Turn a String into a Math Expression?

Question

Lets say I have a method that's declared this way:

public double Calc(String expression) {

// Code

}

I want to take a String expression like

"2 + 4 - (3 * 4)"

Then feed it to Calc() and it should return the value that it gets.

Can you Parse a Math Expression out of a String so that it becomes an expression that Java can understand? Because normally you could just write

return 2 + 4 - (3 * 4);

But that would only work for that single expression.

Take a look at this: http://stackoverflow.com/questions/2605032/using-eval-in-java — Alberto Segura, Feb 26 '13 at 09:03
To roll your own you'd normally use an expression tree http://en.wikipedia.org/wiki/Binary_expression_tree — Daniel Imms, Feb 26 '13 at 09:04
@beto But it seems that doing this, you open up the entire JS engine just to do this one thing. Though it gave me a hint I suppose. Thanks! — OmniOwl, Feb 26 '13 at 09:06
What about using regular expressions to simplify the solution using BODMAS principle and executing the statement. You can iterate through the expression using a loop. Evaluate each of the conditions and finally throw out the answer. JS method is not good as injections are a real threat to your app. — Shouvik, Feb 26 '13 at 09:09
@Shouvik I do not know BODMAS Principle. I guess I'll have to google that. — OmniOwl, Feb 26 '13 at 09:11
@Shouvik Oh right. Yeah okay, I thought it was something else. I know the order in which you calculate an operation, but I can't see how it would make it an easy thing to write in Java, seeing as the loops would have to consider a lot of things I guess. Like brackets in brackets, and just brackets first in general. Multiplication before addition etc. — OmniOwl, Feb 26 '13 at 09:13
Well, best of luck with that! :) [NOM] This question seems relevant: http://stackoverflow.com/questions/114586/smart-design-of-a-math-parser?rq=1 — Shouvik, Feb 26 '13 at 09:17
Google "evaluate infix expression" and you'll find your answer. — Benoît Guédas, Feb 26 '13 at 09:20
have a look at this http://floris.briolas.nl/floris/2008/07/simple-calculator-with-antlr/ — user902383, Feb 26 '13 at 09:43

Jeroen Vannevel · Accepted Answer · 2014-02-06T11:58:44.100

I would suggest using Dijkstra's twostack algorithm.

This should be pretty much what you need:

public class DijkstraTwoStack {
    public static void main(String[] args) {
                Scanner scanner = new Scanner(System.in);
                String exp[] = scanner.nextLine().split(" ");
        Stack<String> ops = new Stack<String>();
        Stack<Double> vals = new Stack<Double>();

        for(int i = 0; i < exp.length; i++) {
                        String s = exp[i];
            if (s.equals("(")) {
            }
            else if (s.equals("+") || s.equals("*")) {
                ops.push(s);
            } else if (s.equals(")")) {
                getComp(ops, vals);
            } else {
                vals.push(Double.parseDouble(s));
            }
        }
        getComp(ops, vals);
        System.out.println(vals.pop());
    }

    private static void getComp(Stack<String> ops, Stack<Double> vals) {
        String op = ops.pop();
        if (op.equals("+")) {
            vals.push(vals.pop() + vals.pop());
        } else if (op.equals("*")) {
            vals.push(vals.pop() * vals.pop());
        }
    }
}

Haven't tested it, but it should be about right.

Not so efficient, but one can use this .... `ScriptEngineManager manager = new ScriptEngineManager(); ScriptEngine engine = manager.getEngineByName("js"); Object result = engine.eval("3+4");` *credits : http://stackoverflow.com/a/2605051/936786 — Jyoti Ranjan, Feb 06 '14 at 11:57
Please don't change the code from existing answers. No iterator was wanted, it would break the algorithm. — Jeroen Vannevel, Feb 06 '14 at 11:59

score 0 · Answer 2 · answered Feb 26 '13 at 09:28

Let me state is as an answer a process which could be adopted as I think though browsing over a couple of questions on SO you should get a fair idea on how to do this without loading a JS compiler.

For starters you need to parse your string through a function that converts your math string to a infix string. Then this you basically evaluate this expression as another function by breaking down the string in a loop to return your answer. A beautifully detailed process can be found here.

Turn a String into a Math Expression?

2 Answers2

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