assign function

Question

Dear developer,

When u and u0 are functions,
what is the difference between
u0.assign(u)
and
assign(u0,u)
?
I am confusing because former case
can't be passed for version 1.4.
1.3 is all right.

Best regards,

Oka

Answer 1

The syntax u0.assign() is typically used when you want to create one function u0 from a superposition of other functions, i.e.

u0, u1, u2 = Function(V), Function(V), Function(V)
u0.assign(FuntionAXPY([(1.0, u1), (2.0, u2)])

is equivalent to u0 = u1 + 2*u2.

On the other hand, 'assign' is used to create a function u0 in a MixedFunctionSpace from functions in the corresponding subspaces, i.e.

W = MixedFunctionSpace([V, V])
u0 = Function(W)
u1, u2 = Function(V), Function(V)
assign(u0, [u1, u2])

will create a function u0 in the mixed function space W, where the subfunctions u0.sub(0) and u0.sub(1) are copied from u1, u2 correspondingly.

Comments

Thank you so much.
I understand, but I saw such script may times,

While (t < tmax)

E = function of u and u0

solve (E == 0, u, dE)

u0.assign(u)

I think
assign(u0,u)
is better according to your answer.
if I can write
u0 = u
it is easily understood.

Oka

---

If you just want to assign a single function, both should work fine, although in this case

u0.assign(u)

should be more efficient. I just noticed that you wrote in the original post, that this didn't work for you in dolfin-1.4. Could you check again, it works here without a problem.

---

Thank you,

But still I am troubling assign function.
Both
u0.asssign(u)
and
assign(u0,u)
doesn't pass in case of 1.4.
1.3 is all right.
It depends on type of "u" ?
Error is (for Mac)
File "/Applications/FEniCS.app/Contents/Resources/lib/python2.7/site-packages/dolfin/cpp/function.py", line 1950, in assign
return _function.assign(*args)
RuntimeError:
u and u0's type is 'dolfin.functions.function.Function'

---

Are both u0 and u functions of the same function space? If not, you need to use interpolate.