Runtime expression evaluation in ActionScript

04/19/2008 06:00 pm by sephiroth.it - flash & php in actionscript

Finally I have a bit of free time to write down this post. Some days ago a guy on the forum asked about how to evaluate at runtime simple mathematical expressions with support for symbols and function calls.

Now that the eval function have been removed from the language for reasons I've not enought time to talk about, we need to parse and execute those expressions manually.

I built a simple Actionscript library that can be used to parse mathematical expressions: it has support for function calls, for variables and it is able to convert the expression into a postfix rappresentation if you may need it. The expression parser is quite simple and have been built manually following some concepts related to programming language compilers; it includes a Scanner (or lexer - however you want to call it) to tokenize the expression, a Parser that convert the expression into an Abstract Syntax Tree and a simple routine that evaluates that AST using a Symbol Table as evaluation context.

There are no comments inside the code right now; I hope to find a little bit of time to write an in depth discussion about this topic. In the mean time you can continue reading the entry for a general explanation about how does the code works and for some examples that may be useful.

Here is a simple example that shows how does the code works:


import it.sephiroth.expr.CompiledExpression;
import it.sephiroth.expr.Parser;
import it.sephiroth.expr.Scanner;

public class Example
{
  public static function run(): void
  {
    var expression: String = "sin( x / ( 8 / 2 + (-0.12 + 2.3) * x / x ) ) * 100";
    var scanner: Scanner = new Scanner( expression );
    var parser: Parser = new Parser( scanner );
    
    var compiled: CompiledExpression = parser.parse();
    
    var context: Object = {
      x:    100,
      sin:   Math.sin,
      cos:  Math.cos
    };
    
    trace( 'Postfix:', compiled.toString() );
    trace( 'Result:', compiled.execute( context ) );
  }
}

Before I forgot, you can download the source code here with a simple example included.

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