Unable to compile C++ code with dijitso, error in expression

1

A= Expression(‘0.5*(1+tanh(100*(cos(0.4pi )/sin(0.4pi )- cos(pi0.1phi )/sin(pi0.1phi))))+1’,degree=2,pi=pi, phi=phi, element = V.ufl_element())

This is how phi is defined.
q,dq = Function(V),TestFunction(V)

pii,phi, qe         = split(q)

I am a newbie may be I am asking very basic. The phi in the argument of cos and sin generating this error. phi is a function. I want to know how to insert phi in the expression.

2

Please add a full minimal working code example that reproduces the error message, and make sure that you use 3x` encapsulation to ensure appropriate formatting.

3

You can index into a mixed Function in an Expression instead of passing the Function components returned by split, e.g.,

from dolfin import *
mesh = UnitIntervalMesh(1)
V = VectorFunctionSpace(mesh,"CG",1,dim=2)
v = project(Constant((1,2)),V)
v1,v2 = split(v)

# Does not work:
#expr1 = Expression("v1",v1=v1,degree=1)
#expr2 = Expression("v2",v2=v2,degree=1)

# Works:
expr1 = Expression("v[0]",v=v,degree=1)
expr2 = Expression("v[1]",v=v,degree=1)

# Verify (should print 1 and 2):
print(assemble(expr1*dx(domain=mesh)))
print(assemble(expr2*dx(domain=mesh)))
2 Likes
4

Thank you, this solution worked for me @kamensky!

However I would change it a tiny bit to

expr1 = Expression("v",v=v.sub(0),degree=1)

This has the advantage, that the ‘0’ can easily exchanged by a variable, making a larger code more readable and easier to maintain.

5

Whether this is more readable is perhaps subjective, but the following eliminates the redundant lines that differ only by index and is easier to generalize to arbitrary dimension:

# Works:
#expr1 = Expression("v[0]",v=v,degree=1)
#expr2 = Expression("v[1]",v=v,degree=1)
d=2
expr = [Expression("v["+str(i)+"]",v=v,degree=1)
        for i in range(0,d)]

# Verify (should print 1 and 2):
#print(assemble(expr1*dx(domain=mesh)))
#print(assemble(expr2*dx(domain=mesh)))
for i in range(0,d):
    print(assemble(expr[i]*dx(domain=mesh)))