# Print assembled matrix

**URL:** <https://fenicsproject.discourse.group/t/print-assembled-matrix/13451>\
**Category:** General\
**Created:** [January 30, 2024, 3:33pm UTC](https://fenicsproject.discourse.group/t/print-assembled-matrix/13451 "2024-01-30T15:33:56Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Geo](https://avatars.discourse-cdn.com/v4/letter/g/f4b2a3/32.png) [@Geo](https://fenicsproject.discourse.group/u/Geo)\
**Post date:** [January 30, 2024, 3:33pm UTC](https://fenicsproject.discourse.group/t/print-assembled-matrix/13451/1 "2024-01-30T15:33:56Z")

</div>

Hi,

I was wondering when I assemble a matrix, say in the Poisson tutorial in the step

```auto
a = ufl.dot(ufl.grad(u), ufl.grad(v)) * ufl.dx
L = f * v * ufl.dx

```

can I print in the screen A and L or transform them to np matrix or array without resorting to the source code?

---

<div class="post-metadata">

**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\
**Post date:** [January 30, 2024, 3:48pm UTC](https://fenicsproject.discourse.group/t/print-assembled-matrix/13451/2 "2024-01-30T15:48:32Z")

</div>

For small matrices you can first assemble the matrix with `A=dolfinx.fem.petsc.assemble_matrix(dolfinx.fem.form(a))`  
and call `print(A[:,:])`.  
For larger matrices one should use the scipy csr matrix. There are plenty of posts with this on the forum.

The vector is easy to print, simply assemble it

```auto
b = dolfinx.fem.petsc.assemble_vector(dolfinx.fem.form(L))
print(b.vector.array)

```

or use

```auto
b = dolfinx.fem.assemble_vector(dolfinx.fem.form(L))
print(b.array)

```

See for instance: [Solving PDEs with different scalar (float) types — DOLFINx 0.7.3 documentation](https://docs.fenicsproject.org/dolfinx/v0.7.3/python/demos/demo_types.html?highlight=csr)

---

<div class="post-metadata">

**Author:** ![Geo](https://avatars.discourse-cdn.com/v4/letter/g/f4b2a3/32.png) [@Geo](https://fenicsproject.discourse.group/u/Geo)\
**Post date:** [January 30, 2024, 4:09pm UTC](https://fenicsproject.discourse.group/t/print-assembled-matrix/13451/3 "2024-01-30T16:09:05Z")

</div>

Thanks, worked perfectly. I will copy-paste a complete code below in case someone needs it in the future.

```auto
from mpi4py import MPI
import dolfinx
import matplotlib.pyplot as plt
import numpy as np
import ufl
from petsc4py.PETSc import ScalarType
import dolfinx.fem.petsc
import scipy

domain = dolfinx.mesh.create_unit_square(MPI.COMM_WORLD, 10, 10, dolfinx.mesh.CellType.quadrilateral)

V = dolfinx.fem.FunctionSpace(domain, ("CG", 1))
uD = dolfinx.fem.Function(V)
uD.interpolate(lambda x: 1 + x[0]**2 + 2 * x[1]**2)

tdim = domain.topology.dim
fdim = tdim - 1
domain.topology.create_connectivity(domain.topology.dim-1, domain.topology.dim)
boundary_facets = dolfinx.mesh.exterior_facet_indices(domain.topology)
boundary_dofs = dolfinx.fem.locate_dofs_topological(V, domain.topology.dim-1, boundary_facets)
print(boundary_dofs)
dof_coordinates = V.tabulate_dof_coordinates()[boundary_dofs]
bc = dolfinx.fem.dirichletbc(uD, boundary_dofs)

u = ufl.TrialFunction(V)
v = ufl.TestFunction(V)
f = dolfinx.fem.Constant(domain, ScalarType(-6))
a = ufl.dot(ufl.grad(u), ufl.grad(v)) * ufl.dx
L = f * v * ufl.dx
problem = dolfinx.fem.petsc.LinearProblem(a, L, bcs=[bc])
uh = problem.solve()

# Assemble the sparce matrix
A=dolfinx.fem.assemble_matrix(dolfinx.fem.form(a))
As = scipy.sparse.csr_matrix((A.data, A.indices, A.indptr))
print(As)

# Convert the scipy sparse matrix to a dense numpy array
A_dense = As.toarray()
# Convert the numpy array to a numpy matrix
A_matrix = np.matrix(A_dense)
print(A_matrix)

b = dolfinx.fem.assemble_vector(dolfinx.fem.form(L))
print(b.array)

```