User defined math expression using values from objects... or something like that

Dear forum readers/writers,

I am looking for some ideas/suggestions/thoughts on the following:

i am creating a tool that does a whole bunch of structural analysis. It reads data from a retail analysis software package and postprocesses that.

I read in that data and store it in an object. Later in the process, i use the data. The issue that that i want to let the user define what objects to use, to build an expression.

So let's say, I have an object X containing data: A, B, C, D E and F (all doubles). I would like the user to be able to define an expression in a form, something like sqrt(A^2 + B^2) + 2*F.

basically a custum expression builder using values of objects....

any ideas?