How to solve a simple arithmetic expression of String, such as 5-2 * 10?

Question

I have a lot of problems trying to do this for some reason. I have a class that wants me to evaluate a complex Java expression such as (3 + 5 [3 * 2-4]) using recursion. I think I have an idea about how I want to approach it, but I can’t understand how to solve something really simple - how

5-2 * 10

I do not know how to do that. They do not allow you to import external scripts, and you cannot convert them to a postfix expression.

I don’t expect anyone to write me the code, but if someone can send me in the right direction or give me some psuedocode, I would really appreciate it - I spent two hours to no avail trying to figure out how I can use string tokenizers and other things to solve it, but I always run into a wall in which I don't know how to get around. Thank you very much in advance!

Answer 1

You can sequentially reduce subexpressions (so-called "redexes") until more reductions are possible.

This replacement of internal expressions can be done using regular expressions:

• "(\ d +) ([* /]) (-? \ D +)"
• "(\ d +) ([+ -]) (-? \ D +)"
• "\ [(-?\D +) \]"
• ...

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Answer 2

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Answer 3

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Answer 4

, "5-2 * 10", :

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;

.

, "", , 5-2 * 10 TextField:

ScriptEngineManager mgr = new ScriptEngineManager();
        ScriptEngine engine = mgr.getEngineByName("JavaScript");
        String r = jTextField1.getText();
        try {
            jTextField1.setText(engine.eval(r).toString());

        } catch (ScriptException ex) {
            Logger.getLogger(MegaCal.class.getName()).log(Level.SEVERE, null, ex);
        }