Problem defining initial conditions : Expression problem?

I/O
2

Custom expressions defined in Python are no longer subclasses of Expression, but, instead, of a separate class, UserExpression. However, I will note that what it looks like you’re trying to do can be accomplished with the standard Expression constructor, by passing a tuple of C++ snippets:

w_init = Expression(2*('1.0*random() + 0.25',),
                    element = W.ufl_element())

I will also add that I’m not sure the C++ random() function is doing what you think it is; this will return very large numerical values. To get random floating point numbers between, say, 0 and 1, you would want something like

random()/((double)RAND_MAX)

because random() on its own returns an integer between 0 and RAND_MAX (which is about 2 billion).

EDIT: Alternatively, if you are trying to use the NumPy random() function (based on the import statements at the top) you could do something like

from numpy.random import random
class IC(UserExpression):
    def eval(self,values,x):
        values[0] = 1.0*random() + 0.25
        values[1] = 1.0*random() + 0.25
    def value_shape(self):
        return(2,)
w_init = IC(element = W.ufl_element())
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