Hello Dr. Kisil, The question from C. Trophime is not about evaluating the function but to define it. Feel++ <http://www.feelpp.org> is a Finite Element library that main goal is to solve the variational problem a(u,v)=f(v) that arises from a lot of physical problems. We use GiNaC to dynamically define the right hand side of our problem, or our boundaries conditions and whatever can be expressed with it. The question - I guess - can be explained like that : I would like to define - f(x) = 2*x if x>0 - f(x) = 3*x if x<=0. Thus, basically, I would like to write ex MyEx = (x<=0)?3*x:2*x To my best knowledge, boolean expression are not handled in GiNaC, isn’t it ? all the best, Vincent H. -- Docteur Ingénieur de recherche CeMoSiS <http://www.cemosis.fr> - vincent.huber@cemosis.fr Tel: +33 (0)3 68 8*5 02 06* IRMA - 7, rue René Descartes 67 000 Strasbourg