Dear all,

In fenics, we can use Constant for a scalar constant, for example `f=Constant(-6.0)`

, and alternatively we can use float for a scalar constant, for example `f=-6.0`

. I wonder what’s the difference between these two usages? Below is the Poisson problem code from fenics tutorial. I changed the `f = Constant(-6.0) `

to `f = -6.0`

, and the result didn’t change. So what’s the meaning of using Constant for a scalar constant? We can just simply use float type for a scalar constant, like this: `f = -6.0`

.

My second question is are the `Constant((0, 1))`

and `as_vector([0, 1])`

the same?

I appreciate any help. Thank you very much in advance.

```
from __future__ import print_function
from fenics import *
import matplotlib.pyplot as plt
# Create mesh and define function space
mesh = UnitSquareMesh(8, 8)
V = FunctionSpace(mesh, 'P', 1)
# Define boundary condition
u_D = Expression('1 + x[0]*x[0] + 2*x[1]*x[1]', degree=2)
def boundary(x, on_boundary):
return on_boundary
bc = DirichletBC(V, u_D, boundary)
# Define variational problem
u = TrialFunction(V)
v = TestFunction(V)
#test Constant and float
# f = Constant(-6.0)
f = -6.0
a = dot(grad(u), grad(v))*dx
L = f*v*dx
# Compute solution
u = Function(V)
solve(a == L, u, bc)
# Plot solution and mesh
plot(u)
plot(mesh)
# Save solution to file in VTK format
vtkfile = File('poisson/solution.pvd')
vtkfile << u
# Compute error in L2 norm
error_L2 = errornorm(u_D, u, 'L2')
# Compute maximum error at vertices
vertex_values_u_D = u_D.compute_vertex_values(mesh)
vertex_values_u = u.compute_vertex_values(mesh)
import numpy as np
error_max = np.max(np.abs(vertex_values_u_D - vertex_values_u))
# Print errors
print('error_L2 =', error_L2)
print('error_max =', error_max)
# Hold plot
plt.show()
```