# Wrong order of plotted node values in matplotlib even after using dofmap().dofs()

**URL:** <https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201>\
**Category:** Uncategorized\
**Created:** [January 12, 2020, 12:14am UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201 "2020-01-12T00:14:17Z")\
**Posts on this page:** 10\
**Page:** 1

<div class="post-metadata">

**Author:** ![alexmm](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@alexmm](https://fenicsproject.discourse.group/u/alexmm)\
**Post date:** [January 12, 2020, 12:14am UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/1 "2020-01-12T00:14:18Z")

</div>

Hi,  
I am trying to save the results of a 1D problem with mixed elements. After using dofmap().dofs() the order is still wrong when plotting. The code is shown below:

```
import matplotlib.pyplot as plt
from fenics import *
import numpy as np

rho_p = 3000.0                
d_p = 100.0e-6;              
m_p = rho_p*pi*d_p**3/6.0   
l = 30.0e-3            
numElems = 100                
abc = 2.0      
dfg = abc/60000.0
n = dfg/m_p
r_0 = 1.0e-3
h_d = 17.0e-3                           
U = 0.017 
Dz_F = 10.0e-3;
PI = 3.1415926535
theta = 10.95*PI/180.0

# Define function space for system
deg = 2
mesh = IntervalMesh(numElems, 0.0,l/2.0)
P1 = FiniteElement('Lagrange', interval, deg)

element = MixedElement([P1, P1])
V = FunctionSpace(mesh, element)

R = Expression("r_0 + (h_d-Dz_F/2.0)*tan(theta)", degree=deg, r_0=r_0, h_d=h_d, Dz_F=Dz_F, theta=theta)
u_ic='tan(theta)*U*x[0]/R'
c_ic='2.0*n/U/pi/pow(R,2)*exp( -2.0*pow(x[0],2)/pow(R,2) )'
w_0 = Expression((u_ic, c_ic), degree=deg, theta=theta, U=U, R=R, n=n, pi=PI)

#Write values in text file
w_p = project(w_0, V)

dof_coordinates = V.tabulate_dof_coordinates()
n = V.dim()
d = mesh.geometry().dim()
dof_coordinates.resize((n, d))

u_dofs = V.sub(0).dofmap().dofs()
c_dofs = V.sub(1).dofmap().dofs()

plt.plot(dof_coordinates[c_dofs], w_p.vector()[c_dofs])
plt.show()

```

Thank you for your time,  
Alex

---

<div class="post-metadata">

**Author:** ![volkerk](https://avatars.discourse-cdn.com/v4/letter/v/9d8465/32.png) [@volkerk](https://fenicsproject.discourse.group/u/volkerk)\
**Post date:** [January 13, 2020, 8:19am UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/2 "2020-01-13T08:19:08Z")

</div>

You can try

```python
plot(w_p.sub(1))

```

and also

```python
print(w_p.sub(1).compute_vertex_values())

```

---

<div class="post-metadata">

**Author:** ![alexmm](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@alexmm](https://fenicsproject.discourse.group/u/alexmm)\
**Post date:** [January 13, 2020, 3:28pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/3 "2020-01-13T15:28:00Z")

</div>

Hi volkerk thank you for the reply.

w\_p.sub(1).compute\_vertex\_values() only works when the degree of the elements is 1 and the values are displayed right to left.

Using w\_p.sub(1) gives the following error:  
NotImplementedError: Cannot take length of non-vector expression

---

<div class="post-metadata">

**Author:** ![volkerk](https://avatars.discourse-cdn.com/v4/letter/v/9d8465/32.png) [@volkerk](https://fenicsproject.discourse.group/u/volkerk)\
**Post date:** [January 13, 2020, 3:42pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/4 "2020-01-13T15:42:54Z")

</div>

```python
w_p.sub(1)compute_vertex_values()

```

works with arbitrary order elements. It calculates the values of the function at the mesh vertices.  
[https://fenicsproject.org/docs/dolfin/latest/python/\_autogenerated/dolfin.cpp.function.html#dolfin.cpp.function.GenericFunction](https://fenicsproject.org/docs/dolfin/latest/python/_autogenerated/dolfin.cpp.function.html#dolfin.cpp.function.GenericFunction)

I do not get the error for w\_p.sub(1) when using your code:

```python
import matplotlib.pyplot as plt
from fenics import *
import numpy as np

rho_p = 3000.0                
d_p = 100.0e-6;              
m_p = rho_p*pi*d_p**3/6.0   
l = 30.0e-3            
numElems = 100                
abc = 2.0      
dfg = abc/60000.0
n = dfg/m_p
r_0 = 1.0e-3
h_d = 17.0e-3                           
U = 0.017 
Dz_F = 10.0e-3;
PI = 3.1415926535
theta = 10.95*PI/180.0

# Define function space for system
deg = 2
mesh = IntervalMesh(numElems, 0.0,l/2.0)
P1 = FiniteElement('Lagrange', interval, deg)

element = MixedElement([P1, P1])
V = FunctionSpace(mesh, element)

R = Expression("r_0 + (h_d-Dz_F/2.0)*tan(theta)", degree=deg, r_0=r_0, h_d=h_d, Dz_F=Dz_F, theta=theta)
u_ic='tan(theta)*U*x[0]/R'
c_ic='2.0*n/U/pi/pow(R,2)*exp( -2.0*pow(x[0],2)/pow(R,2) )'
w_0 = Expression((u_ic, c_ic), degree=deg, theta=theta, U=U, R=R, n=n, pi=PI)

#Write values in text file
w_p = project(w_0, V)

dof_coordinates = V.tabulate_dof_coordinates()
n = V.dim()
d = mesh.geometry().dim()
dof_coordinates.resize((n, d))

u_dofs = V.sub(0).dofmap().dofs()
c_dofs = V.sub(1).dofmap().dofs()

print(w_p.sub(1).compute_vertex_values())

#plt.plot(dof_coordinates[c_dofs], w_p.vector()[c_dofs])
plot(w_p.sub(1))
plt.show()

```

---

<div class="post-metadata">

**Author:** ![alexmm](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@alexmm](https://fenicsproject.discourse.group/u/alexmm)\
**Post date:** [January 13, 2020, 3:47pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/5 "2020-01-13T15:47:26Z")

</div>

Hi thank you the prompt reply.

Its strange I’m getting the same error in my installation even when using plot(w\_p.sub(1))

Also even if plot(w\_p.sub(1)) were working I’m looking to write this into a numpy array. I’m doing the plotting first to check the ordering

I think the issue with w\_p.sub(1).compute\_vertex\_values() is that it prints the vertex values of the elements and not the inside nodes, if you try  
plt.plot(dof\_coordinates[c\_dofs], w\_p.sub(1).compute\_node\_values())  
With deg=2 it doesn’t work because they have a different size

---

<div class="post-metadata">

**Author:** ![volkerk](https://avatars.discourse-cdn.com/v4/letter/v/9d8465/32.png) [@volkerk](https://fenicsproject.discourse.group/u/volkerk)\
**Post date:** [January 13, 2020, 4:02pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/6 "2020-01-13T16:02:32Z")

</div>

Yes, as i wrote, the .compute\_vertex\_values() function computes the values at the mesh vertices, not the dof nodes. Please describe your desired result precisely.

If you just want to save the function, why not use

```python
function_file = XDMFFile(MPI.comm_world, './solution.xdmf')
function_file.write_checkpoint(w_p.sub(1), 'w_p.sub(1)')
function_file.close()

```

You can then view the solution e.g. in paraview.

I don’t know why w\_p.sub(1) does not work for you, but you could try

```python
w_0, w_1 = w_p.split(True)

```

and then work with w\_1.

---

<div class="post-metadata">

**Author:** ![alexmm](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@alexmm](https://fenicsproject.discourse.group/u/alexmm)\
**Post date:** [January 13, 2020, 4:16pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/7 "2020-01-13T16:16:24Z")

</div>

Hi, using the w\_p.split(True) method compiles without error but it still gives the same error in the ordering as when using the original code.

I would like to export the results to MATLAB that’s why I would like to save a numpy array of the results in a text file.

---

<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 15, 2020, 12:01pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/8 "2020-01-15T12:01:17Z")

</div>

The dofs are not ordered in increasing direction. You need to sort them and the output. For instance, consider:

```auto
import numpy
asc_order = numpy.argsort(w_p.vector()[c_dofs])
permuted_dofs = [dof_coordinates[c_dofs][i] for i in asc_order]
permuted_solution = [w_p.vector()[c_dofs][i] for i in asc_order]
plt.plot(permuted_dofs, permuted_solution, "-go")

```

---

<div class="post-metadata">

**Author:** ![alexmm](https://avatars.discourse-cdn.com/v4/letter/a/b9bd4f/32.png) [@alexmm](https://fenicsproject.discourse.group/u/alexmm)\
**Post date:** [January 15, 2020, 5:35pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/9 "2020-01-15T17:35:01Z")

</div>

Hi dokken, thank you for the reply.

Since the answer might not be always decreasing I used the same method but with the dofs. I had to take one of the size 1 dimensions to get it working:

```
dofs = np.squeeze(dof_coordinates[c_dofs])
asc_order = np.argsort(dofs)
solution = w_p.vector()[c_dofs]

plt.plot(dofs[asc_order], solution[asc_order])
plt.show()

```

I’m still not 100% sure why the plot shows the wrong ordering, even if the dofs are ordered wrong the corresponding solution values are not ordered in the same way woudn’t that give you the right plot anyways?

---

<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 15, 2020, 6:04pm UTC](https://fenicsproject.discourse.group/t/wrong-order-of-plotted-node-values-in-matplotlib-even-after-using-dofmap-dofs/2201/10 "2020-01-15T18:04:58Z")

</div>

I agree that you should use argsort on the x-coordinate (a bit too fast from my side). To elaborate,  
if you plot it with the option `plt.plot(dof_coordinates[c_dofs], w_p.vector()[c_dofs], "ro")` you will observe that the dofs are at the correct places. It is just that matplotlib by default uses lines to Connect points linearly (equivalent to “-ro”), Which makes the plot look funky.