# How to Evaluate a Math Expression Given in String Form?

Do you need to write a Java routine to evaluate math expressions given in string form? Are you looking for a solution that doesn't involve a lot of if-then-else statements? You're in the right place! In this article, we will explore different approaches to solving this problem.

## Understanding the Problem

Before we dive into the solution, let's make sure we understand the problem clearly. We are given math expressions in string form, such as "5+3" or "10-4*5". We need to evaluate these expressions and obtain the correct result. The solution should avoid excessive if-then-else statements, as they can make the code complex and difficult to maintain.

## Approach #1: Using ScriptEngine

One way to evaluate math expressions in Java is by using the `ScriptEngine`

class from the Java Scripting API. This API provides a way to execute scripts in different scripting languages, including JavaScript and Python. Here's how you can use it to evaluate math expressions:

```
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
public class MathExpressionEvaluator {
public static double evaluate(String expression) throws ScriptException {
ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("JavaScript");
return (double) engine.eval(expression);
}
}
```

With this approach, you can evaluate math expressions using JavaScript syntax, which is widely supported and provides powerful math capabilities.

## Approach #2: Using a Library

If you prefer to use a library instead of the Java Scripting API, you can consider using a math expression evaluation library like JEval or exp4j. These libraries provide convenient methods to parse and evaluate math expressions. Here's an example using JEval:

```
import org.cheffo.jeplite.JEP;
import org.cheffo.jeplite.ParseException;
public class MathExpressionEvaluator {
public static double evaluate(String expression) throws ParseException {
JEP parser = new JEP();
parser.parseExpression(expression);
return parser.getValue();
}
}
```

Both JEval and exp4j allow you to evaluate complex math expressions without manually parsing the string.

## Approach #3: Building a Custom Parser

If you prefer to implement your own solution without relying on external libraries, you can build a custom parser for math expressions. This approach gives you full control over the parsing and evaluation process. Here's an example implementation:

```
public class MathExpressionEvaluator {
public static double evaluate(String expression) {
char[] tokens = expression.toCharArray();
// Iterate over the characters of the expression
// and perform the appropriate operation
...
// Return the final result
...
}
}
```

In this approach, you would need to implement the logic to handle different operators, parentheses, and precedence rules. This can be more challenging, but it offers the most flexibility.

## Conclusion

In this article, we explored different approaches to solve the problem of evaluating math expressions given in string form. We discussed using the `ScriptEngine`

class from the Java Scripting API, using a library like JEval or exp4j, and building a custom parser. Depending on your requirements and preferences, you can choose the approach that best suits your needs.

Remember, by leveraging existing libraries or APIs, you can save time and effort in implementing complex parsing and evaluation logic. Additionally, consider the extensibility and flexibility of your solution to accommodate future requirements.