Hi Francis, What is the function space you are using? You can build a piecewise constant function using the C-string Expression via the ?: ternary operator in C. all the best --cjc On 26 February 2017 at 22:05, 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 <(519)%20888-4567>

-- http://www.imperial.ac.uk/people/colin.cotter www.cambridge.org/9781107663916