evaluating expressions containing functions in java

Question

I have written a project that calculates different functions like sine,MCM etc without using existing packages like math in java now I want to get an expression from the user in form of a string and then print out the result
like :

import java.util.Scanner;

public class Phase2main {

public static void main(String[] args) {

    Scanner s = new Scanner(System.in);
    String expression   ;
    double result = 0 ;

    System.out.println(" Enter your desired expression from the available functions ");

    expression = s.nextLine();


    System.out.println("Result is : " + result);                
 }
}

then it should run like this: Enter an Expression: ADD(DIV(SIN(FACT(3)),CEIL(TAN(MUL(1.5,FIB(4))))),GCD(2,10)) The Result is: 1.94

how can I make the program to identify my functions like CEIL and their input ? I've checked many of the similar questions but the ones that I found are rather libraries that are too complex for me to understand or do basic arithmetic without identifying functions and their inputs

so how can I write a simple evaluator for this specific problem?

Answer 1

May be use JavaScript interpreter?

First create engine instance and init:

// Manager creates engines by mime/language names.
// It has own global scope for engiges created by it.
ScriptEngineManager manager = new ScriptEngineManager();
// Create JavaScript interpreter instance.
// (Nashorn is bundled JavaScript interpreter)
ScriptEngine scope = manager.getEngineByName("JavaScript");
// Define functions you need
String initialScript = "cos = Math.cos;" // ; or \n
    + "sin = Math.sin;"
    + "tg  = Math.tan;"
    + "PI  = Math.PI;"
// Any other function
    + "ctg = function (x) { return cos(x)/sin(x); };";
// ...

try {
    // Add these functions to scope
    scope.eval(initialScript);
} catch(ScriptException ex) {
    // Evaluating exceptions, syntax errors are thrown here
}

And then you can evaluate expressions in the "scope" many times:

try {
    double d = (double)scope.eval("sin(PI/2) + cos(PI/2)");
    System.out.println("Calculated: " + d);
} catch(ScriptException e) {
    // ...
}

Be warned:

1. There is language interpreting - user can pass any script and...
2. ... it can reduce perfomance of application.

You can also use, for example, Jython or JRuby as interpreter.

Answer 2

You could use the Java Scripting API, and for instance use JavaScript, or BeanShell (java like).

    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByMimeType("text/javascript");
    try {
        engine.eval("print('Result: ' + java.lang.Math.sin(0.8))");
        double y = ((Number) engine.eval("java.lang.Math.sin(0.8)")).doubleValue();
    } catch (ScriptException ex) {
        Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
    }

JavaScript can use imports. As you can see, calling java is quite simple.

Answer 3

I'm not sure if there is any simple way to identify and evaluate generic inputed expressions without using libraries.

If you want to develop your own way to evaluate math expressions take a look on this page, it should show you the way to go: http://cogitolearning.co.uk/?p=565

And if you change your mind and want to use an library, check this link: Evaluating a math expression given in string form

I've done this when I built a java compiler, that way I could read and evaluate any expression, but it wasn't simple...