I am solving Poisson’s equation. and I want to print the u solution to a text file. The solution printed include the solution at nodes and at cells as well.

How can I print the solutions at the nodes only.
This is my code to save the solution

Remark, what is a node for you, if a node as a DoF (degree of freefem++) you print the correct thing, but
iT depend of the type on your finite element`
for (P1) your get value a vertex,
for (P2) the value are Vertex and middle edge with a strange numbering.
for other you can more complexe stuff.

So what you want the value at vertices only, or get the value of the coordinate of middle edge.

if you do this:

mesh Th=square(3,3);
fespace Vh(Th,P2);
Vh u = x*x+y*y; // just for the test
int[int] I(Vh.ndof),II(Vh.ndof);
I=-1;
II=-1;
for(int k=0; k< Th.nt; ++k)
for(int j=0; j<6; ++j)
{
int dof = Vh(k,j); // dof number of dof j ok k
// the 3 first dof are 3 vertices
// the 3 first dof one 3 edges
if( j <3 ) I[dof] =Th[k][j]; // the vertex number
else { //on edge e =j-3
int e = j-3;
int i1 = (e+1)%3,i2=(e+2)%3; // vertex of edeg on triangle
int j1= Th[k][i1],j2= Th[k][i2]; // the 2 vertex number of edge
if( j2 < j2) swap(j1,j2);
I[dof] =j1;
II[dof] =j2;
}
}
for(int dof =0; dof< I.n;++dof)
cout << dof << " :: " << I[dof] << " " << II[dof] << endl;