userdefine

Expression4J

Sample > User define function

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Agregated expression
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User define function
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Create an agregated expression

An user define function is constitued by :

a specific class who implement Expression interface.

the sample bellow indicate how to construct the "factoriel" user define function. "Factoriel" is the class who implement the factoriel function.

My first user define function with expression4j

Factoriel class code:

public static class Factoriel extends AbstractFunction {
	 /**
	  * compute the factoriel of a number
	  * @param x number to compute
	  * @return factoriel of the number.
	  */
	 private double fact(double x) {
		 if (x == 0) {
			 return 1;
		 }
		 return x * fact(x-1);
	 }

	 /**
	  * evaluation method called by the Expression4j when needed.
	  * 
	  * @param parameters parameter given to the function for the evaluation.
	  */
	 public MathematicalElement evaluate(Parameters parameters) 
		throws EvalException {
		try {
			MathematicalElement me = parameters.getParameter("x");
			double tmpValue = me.getRealValue(); 
			if (tmpValue < 0) {
				throw new EvalException(
					"Cannot evaluate fact of " + 
					"negativ number.");
			}
			double result = fact(tmpValue);
			return NumberFactory.createReal(result);
		}
		catch (ParametersException pe) {
			throw new EvalException("Cannot evaluate fact(x). " + pe);
		}
	}

	public MathematicalElement evaluate(OperatorManager operatorManager,
				Parameters parameters) throws EvalException {
		return evaluate(parameters);
	}

	public Catalog getCatalog() {
		return ExpressionFactory.getCatalog();
	}

	/**
	 * get the function name
	 */
	public String getName() {
		return "fact";
	}

	/**
	 * get parameter list of the function.
	 */
	public List getParameters() {
		//this util method get one parameter "x".
		return ParameterUtil.generateXParameters();
	}

	public ExpressionModel getExpressionModel() {
		// TODO Auto-generated method stub
		return null;
	}
}

To use this user define function in Expression4j, you must put it in the classpath and use it like this:

public static void main(String[] args) {
	try {
		//define a specific catalog (not necessary,
		//we can use the default catalog instead)
		Catalog catalog = ExpressionFactory.createCatalog("catalog");

		//add the user define function to the catalog
		catalog.addExpression(new Factoriel());

		//create the expression who use the user define function
		//dont forget to put the catalog of the user define function
		//if default catalog is not used.
		Expression expression1 = 
			ExpressionFactory.createExpression("f(x)=fact(x+1)",catalog);
		System.out.println("Expression name: " + expression1.getName());
		System.out.println("Expression : " + expression1);
		System.out.println("Expression parameters: " + 
			expression1.getParameters());

		//compute a value
		MathematicalElement me1 = NumberFactory.createReal(5);
		Parameters parameters = ExpressionFactory.createParameters();
		parameters.addParameter("x",me1);
		System.out.println("Value of expression h :" + 
			expression1.evaluate(parameters));
	}
	catch (Exception e) {
		System.out.println("Error: " + e);
	}
}