What is the syntax to import conditional operators from ufl package?

1

I am little confused how to fix the issue in the function H(x). The minimal example is given below.

    from fenics import *
    q_s =  75                               # Column saturation moisture content

    # Create mesh and define function space
    nx = ny = 10
    mesh = RectangleMesh( Point(0, 0), Point(5120, 6400), nx, ny)

    # Define function space for velocity
    W = VectorFunctionSpace(mesh, 'P', 2)

    #Define function space for system of equations

    P1 = FiniteElement('P', triangle, 1)
    element = MixedElement( [P1, P1] )
    V = FunctionSpace ( mesh, element)

    # Define test functions
    v_1, v_2= TestFunctions(V)

    # Define functions for velocity and PDE
    u = Function(V)

    # Split system functions to access components
    u_1, u_2 = split(u)

    # Define expressions used in variational forms
    q_s = Constant(q_s)

    #Define Latent heat source
    from ufl.algebra import Sum
    def P(u_1):
        return ( alpha*(u_1-q_c) * H(u_1 - q_c) )

    # Define Heaviside function H
    def H(x):
        if ( x>0):
            return(1)
        else :
            return (0)

    # Define variational problem
    F = P(u_1)*v_1*dx


The following error occurred.

    File "exp.py", line 105, in <module>
        + P(u_1)*v_1*dx + Q(u_1)*v_1*dx - M1*K(u_1, u_2)*u_1*v_1*dx \
      File "exp.py", line 75, in P
        return ( alpha*(u_1-q_c) * H(u_1 - q_c) )
      File "exp.py", line 96, in H
        if ( x>0):
      File "/Users/kapilchawla/opt/anaconda3/envs/fenicsproject/lib/python3.8/site-packages/ufl/conditional.py", line 46, in __bool__
        error("UFL conditions cannot be evaluated as bool in a Python context.")
      File "/Users/kapilchawla/opt/anaconda3/envs/fenicsproject/lib/python3.8/site-packages/ufl/log.py", line 172, in error
        raise self._exception_type(self._format_raw(*message))
    ufl.log.UFLException: UFL conditions cannot be evaluated as bool in a Python context.


I did not find the name of the operator in ufl package.
2

Use a ufl.conditional, see: How to implement function with condition? - #3 by dokken

3

It does not work for me. Can you write how you import from ufl.conditional?

4

Conditional is actually import with dolfin, so if you have used from dolfin import *, simply use conditional(gt(x, 0), 1, 0).