expression-parser | parse math expressions into a syntax tree | Parser library

I'm trying to parse some expression. I started from impressive answers of llonesmiz and Sehe

I wanted to add some:

(1) defined parameters. These prams are given as map by another class. They may have no argument (integer) one or two :

• imf ---> $imf
• imf(1) ---> $imf(1)
• imf(1,2) ---> $imf(1,2)

I'm traying to fist get the parameter name "imf" an then its arguments if they exist (1), (2,2) ...

(2) defined function given as a map by another class. They may have one, two or three argument :

• cos(1) ---> cos(1)
• cross(imf(1),1) ---> cross($imf(1),1)
• fun3(1,2,1) ---> fun3(1,2,1)

custom_fold_directive.hpp

I faced with situation described further. I have a SBT project with traditional catalog structure in which I would like to use this library. Problem occurring when I try to parse the string directly in the program.

I know there are libs like boost.spirit or the expression parser from http://jamesgregson.blogspot.de/2012/06/mathematical-expression-parser-in-c.html and many others.

However, my expression is to be evaluated quite a lot of times inside a loop and it seems quite inefficient to run the parser every time.

Suppose my expression string is "exp(-0.5*(t-t_0)^2/(b^2))" (simple gaussian), where t_0 and b are constants and only t (=time) varies within the loop.

instead of calling the parser millions of times, I'd like to be able to save the expression somehow into a function accepting one argument (for the variable t) which then evaluates the expression.

Do you have an idea know how this can be done or if it can be done at all?