How to multiply 2 array items

Question

I am making an Android app that will eventually be used as a dice roller for a game. I have a keypad that enters an equation into a String. I then parse the String into an array and need to compute the equation. I am using a for loop to go through the array, but I'm not sure how to handle the multiplication when it comes up.

As an example:

String theEquation = "2 + 3 * 6";
String[] result = theEquation.split("\\s+");

So I now have an array that has 2, +, 3, *, 6 in it. How would I go about turning that in to the answer of 20?

I initially used a for loop to just add the entire array together. That works fine when it is just addition. But things get hinky when i add in multiplication.

This is a broad topic. You need to specify which kind of operations you want to support, what input values are allowed etc. Out of curiousity: Why do you need such a thing for a dice game? — J Fabian Meier, Aug 12 '16 at 11:13
Check this: http://stackoverflow.com/questions/1432245/how-to-parse-a-mathematical-expression-given-as-a-string-and-return-a-number — beeb, Aug 12 '16 at 11:14
It is for Dungeons and Dragons. I use it in place of the various different dice. As an example, I need to roll 5- 6 sided dice for one thing that i do often. I only have 1 actual 6 sided die. Its much easier to open this app and hit "5*d6" and have it compute the roll. — Inessaria, Aug 12 '16 at 11:20
You could check if your result array contains * to make the operation in first place. Then remove from your array the elements before and after the * and continue with operations. — Manu AG, Aug 12 '16 at 11:29
Fairly low tech and easy to implement.....Perfect for something like this. I will try that out, thank you. — Inessaria, Aug 12 '16 at 11:33

Raman Sahasi · Answer 1 · 2016-08-12T11:14:08.217

2

You can use javax.script.ScriptEngine

Output

See this code:

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

public class EvaluateExpr {
  public static void main(String[] args) throws Exception{
    ScriptEngineManager se = new ScriptEngineManager();
    ScriptEngine scriptengine = se.getEngineByName("JavaScript");
    String theEquation = "2 + 3 * 6";
    System.out.println(scriptengine.eval(theEquation));
    } 
}

edited Aug 12 '16 at 11:14

answered Aug 12 '16 at 11:13

Raman Sahasi

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Just blatantly giving code to a script engine is just asking for code injection attacks. – Mark Jeronimus Aug 12 '16 at 11:14
I don't think this (StackOverflow) is right place to do the validations. That should be a different question. I'm just supplying the logic. – Raman Sahasi Aug 12 '16 at 11:14
Just pointing out the common pitfalls/hazards – Mark Jeronimus Aug 12 '16 at 11:15
You could probably search the web for a good regex that tells if a string is a formula or not. – Mark Jeronimus Aug 12 '16 at 11:17
Using `eval` is not good idea, you are just asking for code injection attacks – Kamil Witkowski Aug 12 '16 at 11:19
@MarkJeronimus How about [this answer](http://stackoverflow.com/a/11009403/2815219). Visit http://regexr.com/31884 to see the output. – Raman Sahasi Aug 12 '16 at 11:19
Those two imports both result in a "cannot resolve symbol" message? – Inessaria Aug 12 '16 at 11:26
@Inessaria which java version are you using? `ScriptEngine` requires atleast 1.6 above. – Raman Sahasi Aug 12 '16 at 11:30
I currently have V 1.8 – Inessaria Aug 12 '16 at 11:32
are you using android studio? The above code is compiled and tested so I guess it should work. However the error that you're addressing seems to be android studio specific error. Please view this question once: http://stackoverflow.com/q/24970646/2815219 – Raman Sahasi Aug 12 '16 at 11:57
Thank you. I am just learning Android Studio and don't quite know my way around yet. I will update it and see if that resolves my issues. – Inessaria Aug 12 '16 at 12:22
That didn't work, it still doesn't recognize that import. :( Such a fairly simple problem is turning in to a large headache for me. – Inessaria Aug 12 '16 at 12:51
Java 1.7 and before use the Rhino JavaScript engine. Java 1.8 and above use the Nashorn JavaScript engine. These are incompatible. – Mark Jeronimus Aug 13 '16 at 13:10

score 0 · Answer 2 · edited Jun 20 '20 at 09:12

You are asking to implement some kind of an interpreter basically interprets the simple math notations and gives true results.

If you really wanna develop a basic level interpreter on your own, you can find a very good sample written with Python at here (don't worry, python is very easy to read and understand).

The Python code at the link simply works like this:

$ python calc1.py

calc> 3+4

7

calc> 3+5

8

calc> 3+9

12

calc>

There is also another sample written in Java here.

Bu anyway, what you need is to somehow calculate the math given in String so @Raman's answer is very good for the solution of your problem.

How to multiply 2 array items

2 Answers2