hello,first thank you for your attention! i am modeling a water vapor transport between two gas channel though a ployer membrane.the diffusion coefficients of the mixture gas are different in the membrane and in the gas channel.i am thinking about that i can write a udf like this using if-else function:if a<x<b and c<y<d,coefficient=.......;else .....,is that feasible? and another question is that can i use a specie's name instead of its specie index when i define the coefficient of this specie in my udf,because i don't know its index or how to get its index.

go on........... if a<X<b and c<y<d,coefficicent=.......;else coefficient=...... is this feasible? another question,can i use a specie's name instead of its specie index when i define mass diffusion coefficient because i don't its index or how to get its index,for example,i use water-vapor instead of its index.