# Maybe the new version of the 2019pde toolbox has bugs when building the grid.

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NAN YANG on 11 Jun 2019
Commented: NAN YANG on 16 Jun 2019
r=6;rr=0.2;
a=0.8;b=0.4;
%% Geometry
scattering_model = createpde;
%%
% Define a circle in a rectangle, place these in one matrix, and create a
% set formula that subtracts the circle from the rectangle.
C1 = [1,0,0,r]';
C2 = [1,0,0,rr]';
E2 = [4,0,0,a,b,0]';
C1 = [C1;zeros(length(E2)-length(C1),1)];
C2 = [C2;zeros(length(E2)-length(C2),1)];
gm = [C1,E2,C2];
sf = 'C1+E2+C2';
%%
% Create the geometry.
ns = char('C1','E2','C2');
ns = ns';
g = decsg(gm,sf,ns);
%%
% Include the geometry in the model and plot it.
geometryFromEdges(scattering_model,g);
pdegplot(scattering_model,'EdgeLabels','on')
axis equal
generateMesh(scattering_model,'Hmax',0.1);
pdeplot(scattering_model)
%%
V=scattering_model.Mesh.Nodes';
F=scattering_model.Mesh.Elements';
F1=F(:,1:3);F2=F(:,4:6);F=[F1;F2];
F=unique(F,'rows');
When I was using matlab 2016b, there was no problem in building the grid. The same program appeared in the 2019a version with a significant error. The 2019a has the wrong grid as follows：
The green point in the figure is the adjacent point of the red point. There should be no such arrangement in the correct grid F.
And this kind of grid is not allowed in finite elements, right? Caused a lot of trouble for subsequent programming.

Ravi Kumar on 13 Jun 2019
I am not sure I understand the issue. Can you please elobrate what I should be looking to notice to problem?
NAN YANG on 16 Jun 2019
Hi kumar, in the 2019a version, generateMesh iterates out the grid shown in the last picture. I think this kind of grid can't be used as a finite element mesh, and the behavior of storage of elements information compared with which in the previous version 2016b is not quite the same, this has caused a lot of trouble for subsequent programming.

Gaurav Tyagi on 11 Jun 2019
Yes

R2019a

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