What's the best way to achieve the summation operator in expression?

Question

Want to make an expression for

(typos, should be i=1 in the figure, and missed right bracket as well)
where Ri is a random number from [-0.1,0.1]

I understand I could use random.uniform(-0.1,0.1) to get the random number, but I don't know how i could do it in the Expression(" "). And I don't know how to use do the summation operator either.

Please help.

Answer 1

I suggest you do a subclassing of the Expression class, as it is explained in the documentation of the Expression.

class MyExpression0(Expression):
    def eval(self, value, x):
        # your python code goes here like
        xval, yval = x[1], x[0]
        value[0] = xval # here you have to set the function values
        value[1] = yval # ... 
    def value_shape(self):
        return (2,)

f0 = MyExpression0()  # instantiate your Expression for use in dolfin