# List of edges on the mesh

**URL:** <https://community.freefem.org/t/list-of-edges-on-the-mesh/1841>\
**Category:** General Discussion\
**Created:** [June 23, 2022, 5:30am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841 "2022-06-23T05:30:49Z")\
**Posts on this page:** 16\
**Page:** 1

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**Author:** ![mariancantal](https://avatars.discourse-cdn.com/v4/letter/m/a9adbd/32.png) [@mariancantal](https://community.freefem.org/u/mariancantal)\
**Post date:** [June 23, 2022, 5:30am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/1 "2022-06-23T05:30:49Z")

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Hi! Is there a command in freeFEM where we can list all edges on the mesh? both internal and on the boundary?

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<div class="post-metadata">

**Author:** ![larismartins](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/larismartins/32/2712_2.png) [@larismartins](https://community.freefem.org/u/larismartins)\
**Post date:** [September 8, 2023, 2:55pm UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/2 "2023-09-08T14:55:54Z")

</div>

Did you found out how to do this?

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<div class="post-metadata">

**Author:** ![ksugahar](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/ksugahar/32/2115_2.png) [@ksugahar](https://community.freefem.org/u/ksugahar)\
**Post date:** [September 10, 2023, 5:48am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/3 "2023-09-10T05:48:36Z")

</div>

If you use the “savemesh” command, all the edges are listed up in the mesh file.

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<div class="post-metadata">

**Author:** ![larismartins](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/larismartins/32/2712_2.png) [@larismartins](https://community.freefem.org/u/larismartins)\
**Post date:** [September 25, 2023, 10:23am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/4 "2023-09-25T10:23:28Z")

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but only the boundary edges, not the internal ones

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<div class="post-metadata">

**Author:** ![marchywka](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@marchywka](https://community.freefem.org/u/marchywka)\
**Post date:** [September 25, 2023, 10:43am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/5 "2023-09-25T10:43:27Z")

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There is “intAllEdges” . In the saved mesh you also get triangles so you could  
look at every pair of points for each element in some other code. I wrote  
c++ code to load the meshes for my own stuff and it seems to be  
pretty simple but its not part of FF and not sure if there is another way  
to do what you want.

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<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [September 25, 2023, 3:41pm UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/6 "2023-09-25T15:41:09Z")

</div>

The idea is simple, use a finite element with one DOF par edge ,  
for example P0edge  
a exemple

[buildedge.edp](https://community.freefem.org/uploads/short-url/rhKknUSVWUAsDGd7Qo7vKVSVHqC.edp) (377 Bytes)

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<div class="post-metadata">

**Author:** ![marchywka](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@marchywka](https://community.freefem.org/u/marchywka)\
**Post date:** [September 25, 2023, 7:27pm UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/7 "2023-09-25T19:27:56Z")

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So Eh(int,int) returns an int and the parameters are element and node rahter than  
x,y as is the case in Ed(real,real)?  
That is good concise syntax but may not be obvious.  
Thanks.

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<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [September 28, 2023, 9:07am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/8 "2023-09-28T09:07:14Z")

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You want to have for a space position the edge number,

do can do some thing like:

```auto
Eh num;
num[]=0:num[].n-1; // put lhe value of the dof as the dof number. ie : num[][i] == i

so if x,y is on edge the num(x,y) is the dof number but due to roundoff error 
use round(num(x,y)) if abs( round(num(x,y)) -num(x,y) ) < 1e-3 . 

```

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<div class="post-metadata">

**Author:** ![larismartins](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/larismartins/32/2712_2.png) [@larismartins](https://community.freefem.org/u/larismartins)\
**Post date:** [September 28, 2023, 9:05pm UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/9 "2023-09-28T21:05:16Z")

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I’m trying to extract all edges from this exemple. But when I use the comand “extract” it only gives me the boundary edges and not all 5 edges from the mesh Th. Is there a way I can build a meshL with all five edges?

```auto
load "msh3"

mesh Th = square(1,1);

int NbTriangles = Th.nt;

plot(Th, wait=1, fill=1);

meshL ThEdge = extract(Th);
ThEdge = trunc(ThEdge, 1, split=1);

plot(ThEdge, wait=1);

fespace Eh(ThEdge, P0);
Eh xx = x, yy=y;

int nDoFl0 = Eh.ndof;
int nDoFl0K = Eh.ndofK;

cout << nDoFl0 << "\n";

```

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<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [September 29, 2023, 6:26am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/10 "2023-09-29T06:26:04Z")

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Yes the command extract extraterrestrial the boundary mesh.  
this is why a given a exemple to build all edges. cf [buildedge.edp](https://community.freefem.org/uploads/short-url/rhKknUSVWUAsDGd7Qo7vKVSVHqC.edp)

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<div class="post-metadata">

**Author:** ![larismartins](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/larismartins/32/2712_2.png) [@larismartins](https://community.freefem.org/u/larismartins)\
**Post date:** [September 29, 2023, 10:12am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/11 "2023-09-29T10:12:34Z")

</div>

and is it possible to build a mesh from this example you provided?

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<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [September 29, 2023, 10:55am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/12 "2023-09-29T10:55:23Z")

</div>

So you want a meshL build for the edge of a mesh, to day not simple way

But why you need that?

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<div class="post-metadata">

**Author:** ![larismartins](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/larismartins/32/2712_2.png) [@larismartins](https://community.freefem.org/u/larismartins)\
**Post date:** [September 29, 2023, 10:57am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/13 "2023-09-29T10:57:59Z")

</div>

Yes, I need a meshL from all the edges of a mesh.  
I have a method that is defined on the boundaries so I can build a FE space using edge partition

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<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [October 1, 2023, 8:45am UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/14 "2023-10-01T08:45:49Z")

</div>

To day the only is to build a file form my example an read the girl like in example:

3dCurve/extractMeshes.edp

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<div class="post-metadata">

**Author:** ![larismartins](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/larismartins/32/2712_2.png) [@larismartins](https://community.freefem.org/u/larismartins)\
**Post date:** [October 4, 2023, 7:06pm UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/15 "2023-10-04T19:06:37Z")

</div>

I’ve managed to create a meshL from all the edges, it works perfectly fine when I build a P0 finite element space over this mesh, but when I try with P1, it does not work, the number of DoFs should be 2 per edge, so 10, and it’s giving me 4

Am I doing something wrong?

```auto
load "msh3"
int EkOrder = 1;

//------------------Global Mesh---------------------
int globalH = 2^0;
mesh calP = square(globalH,globalH);

int NbTriangles = calP.nt; //number of triangles
int NbVertices = calP.nv; //number of vertices
int NbBndyEdges = calP.nbe; //number of boundary edges
int NbBndy = 3*NbVertices - 3 - NbBndyEdges; //number of edges
int NbIntEdges = NbBndy - NbBndyEdges; //number of internal edges

//-------------------Mesh info---------------------
//-------------------------vector: nodes in elem K
real[int,int] elemNodesGlobal(NbTriangles,3);
for (int i = 0; i < NbTriangles; i++){
  for(int j = 0; j < 3; j++)
    elemNodesGlobal(i,j) = int(calP[i][j]);
}
//cout << elemNodesGlobal <<"\n";

real[int,int] elemCoordNodesGlobal(NbVertices, 2);
for(int i=0; i<NbVertices; i++){
    elemCoordNodesGlobal(i,0) = calP(i).x;
    elemCoordNodesGlobal(i,1) = calP(i).y;
}
// cout << elemCoordNodesGlobal << "\n";

// ----------------DoF of V0 space-------------------
fespace V0(calP,P0);
V0 u0;
V0 u0x = x, u0y = y;

int nDoFu0 = V0.ndof; //number of DoF
int nDoFu0K = V0.ndofK; //number of DoF per elem

//function to identify global elements
for(int i = 0; i < NbTriangles; i++){
  u0[][i] = i;
}

//----------------edge orientation matrix per elem
int[int,int] edgeOrient(NbTriangles, 3);
for (int k=0; k<NbTriangles;k++){
    for(int i=0; i<3;i++){
        int i1 = calP[k][(i+1)%3];
        int i2 = calP[k][(i+2)%3];
        int orientation = i1 < i2 ? 1 : -1;
        edgeOrient(k,i) = orientation;
    }
}
// cout << "Edge sign" << "\n";
// cout << edgeOrient << "\n";

//------------------------------- local meshes
mesh[int] Th(NbTriangles);
meshL[int] EdgeH(NbTriangles);

//mesh size
real hSubMesh;

int[int] edgeLabel(6);
int[int,int] edgeSignG(NbTriangles, 6);

for(int K = 0; K < NbTriangles; K++){
    real coordX0, coordX1, coordX2;
    real coordY0, coordY1, coordY2;

    for(int i = 0; i< NbVertices; i++){
        if(elemNodesGlobal(K,0) == i){
            coordX0 = calP(i).x;
            coordY0 = calP(i).y;
        }
        if(elemNodesGlobal(K,1) == i){
            coordX1 = calP(i).x;
            coordY1 = calP(i).y;
        }
        if(elemNodesGlobal(K,2) == i){
            coordX2 = calP(i).x;
            coordY2 = calP(i).y;
        }
    }

    real[int] elemNodesGlobaltoLocal = elemNodesGlobal(K,:);
    // cout << elemNodesGlobaltoLocal << "\n";

    real[int, int] nodes = [[coordX0, coordY0], [coordX1,coordY1], [coordX2, coordY2]];

    border f0(t=0,1){P.x=nodes(2,0)*t + nodes(1,0)*(1-t);P.y=nodes(2,1)*t + nodes(1,1)*(1-t); label=11;};
    border f1(t=0,1){P.x=nodes(0,0)*t + nodes(2,0)*(1-t);P.y=nodes(0,1)*t + nodes(2,1)*(1-t); label=22;};
    border f2(t=0,1){P.x=nodes(1,0)*t + nodes(0,0)*(1-t);P.y=nodes(1,1)*t + nodes(0,1)*(1-t); label=33;};

    mesh ThK = buildmesh(f0(1) + f1(1) + f2(1));
    Th[K] = trunc(ThK, abs(u0 -K) < 1e-5, split=2^0);
    mesh auxTh = Th[K];
    int NbTrianglesLocal = Th[K].nt;

    fespace V0k(auxTh, P0);
    V0k hk = hTriangle;

    hSubMesh = hk[].max;

    int sizeVecEdge;

    if(EkOrder == 0){
        edgeLabel = [11,22,33];
        sizeVecEdge = 3;
    }
    if(EkOrder == 1){
        edgeLabel = [11,11,22,22,33,33];
        sizeVecEdge = 6;
    }
    edgeLabel.resize(sizeVecEdge);
  

    for(int i=0; i<edgeLabel.n; i++){
        if(edgeLabel(i) == 11){
            edgeSignG(K,i) = edgeOrient(K,0);
        }
        else if(edgeLabel(i) == 22){
            edgeSignG(K,i) = edgeOrient(K,1);
        }
        else if(edgeLabel(i) == 33){
            edgeSignG(K,i) = edgeOrient(K,2);
        }
    }
    edgeSignG.resize(NbTriangles,sizeVecEdge);

    EdgeH[K] = extract(ThK);
    EdgeH[K] = trunc(EdgeH[K], 1, split=2^0);    

}

// cout << edgeLabel << "\n";
// cout << edgeSignG << "\n";

meshL Ehg;
for(int K=0; K<NbTriangles; K++){
    Ehg = Ehg + EdgeH[K];
}
// plot(Ehg, wait=1);

fespace EdgeSpace(Ehg, P1);
EdgeSpace ex = x, ey = y;

int ndofEh = EdgeSpace.ndof;
int NbEdgesK = EdgeSpace.ndofK;
int NbPartinEdge;

cout << "Number of DoFs: " << ndofEh << "\n";
cout << "Number of DoFs in each edge: " << NbEdgesK << "\n";

```

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<div class="post-metadata">

**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [October 4, 2023, 7:52pm UTC](https://community.freefem.org/t/list-of-edges-on-the-mesh/1841/16 "2023-10-04T19:52:12Z")

</div>

You are wrong, for me the for P1, on edges the dof are the vertices not 2 times the edges in this case.

My code more simple I think,

[buildedge-BuildMeshL.edp](https://community.freefem.org/uploads/short-url/zwAh5JJLe4c66A3BAegQubIyI5M.edp) (967 Bytes)