Basic Usage

Getting started

Using the Jep package of classes in your project is simple! The following steps will get you started quickly.

  1. Download the Jep package
  2. Unpack the archive
  3. Move the jep-java-x.x.x.jar file (located in the main directory) to a directory of your choice
  4. IMPORTANT: For the Java compiler to be able to find the Jep classes when compiling your program, it needs to know their location. You will need to set the location of the jar archive in your Java IDE so that the compiler finds the library (refer to your IDE manual for how to do this). If you are not using an IDE, you will need to add the location of the .jar file to your CLASSPATH environment variable (if you don't know how, read this). Your CLASSPATH variable should contain something like C:\java\packages\jep-java-x.x.x.jar, depending on where you place the jar file.
  5. Include the following code in your program
    import com.singularsys.jep.Jep;
     1: Jep jep = new Jep();
     2: try {
     3:    jep.addVariable("x", 10);
     4:    jep.parse("x+1");
     5:    Object result = jep.evaluate();
     7:    System.out.println("x + 1 = " + result);
     8: } catch (JepException e) {
     9:    System.out.println("An error occurred: " + e.getMessage());
    10: }
    Line 1 creates a new Jep parser object (Javadoc class description). Line 3 adds the variable "x" to the parser and initializes it's value to 10. In lines 4 and 5, the expression "x+1" is parsed and evaluated. The result is stored in result. Finally, the result is printed, resulting in "x + 1 = 11" being printed.
  6. You should be able to compile your application and see the result printed when you run your program.

What's next? Browse through the documentation to see what Jep is all capable of. Also have a look at the code of the sample applets. And don't forget to use the Javadocs as handy reference!


Handling errors

Exceptions are thrown by the parse and evaluate methods if an error occurs. Parsing methods throw ParseException instances while evaluation methods throw EvaluationException instances. Both of these classes extend the JepException class. So if you do not need to distinguish between parsing and evaluation errors, simply catch JepException instead.

The exception classes provide information about the error that occurred through the getMessage() method.


Default setup

If you create a Jep instance with default settings you can parse expressions with the all functions listed in the Functions section. You can also use the following constants:

pi 3.1415... The ratio of the circumference of a circle to its diameter (Math.PI )
e 2.718... Euler's constant in double precision (Math.E)
true The boolean constant (Boolean.TRUE)
false The boolean constant (Boolean.FALSE)
i The complex imaginary unit

Evaluation methods

Three methods for evaluating an expression are available:

You may not always know what type the result of the expression is. For example it could be Double, Vector, Boolean, or String depending on the expression parsed. You can use the instanceof operator to identify the type of the result, then cast the result into the appropriate class.


Fast repeated evaluation

It is possible to perform repeated evaluation of expressions while changing variable values without repeatedly parsing the expression. This, of course, is much faster than re-parsing the expression each time it needs to be evaluated.

The following code shows how the addVariable(String, Object) and evaluate() functions can be called repeatedly to change variable values and re-evaluate an expression.

// add variable to allow parsing
jep.addVariable("x", 0);
// evaluate expression for x = 0 to x = 99
for (int i=0; i<100; i++) {
	// update the value of x
    jep.addVariable("x", i);
	// print the result
    System.out.println("Value at x = " + i + ": " + jep.evaluate());

You can use fast repeated evaluation with any evaluator including the RealEvaluator.


Decimal arithmetic

With Jep 3, it is now possible to evaluate expression using decimal arithmetic. The datatype used to represent numbers in this mode is BigDecimal rather than double. Simply use the following code for constructing a new Jep instance:

import com.singularsys.jep.bigDecimals.BigDecComponents;
jep = new Jep(new BigDecComponents());

The difference in the accuracy is best shown through an example. When performing multiplication of two numbers of the double type, 10*0.09 evaluates as 0.8999999999999999. But when performing the same calculation using decimal arithmetic with the BigDecimal type, 10*0.09 evaluates as 0.9.

If you are interested in performing high precision arithmetic, refer to the BigDecimal section of this documentation.


Implicit multiplication

You can enable the implicit multiplication option with setImplicitMul(true). The default setting is true (implicit multiplication allowed).

Implicit multiplication allows expressions such as "2 x" to be interpreted as "2*x". Note that a space is required between two variables for them to be interpreted as being multiplied. The same holds for a variable followed by a number. For example "y 3" is interpreted as "y*3", but "y3" is interpreted as a single variable with the name y3. If a variable is preceded by a number, no space is required between them for implicit multiplication to come in effect.


Handling multiple expressions

Jep can work with multiple expressions at the same time. A simple approach is to call the Jep.parse() method multiple times on multiple strings. The method returns an object of type Node which represents the expression tree. These Nodes can be stored for later use and evaluated using the Jep.evaluate(Node n) method.

Jep jep = new Jep();
try {
	Node n1 = jep.parse("y=x^2");
	Node n2 = jep.parse("z=x+y");
	for (double x=0.0; x<=1.0; x+=0.1) {
		jep.addVariable("x", x);
		Object value1 = jep.evaluate(n1);
		Object value2 = jep.evaluate(n2);
catch(JepException e) { }

Another approach allows all expressions to be contained in a single string. A sequence of equations can be read from a single string or Reader using the Jep.initMultiParse(String s), Jep.initMultiParse(Reader r) and the continueParsing() methods. Equations are separated by semi-colons. The parser is initialized by initMultiParse and the expressions are read in turn using continueParsing(). For example:

Jep jep = new Jep();
String s = "x=1; y=2; x+y";
try {
	while ((Node n = jep.continueParsing()) != null) {
		Object res = jep.evaluate(n);
catch (JepException e) { }

Fast evaluation using RealEvaluator

A significant evaluation speed improvement was made from Jep 3.2 to Jep 3.3 by replacing the default StandardEvaluator component with FastEvaluator. However, if your expressions operate solely on floating point numbers, you can achieve an additional performance boost by using RealEvaluator instead of FastEvaluator.

RealEvaluator can be loaded through the Jep constructor with

Jep jep = new Jep(new RealEvaluator());

or by setting the evaluator after construction:

jep.setComponent(new RealEvaluator());

The performance gain depends on factors such as the expression, the JVM and operating system. In tests, the speedup compared to FastEvaluator was approximately 30%.