# Evalute derivative of a function

Asked by Melanie Jahny on 2013-03-21

I've got a function defined on a 2D array. I compute the spacial derivative of this function
by using the "dx(i)" function from FEniCS. It works.
But I further have to evalute the derivative function on a certain mesh point. How can I do that? I tried the
"eval" function but I did not succeed. I'm not sure if "eval" is the right function here.

Here is an example

mesh = Rectangle(0,0,1,1,10,10)
V = FunctionSpace(mesh, "CG", 2)
c = Function(V)

deriv_c = c.dx

# I like to get "deriv_c" at mesh point (0.1,0.1)

cord = [0.1, 0.1]
deriv_c.eval(values,cord) # does not work

Thanks for any help!

## Question information

Language:
English Edit question
Status:
Solved
For:
DOLFIN Edit question
Assignee:
No assignee Edit question
Solved by:
Jan Blechta
Solved:
2013-03-21
Last query:
2013-03-21
2013-03-21
 Jan Blechta (blechta) said on 2013-03-21: #1

c.dx is UFL expression - so you need to project it to some Function (DG1 would be appropriate here) and use eval on it.
Vd = FunctionSpace(mesh, "DG", 1)
deriv_c = project(c.dx, Vd)

Second option is to use ufl.expr.Expr.evaluate() which, I believe, enables you to calculate values of derivatives of UFL expressions. But it is intended only for some small investigation or debugging, not for assembling forms.

Jan

 Melanie Jahny (melanie-jahny) said on 2013-03-21: #2

But unfortunately I did not succeed in applying the eval function.
I don't understand which arguments it needs, if the coordinates
where I want to evaluate deriv_c are (0.5, 0.1) for example.
I tried:

mesh = Rectangle(0,0,1,1,10,10)
V = FunctionSpace(mesh, "CG", 2)
c = Function(V)
Vd = FunctionSpace(mesh, "DG", 1)
deriv_c = project(c.dx(0), Vd)

cords = np.array([0.5, 0.1])
values = np.array([])
deriv_c.eval(values, cords) # I don't understand the first argument Jan Blechta (blechta) said on 2013-03-21: #3

On Thu, 21 Mar 2013 16:56:08 -0000
Melanie Jahny <email address hidden> wrote:
> Question #224828 on DOLFIN changed:
>
>
> Melanie Jahny is still having a problem:
> But unfortunately I did not succeed in applying the eval function.
> I don't understand which arguments it needs, if the coordinates
> where I want to evaluate deriv_c are (0.5, 0.1) for example.
> I tried:
>
> mesh = Rectangle(0,0,1,1,10,10)
> V = FunctionSpace(mesh, "CG", 2)
> c = Function(V)
> Vd = FunctionSpace(mesh, "DG", 1)
> deriv_c = project(c.dx(0), Vd)
>
> cords = np.array([0.5, 0.1])
> values = np.array([])
> deriv_c.eval(values, cords) # I don't understand the first argument
>

values shape needs to correspond with deriv_c value shape:
values = np.array([0.0])

 Melanie Jahny (melanie-jahny) said on 2013-03-21: #4

Thanks a lot, it works!

 Johan Hake (johan-hake) said on 2013-03-21: #5

On 03/21/2013 05:56 PM, Melanie Jahny wrote:
> Question #224828 on DOLFIN changed:
>
>
> Melanie Jahny is still having a problem:
> But unfortunately I did not succeed in applying the eval function.
> I don't understand which arguments it needs, if the coordinates
> where I want to evaluate deriv_c are (0.5, 0.1) for example.
> I tried:
>
> mesh = Rectangle(0,0,1,1,10,10)
> V = FunctionSpace(mesh, "CG", 2)
> c = Function(V)
> Vd = FunctionSpace(mesh, "DG", 1)
> deriv_c = project(c.dx(0), Vd)
>
> cords = np.array([0.5, 0.1])
> values = np.array([])
> deriv_c.eval(values, cords) # I don't understand the first argument

Try:

deriv_c(0.5, 0.1)

Johan

 Melanie Jahny (melanie-jahny) said on 2013-03-22: #6

That also works, thank you!
I tried that in my first tests, but without the projection
onto V_d I always got errors. But it is really an easy solution.
thank you.