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Index › Programming Questions & Help › Syntax Questions › Evaluation of Strings as Mathematical Functions
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Evaluation of Strings as Mathematical Functions (Read 622 times)
tomgupper
Evaluation of Strings as Mathematical Functions
Oct 19th, 2007, 1:51pm  
Hi there!
Quick question,

Say I've got the following :

Code:

String myFunction = "random(255)";
float theAnswer;


As far as I can see, there's no way for me to evaluate "myFunction" so that I can get the value into theAnswer is there ? Flash has eval() ( or It used to, I think AS3 dropped it ), I take it there's no equivilent in flash ?

So what would be the best way to tackle this ?

I'm asking because I'm coding a visualisation creation tool, and I'd like users to be able to enter in formulas to control movement/position/shape etc.
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antiplastik
Re: Evaluation of Strings as Mathematical Function
Reply #1 - Oct 19th, 2007, 2:59pm  
I think there is no easy way.

maybe have a look at this site.
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tomgupper
Re: Evaluation of Strings as Mathematical Function
Reply #2 - Oct 19th, 2007, 3:38pm  
darn.
ah well, thanks! Guess I'd better get to it!
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ewjordan
Re: Evaluation of Strings as Mathematical Function
Reply #3 - Oct 19th, 2007, 10:29pm  
One thing you can do is write your own mini-parser that reads the strings, figures out which functions to delegate the results to, and handles outputting the results to a float variable.

If the number of functions you're planning to allow is fairly small (basic math functions + a few other simple things), this is probably manageable, though you have to be able to figure out how to handle things like nested calls and stuff like that.

The main problem with dynamic compilation, other than the fact that it's a bit difficult, is that you wouldn't be able to deploy as an applet very easily (at the very least you'd have to sign the applet).
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tomgupper
Re: Evaluation of Strings as Mathematical Function
Reply #4 - Oct 20th, 2007, 1:30am  
Yeah ewjordan, That's what I decided to do last night.

So I've written one that can do basic operations ( + - / * ) understands random(number), and fills in the values for a set of variables.  Just made it so it splits a string based on spaces.  So as long as the user enters : 2 + 2 it can figure it out.

It works for now, should be enough to impress my lecturers Wink
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fjen
Re: Evaluation of Strings as Mathematical Function
Reply #5 - Oct 20th, 2007, 9:57am  
pattern matching (regular expressions) is really helpfull in such cases. processing has a new function called match() (as of 126) which does that:

Code:

String eq = "100 * 0.005";

String[] matches = match( eq, "([0-9]+(\\.[0-9]+)?)[ ]*([-+*/]{1})[ ]*([0-9]+(\\.[0-9]+)?)" );

if ( matches == null || matches.length < 5 ) {
       println( "Err" );
       return;
}

float result;
float left = float(matches[0]);
float right = float(matches[3]);

switch ( matches[2].charAt(0) ) {

    case '-':
       println( left - right );
       break;

    case '+':
       println( left + right );
       break;

    case '*':
       println( left * right );
       break;

    case '/':
       println( left / right );
       break;
}



this might be a nice example for the new (and yet undocumented) match() function ..

F
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