expr

This package parses and evaluates mathematical expressions over floating-point numbers, like 2 + 2 or cos(x/(2*pi)) * cos(y/(2*pi)).

The design priorities were ease of use with helpful error messages, ease of integration into applets, and good performance: fast evaluation and short download times. Features and flexibility were not high priorities, but the code is simple enough that it shouldn't be hard to change to your taste.

Javadoc comments in the source files give full documentation, and user-doc.html gives a user's-eye view.

Installing it

To install, put expr.jar in your classpath. (You'll need to create it first by running make, if you downloaded this from GitHub. If you don't have make, then run javac -O expr/*.java and then jar cf expr.jar expr/*.class.)

To try it out, put the expr directory in your classpath. Then

java expr.Example '3.14159 * x^2' 0 4 1

should write the output given in Example.java.

To incorporate this code into an applet, put expr.jar somewhere accessible to your webserver, and reference it with the ARCHIVE attribute in your HTML:

<APPLET ARCHIVE="/path/to/expr.jar"
	CODE="MyApplet.class"
	...>
</APPLET>

To provide documentation for your users, put user-doc.html where they can read it.

Using it

To get started quickly without reading the documentation, see the example code in Example.java. Here are some excerpts from it, with each bit preceded by an explanation:

expr is an object representing a parsed expression.

Expr expr;

Parse the string in args[0] and set expr to the representation of the result. We only parse the string once, so that later on we won't have to parse it each time we evaluate it.

try { expr = Parser.parse(args[0]); }

If the string couldn't be parsed, complain and abort. The e.explain() tries to describe exactly what went wrong.

catch (SyntaxException e) {
    System.err.println(e.explain())
    return;
}

Create a variable-object for x, so that we can control the value that x takes each time we evaluate the expression. For example, if the expression is parsed from 1 + x * x, its value will depend on what we set x to.

Variable x = Variable.make("x");

For values of x in the range from low to high, increasing by step, print out the value of the expression.

for (double xval = low; xval <= high; xval += step) {
    x.setValue(xval);
    System.out.println(expr.value());
}

There's another included example: the graphing applet in example.html.

Other features

The above is the simplest code you can write to get going. With a few more lines, you can help the parser catch more errors. By default, it allows any variable to be in the input expression, even variables you haven't defined. Here's how to tell it what's allowed:

Variable x = Variable.make("x");
Parser parser = new Parser();
parser.allow(x);

Expr expr;
try {
    expr = parser.parseString(args[0]); 
} catch (SyntaxException e) {
    // the rest is the same as before
}

You can disallow all variables, if for some reason you want to, by changing parser.allow(x) to parser.allow(null).

Contact

See the file COPYING for copyright info. Send questions and bug reports to Darius Bacon darius@wry.me.