need to calculate formulae given as string expression.

[john sal]

I have two String expression of a formulae like

String s = "max(1, 5)" ;

and
int a=2;
int b=2
String s2 = "a * b";

Is there any API in java which takes these string and calculate it as a mathematical expression?

[Ulf Dittmer]

Search the FAQ pages for a library named JEP; it does this.

[john sal]

It is not a freeware ...

[Rob Spoor]

If the formulas are not too complex you could try to use a JavaScript engine. See http://download.oracle.com/javase/6/docs/technotes/guides/scripting/programmer_guide/index.html for more information.

Edit: I just tried it, and it can handle a * b just fine:

import javax.script.*;

public class Test
{
    public static void main(String[] args) throws Exception
    {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("JavaScript");

        String[] formulas = {
            //"max(a, b)",
            "a * b",
            "a + b",
        };
        Bindings b = engine.getBindings(ScriptContext.ENGINE_SCOPE);
        b.put("a", 2);
        b.put("b", 3);
        for (String formula: formulas)
        {
            int result = ((Number)engine.eval(formula)).intValue();
            System.out.println(formula + " = " + result);
        }
    }
}

The max function is undefined in JavaScript however. You can solve that by adding it as a function after line 8:

engine.eval("function max(a, b) { return a < b ? b : a; }");

You may want to add a few similar functions like that, like min.

Also, the eval method returns a Double, but the cast to Number and then calling its intValue() method is a bit safer to get the int result.

[Ulf Dittmer]

john sal wrote:It is not a freeware ...

It is, if you use the version linked in the JavaRanch FAQ (which is GPLed).

[john sal]

can you please guide me to the link for GPLed as I am not able to find it..

Thanks.

[john sal]

Hi Rob thanks a lot it worked fine and is vry helpful ..... Just have a doubt how can we make it work for the following expression?

"max(a,b)/c"

The only way i found is to send the c as well as an argument and do all the calculations in the method itself..is there any other possible and better way of doing this?

[john sal]

Hey Rob thanks ... It is working fine for this expression as well.....I was executing it in a wrong way hence was not getting the expected result....

Thanks a lot

[Rob Spoor]

You're welcome