# Is it possible to access values of a P1 element at the nodes?

Hi there!
Is it possible to access values of a P1 function at the nodes and put them in an array?

Thank you.

1 Like

Finite element function can be automatically converted in array, you have to write:

``````fespace Uh(Th, P1);
Uh u = ...;

cout << u[] << endl;
cout << u[][5] << endl;
``````

`u[]` is the array of `u`, `u[][5]` is the 6th element of the array.

2 Likes

Thank you for the quick answer.

I have another question. Given a mesh Th and a function u in Vh, how do I get the value of u at the node Th[i][j]?

The mesh node is obtained using `Th(i)` (not `Th[i][j]`, this is triangle `i`, vertex `j`, see documentation).

You have access to the value at node `i` using `u[][i]`, or alternatively `u(Th(i).x, Th(i).y)` (using interpolation).

1 Like

How about in the case of a vector

``````fespace Vh(Th,[P1,P1]);
Vh [ux, uy];
``````

I gues it should be the same but when I try to output this to file like the following code:

``````{
ofstream cupf("file.txt");
cupf.precision(8);
cupf.scientific;
for (int i=0; i < Th.nv; i++)
{
cupf << Th(i).x << " " << Th(i).y << " " << ux[][i] << " " << uy[][i] << endl;
}
cupf.flush;  //to flush the buffer of file
}
``````

It gives me the exact same values for ux than for uy, which I know is not true because I also plot this vector field and it looks OK. Maybe I’m doing something wrong with the way I access the values since it is a vector? Any ideas are highly appreciated!

You have to write:

``````ux[][2*i] << " " << ux[][2*i+1]
``````

Remark: `ux[]` and `uy[]` are the same objects

1 Like

Thanks a lot! It worked. I think this should be in the manual since it is quite confusing that you can define the vectors using component names for the variables but accessing the elements has a different format.

I will add a clearer description of vectorial finite elements soon in the documentation

Your welcome and thanks to you.

Another related question, now for P2 elements. In the same script along with the vectors I also have P2 scalar variables:

``````fespace Vh(Th,[P1,P1]);
fespace Sh2(Th, P2);
Vh [ux, uy];
Sh2 c;
``````

But when I try to write the values to file in the same way and together with the vector values like this, using your suggestion

``````{
ofstream cupf("file.txt");
cupf.precision(16);
cupf.scientific;
for (int i=0; i < Th.nv; i++)
{
cupf << Th(i).x << " " << Th(i).y << " " << c[][i] << " " << ux[][2*i] << " " << ux[][2*i+1] << endl;
}
cupf.flush;     //to flush the buffer of file
}
``````

The vector component values seem OK now but the scalar value, c variable, does not. The plot directly from FreeFem built in command looks fine but if I plot the scalar c from the data file (I’m working with Matplotlib in Python) it looks like the same values but different distribution, like scrambled…

maybe the order is different because it’s a different type of element? even if the mesh is the same?

It’s more complicated with a P2 finite element: your degrees of freedom are not only the mesh nodes.
A simple way to do that is to write:

``````c(Th(i).x, Th(i).y)
``````

(That use interpolation)