use variables in Lemon parser?

Question

I want to allow mathematical variables in my Lemon parser-driven app. For example, if the user enters x^2+y, I want to then be able to evaluate this for 100000 different pairs of values of x and y, hopefully without having to reparse each time. The only way I can think of to do that is to have the parser generate a tree of objects, which then evaluates the expression when given the input. Is there a better/simpler/faster way?

Performance may be an issue here. But I also care about ease of coding and code upkeep.

Answer 1

If you want the most maintainable code, evaluate the expression as you parse. Don't build a tree.

If you want to re-execute the expression a lot, and the expression is complicated, you'll need to avoid reparsing (in order of most to least maintainable): build tree and evaluate, generate threaded code and evaluate, generate native code and evaluate.

If the expressions are generally as simple as your example, a recursive descent hand-coded parser that evaluates on the fly will likely be very fast, and work pretty well, even for 100,000 iterations. Such parsers will likely take much less time to execute than Lemon.

Answer 2

That is indeed how you would typically do it, unless you want to generate actual (real or virtual) code. x and y would just be variables in your case so you would fill in the actual values and then call your Evaluate function to evaluate the expression. The tree node would then contain pointers to the variables x and y, and so on. No need to parse it for each pair of test values.