Exporting face-vertex mesh to STL - matlab

Question

I'm trying to add thickness to a gyroid mesh and export this to STL. I can successfully plot the thickened gyroid mesh as shown below, but this is because I wrote a function to remove undesirable faces, namely, faces which have edges longer than a certain limit. I should also mention that all faces are triangular, so both the vertices and faces matrices are in N x 3 form.
Here's the code which checks for edge lengths and removes those which are too_long:
function [F] = remove_bad_faces(F,V,too_long)
%% Initiate various counters/storage matrices
bad_face_count = 0; % Number of bad faces
bad_faces = []; % Matrix of bad faces
bad_vertices = []; % Matrix of bad vertices
bad_face_indexes = []; % Matrix of bad face row indices. I know that the plural of index is not indexes.
%% Find all bad faces
for i=1:size(F,1)
face = F(i,:);
v1 = V(face(1),:);
v2 = V(face(2),:);
v3 = V(face(3),:);
if norm(v1-v2) >= too_long || norm(v2-v3) >= too_long || norm(v1-v3) >= too_long
bad_face_count = bad_face_count + 1;
bad_faces = [bad_faces; face];
bad_vertices = [bad_vertices;v1;v2;v3];
bad_face_indexes = [bad_face_indexes,i];
end
end
%% Remove bad faces
for j=1:size(bad_face_indexes,2)
index = bad_face_indexes(j);
F(index,:) = nan;
end
end
As you can see, I'm replacing "bad faces" (i.e. rows of the F matrix) with NaN, but this only works for visualisation. When I try to pass the resultant faces and vertices to an STL generating function (see here), I get the following error:
Index in position 2 is invalid. Array indices must be positive integers or logical values.
Error in stlwrite (line 76)
facets = reshape(facets(:,faces'), 3, 3, []);
Error in Gyroid_Mesh_Script (line 60)
stlwrite('gyroid_test.stl', F, V)
So clearly the NaN is the problem, and I've tried swapping it with [] and 0, but neither has worked. Any help is much appreciated, thanks in advance!
Edit:
When I use [] instead of NaN, I can export to STL, but it includes these "bad faces", shown below (zoomed-in view):

Answer 1

Thanks to #benJephunneh's response! That fixed everything. I'm posting the updated function which removes faces whose edges are too_long:
function [F] = remove_bad_faces(F,V,too_long)
%% Initiate various counters/storage matrices
bad_faces = []; % Matrix of bad faces
bad_vertices = []; % Matrix of bad vertices
bad_face_indexes = []; % Matrix of bad face row indices. I know that the plural of index is not indexes.
%% Find all bad faces
for i=1:size(F,1)
face = F(i,:);
v1 = V(face(1),:);
v2 = V(face(2),:);
v3 = V(face(3),:);
if norm(v1-v2) >= too_long || norm(v2-v3) >= too_long || norm(v1-v3) >= too_long
bad_faces = [bad_faces; face];
bad_vertices = [bad_vertices;v1;v2;v3];
bad_face_indexes = [bad_face_indexes,i];
end
end
bad_face_count = numel(bad_face_indexes);
%% Remove bad faces
F(bad_face_indexes,:) = [];
end
It also successfully exports to STL using stlwrite('gyroid_test.stl', F, V)