                    }
                }
            }
        } 
        vals.push(v);
    }

    private static void repeatOps(String refOp) { 
        while ((vals.size() > 1) && (prec(refOp) <= prec(ops.peek()))) {
            doOP();
        } 
    }

    private static int prec(String refOp) {

        String[] bodmas = new String[] {  "(", ")", "/", "*", "+", "-", "$" };
        int i = 0;
        for (String operation : bodmas) {
            if (refOp.equals(operation)) {
                return i;
            }
            i++;
        }
        return -0;
    }

    private static boolean isNumber(String number) {
        try {
            @SuppressWarnings("unused")
            double d = Double.parseDouble(number);
        } catch (Exception e) {
            return false;
        }
        return true;
    }

    private static Double evalExp(String exp) {
        for (char z : exp.toCharArray()) {           
            if (isNumber(z + "")) {
                vals.push(Double.parseDouble(z + ""));
            } else {
                repeatOps(z + "");
                ops.push(z + "");
            }
        }
        repeatOps("$");
        return vals.peek();
    }

    public static void main(String[] args) {
        System.out.println(evalExp("3*2")); 
    }
}

This problem has been solved. If you'd like to do something more advanced that simple arithmetic, I'd recommend looking into lexers/parsers and grammars; think Antlr. — duffymo
– duffymo, Commented Sep 12, 2015 at 14:25

Andreas · Accepted Answer · 2015-09-12 13:08:57Z

1

Your prec() method is inverted and incorrect.

$ should be lowest precedence, meaning lowest value, e.g. 0.

+ and - should have same precedence.

* and / should have same precedence.

( and ) are not binary operators, and cannot be handled with the described logic.

So, fix prec to return 0 for $, 1 for + and -, and 2 for * and /.

answered Sep 12, 2015 at 13:08

Andreas

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