A function plus a function

Question

Hi, everybody. I use the c++ interface, and want to ask a simple question:
The solution is the summation of two finite element solutions u, w. How can I add u and w together? The straightforward formula U_sum= u+w does not work.
Thanks in advance.

Comments

Could you specify, whether you are using python orC++ as well as the code you are using and any warnings/errormessages that your program creates? Also, maybe this could help you http://fenicsproject.org/qa/430/how-to-sum-functions

Answer 1

Yes, this is possible, u + w creates a new Expression which can be used in a variational problem.

However, if you want to have it as a Function, then you need to project it to a finite element space:

v = project(u + w, V)

If you know the two Functions are in the same space, you could also directly sum the degrees of freedom:

v = Function(V)
v.vector()[:] = u.vector()[:]
v.vector() += w.vector()

(Example code is untested Python but should work with minor modifications in C++.)

Comments

Hi Logg

The code :

v = Function(V)
v.vector()[:] = u.vector()[:]
v.vector() += w.vector()

does not work for c++ interface. But, I found a way to cope with it:

Function U_sum(V);
*U_sum.vector()+=*u.vector();
*U_sum.vector()+=*v.vector();

Thank you very much!