In FEniCS there are two ways of defining the sub-domains or different zones in a mesh. One is the python way and other is the C++ way. In this post I will find the speed difference between the two. I will follow along this article from the official FEniCS website.

```
from fenics import *
lang = "p"
# Create mesh and define function space
mesh = UnitSquareMesh(1000, 1000)
V = FunctionSpace(mesh, 'P', 1)
# Define boundary condition
u_D = Expression('1 + x[0]*x[0] + 2*x[1]*x[1]', degree=2)
if lang == "p":
print("using python")
def boundary(x, on_boundary):
return on_boundary
else:
print("using c++")
boundary = CompiledSubDomain("on_boundary")
bc = DirichletBC(V, u_D, boundary)
# Define variational problem
u = TrialFunction(V)
v = TestFunction(V)
f = Constant(-6.0)
a = dot(grad(u), grad(v))*dx
L = f*v*dx
# Compute solution
u = Function(V)
solve(a == L, u, bc)
```

For expressions, I went throught the discourse post here.