How to calculate mathematical expressions in a string?

Question

I’m making an application picks up an expression. How do I calculate the result of an expression? For example: 3(-9)+50/2. This expression is typed in TextView.

Answer 1

To capture the value of TextView you must use the method getText() and convert to string using toString()

TextView myTextView;
...
String data = myTextView.getText().toString();

Parse and evaluate with expr

You can use the expr package (https://github.com/darius/expr), this package interprets and calculates the mathematical expressions over floating point numbers, such as 2 + 2 or cos(x/(2*pi)) * cos(y/(2*pi))

1. Installing:

   To install import the package expr.jar for your project (you will need to compile the .java first if you download from Github. If you don’t have the make, then run the following command in the folder you downloaded from Github run javac -O expr/*.java and then jar cf expr.jar expr/*.class.)

2. Adding to Activity:

   To use the package you can include it in MainActivity.java (or to another Activity) using like this import exp; (or something similar)

3. Using:

   To use with the TextView, would be something like:

   String data = myTextView.getText().toString();
   
   try {
       expr = Parser.parse(data);
       myTextView.setText(expr.value());
   } catch (SyntaxException e) {
       myTextView.setText(e.explain());
   }
   

Scriptengine

As the user said @Walkin (using Javascript engine can be overkill for a "simple" task), but still you can use it.

The ScriptEngine is an Ecmascript/Javascript engine to perform the calculation/operation, according to Soen, you will have to use the javax.script.*, which is available from the JDK1.6.

Example with TextView:

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

ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");

String data = myTextView.getText().toString();
System.out.println(engine.eval(data));

To set your own TextView or to another TextView, use:

TextView myTextView;
...
String data = myTextView.getText().toString();
myTextView.setText(engine.eval(data));

Answer 2

You need to build a appraiser of mathematical expressions. I did one in C, but it was for reverse polish notation (which I don’t know if it suits you).

The TextBox will have to return a string which is the expression that needs to be evaluated, so you basically read the line of text and as you go finding an operator or number, you keep it in a stack/tree to be able to perform the operations in the proper order, for example: 1 + (2 * 5).

• You read the first item: 1 is a number, so put it in the stack.
• You read the second item: + is an operator that will be executed with the 1, stack of operators.
• You read the third item: ( - parentheses - so here comes a new expression that I must evaluate before returning the result to operate with the previous one - opens a branch in the decision tree.
• you read the fourth item: 2 is a number so put on the new stack.
• you read the fifth item: * is an operator - puts in the stack of operators, will be executed with the 2.
• you read the last item: ) - parentheses, is the last and closes a branch of the tree, so you pull the stack and solve the expressions.

I don’t know if you got it right, but it’s basically a data structure exercise (piles and trees) where you find expressions that will operate 2 in sequence.

The precedence of operators, is the mathematics that says and the trees are bifurcations of the nested expressions. If an operator or parentheses are missing, the tree/stack will be incomplete and the expression automatically fails.

If I’m not mistaken, you can represent this as a Deterministic Finite Automaton, I can’t remember exactly.

A code I found on the net:

http://www.vivaolinux.com.br/script/Avaliacao-de-expressoes-matematicas

Answer 3

Pragmatic response (do not re-invent the wheel). Use one of many libraries to evaluate expressions in Java.

Links:

• Jep (owner)
• expr
• Jeval
• Matheval
• exp4j
• Evalex
• Expression-Evaluator-demo

There are so many that it is easier to search directly on Google. Your challenge is to select the best library for your particular case.

---

Sources

• Soen: Evaluating a Math Expression Given in string form
• Soen: What’s a good library for Parsing Mathematical Expressions in java?