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.
Installing it
To install, put expr.jar in your classpath. (You'll need to create
it first by running ant, if you downloaded this from GitHub.)
To try it out, put the expr directory in your classpath. Then
java expr.test.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>
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. This is done by creating an
expr.Parser() object and calling parserString off of it.
try { expr = (new Parser()).parseString(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 = VariableFactory.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());
}
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 = VariableFactory.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 Joseph Booker joe@neoturbine.net. Most of this library is originally by Darius Bacon darius@wry.me.