Hi Francis,

There are two things here. The first is the proximate cause of your exception. If f is a FunctionSpace then it doesn't have an interpolate method. Instead, you would call the interpolate function:

c = interpolate(MyExpression(), f).

However, creating a subclass of Expression is not the best way to solve this problem. Instead you should interpolate a UFL expression:

x = SpatialCoordinate(f.mesh())

c = interpolate(conditional(le(x[0], 0.5), 1.0, 0.0), f)

Regards,

David

On Sun, 26 Feb 2017 at 22:06 Francis Poulin <fpoulin@uwaterloo.ca> wrote:

Hello,

I am trying to define an expression piecewise and see that I need to do that by defining a class.    Below is something that I tried, building on what I read, but it has an error, see below.

class MyExpression(Expression):
    def eval(self, value, x):
        if x[0] <= 0.5:
            value[0] = 1.0
        else:
          value[0] = 0.0

    def value_shape(self):
        return (1,)

f.interpolate(MyExpression())

It fails with the following error:
AttributeError: 'FunctionSpace' object has no attribute 'interpolate'
Can someone maybe point me in the right direction on how to do tis properly?

Cheers, Francis

------------------

Francis Poulin
Associate Professor
Department of Applied Mathematics
University of Waterloo

email:           fpoulin@uwaterloo.ca
Web:            https://uwaterloo.ca/poulin-research-group/
Telephone: +1 519 888 4567 x32637