How to convert String to math expression and evaluate using a variable?

Question

So I know that you can use

    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");
    String infix = "3+2*(4+5)";
    System.out.println(engine.eval(infix));

which is all great but what if I wanted to evaluate a variable in that expression call it x and make that a random variable between 0 and 1. This is an idea to evaluate definite integrals using a monte carlo method by using input given by the user. So if we had something like say

    String infix = x^2+2;

What's the problem?

shmosel
– shmosel

2016-12-22 21:21:58 +00:00
Commented Dec 22, 2016 at 21:21 — shmosel
– shmosel, Commented Dec 22, 2016 at 21:21
clearly state your problem.

Wasi Ahmad
– Wasi Ahmad

2016-12-22 21:22:46 +00:00
Commented Dec 22, 2016 at 21:22 — Wasi Ahmad
– Wasi Ahmad, Commented Dec 22, 2016 at 21:22

user4668606 · Accepted Answer · 2016-12-22 21:24:58Z

1

Just use ScriptEngine#put to set the respective variable. E.g.:

...
engine.put("x", 25);
System.out.println(engine.eval("x * x + 3 * x + 5"));

Alternatively you could make use of the fact that the context of the ScriptEngine isn't switched between two eval-calls:

...
engine.eval("var x = 25");
System.out.println(engine.eval("x * x + 3 * x + 5"));

answered Dec 22, 2016 at 21:24

user4668606

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1 Comment

user4668606 Over a year ago

@jc7553a glad to help :). If the answer solved your problem consider accepting it (the tick next to the up-/downvote arrows) to mark the problem as solved ;)

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