# Writing symmetric tensor function fails

**URL:** <https://fenicsproject.discourse.group/t/writing-symmetric-tensor-function-fails/1136>\
**Category:** I/O\
**Created:** [July 15, 2019, 1:09pm UTC](https://fenicsproject.discourse.group/t/writing-symmetric-tensor-function-fails/1136 "2019-07-15T13:09:58Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![lamBOO](https://avatars.discourse-cdn.com/v4/letter/l/8dc957/32.png) [@lamBOO](https://fenicsproject.discourse.group/u/lamBOO)\
**Post date:** [July 15, 2019, 1:09pm UTC](https://fenicsproject.discourse.group/t/writing-symmetric-tensor-function-fails/1136/1 "2019-07-15T13:09:59Z")

</div>

Hi guys,

Writing a symmetric tensor to a file fails with an error message. Can someone explain me this behavior?  
Is there a way to output Functions defined on a TensorFunctionSpace (or constructed with TensorElement)?

Error:

```auto
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
 in 
     12 v = Function(TensorFunctionSpace(mesh, "Lagrange", 1, symmetry=True))
     13 v.vector()[:] = 1.0
---> 14 File("v.pvd") << v
     15 print("wrote v")

/usr/local/lib/python3.6/dist-packages/dolfin/io/ __init__.py in __lshift__ (self, u)
     12 
     13 if isinstance(u, dolfin.function.function.Function):
---> 14 self.write(u._cpp_object)
     15 elif isinstance(u, tuple):
     16 if isinstance(u[0], dolfin.function.function.Function):

RuntimeError: interpolate_vertex_values: Failed to set physical value size correctly for subelements.

```

MWE (as done in the test file `dolfin/test/unit/python/io/test_vtk.py`):

```auto
from dolfin import *

mesh = UnitSquareMesh(16, 16)

# works
u = Function(TensorFunctionSpace(mesh, "Lagrange", 1))
u.vector()[:] = 1.0
File("u.pvd") << u
print("wrote u")

# fails
v = Function(TensorFunctionSpace(mesh, "Lagrange", 1, symmetry=True))
v.vector()[:] = 1.0
File("v.pvd") << v
print("wrote v")

```

---

<div class="post-metadata">

**Author:** ![bleyerj](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/bleyerj/32/6020_2.png) [@bleyerj](https://fenicsproject.discourse.group/u/bleyerj)\
**Post date:** [July 16, 2019, 1:54pm UTC](https://fenicsproject.discourse.group/t/writing-symmetric-tensor-function-fails/1136/2 "2019-07-16T13:54:38Z")

</div>

Hello,  
many features of `symmetry=True` option for Tensor elements seem to be broken (see [https://bitbucket.org/fenics-project/ffc/issues/78/tensorelement-symmetry-true-is-totally](https://bitbucket.org/fenics-project/ffc/issues/78/tensorelement-symmetry-true-is-totally)). They should be used with much care.

---

<div class="post-metadata">

**Author:** ![lamBOO](https://avatars.discourse-cdn.com/v4/letter/l/8dc957/32.png) [@lamBOO](https://fenicsproject.discourse.group/u/lamBOO)\
**Post date:** [July 16, 2019, 2:14pm UTC](https://fenicsproject.discourse.group/t/writing-symmetric-tensor-function-fails/1136/3 "2019-07-16T14:14:41Z")

</div>

Hi, I solved the problem in the meantime with a projection into a non-symmetric tensor space before writing the files.

MWE:

```auto
"This solves the problem with a hack"

from dolfin import *

mesh = UnitSquareMesh(4, 4)

# works
u = Function(TensorFunctionSpace(mesh, "Lagrange", 1))
u.vector()[:] = 1.0
File("u.pvd") << u
print("wrote u")

# fails
v = Function(TensorFunctionSpace(mesh, "Lagrange", 1, symmetry=True))
v.vector()[:] = 1.0
# v.assign(u) # also does not work because v afterwards lives in symmetric space
# File("v.pvd") << v # fails
# print("wrote v")

# projection works up to machine precision
vp = project(v, TensorFunctionSpace(mesh, "Lagrange", 1))
error = vp.vector()[:] - u.vector()[:]
print(error)
File("vp.pvd") << vp
print("wrote vp")

```