Correct indices of vertices in face matrix

Hi,

I have to split up a mesh, into region F_feCarOUT and F_feCarIN acording to the array logicFCinside.

But i don't know how to adjust the indices of the vertices in the Faces matrix in a fast manner (not using the for loop).

Thank you in advance

F_feCarOUT=F_feCar(any(~logicFCinside(F_feCar),2),:);

F_feCarIN=F_feCar(all(logicFCinside(F_feCar),2),:);

V_feCarOUT_Cor=V_feCar(unique(F_feCarOUT),:);

for i= 1:length(F_feCarOUT(:))

F_feCarOUT(i)=find(ismember(V_feCarOUT_Cor,V_feCar(F_feCarOUT(i),:),'rows'));

end

Answer 1

[Yash]

Hi Frederic,

I understand that you want to store the indices efficiently and want to eliminate the "for loop". Here is a way to do this:

F_feCarOUT=F_feCar(any(~logicFCinside(F_feCar),2),:);
F_feCarIN=F_feCar(all(logicFCinside(F_feCar),2),:);
V_feCarOUT_Cor=V_feCar(unique(F_feCarOUT),:);
[~,F_feCarOUT(:)] = ismember(V_feCar(F_feCarOUT(:),:),V_feCarOUT_Cor,'row');

We can measure the elapsed time using "tic" and "toc" functions to measure the improvement from the "for loop".

tic
for i= 1:length(F_feCarOUT(:))
    F_feCarOUT(i)=find(ismember(V_feCarOUT_Cor,V_feCar(F_feCarOUT(i),:),'rows'));
end
toc
% Elapsed time is 26.955875 seconds.

In order to assign each element of "F_feCarOUT", first convert "F_feCarOUT" into a column vector using the colon operator and assign the output of the ismember function to this column vector. The colon operator (:) just reshapes all the elements of the array into a single column vector.

tic
[~,F_feCarOUT(:)] = ismember(V_feCar(F_feCarOUT(:),:),V_feCarOUT_Cor,'row');
toc
% Elapsed time is 0.014447 seconds.

Refer here for more information on colon operator: https://www.mathworks.com/help/matlab/ref/colon.html#:~:text=A(%3A)%20reshapes%20all%20elements%20of%20A%20into%20a%20single%20column%20vector.%20This%20has%20no%20effect%20if%20A%20is%20already%20a%20column%20vector.

I hope this helps!