Conversion to struct from double is not possible - matlab

Question

I have two questions
(1) Why is the following code generating this error ? where does double come from?
Error using horzcat
The following error occurred converting from double to struct:
Error using struct
Conversion to struct from double is not possible.
Error in remove_scratch (line 34)
old1 = [lines1, max_vertex1] ;
(2) how to get rid of this error?
Relevant Source Code
for n = 1:N
% take n-th image
hough_trf_input = monochrome_image(:,:,n);
% find straight lines in the image
[hough_lines, thetas] = hugh_transform(hough_trf_input);
%////////////////////////////////////////////////////////////
[lines1, max_vertex1] = find_lines(hough_lines, thetas(1));
[lines2, max_vertex2] = find_lines(hough_lines, thetas(2));
[lines3, max_vertex3] = find_lines(hough_lines, thetas(3));
if(n==1)
old1 = [lines1, max_vertex1];
old2 = [lines2, max_vertex2];
old3 = [lines3, max_vertex3];
else
oldlen1 = vertex_length(old1(2));
oldlen2 = vertex_length(old2(2));
oldlen3 = vertex_length(old3(2));
newlen1 = vertex_length(max_vertex1);
newlen2 = vertex_length(max_vertex2);
newlen3 = vertex_length(max_vertex3);
if(newlen1 > oldlen1)
old1 = [lines1, max_vertex1] ;
end
if(newlen2 > oldlen2)
old2 = [lines2, max_vertex2] ;
end
if(newlen3 > oldlen3)
old3 = [lines3, max_vertex3] ;
end
end
end
Here find_lines() returns a vector whose 1st element is a vector of line-structs, and the second element is a vector of two vectors where each vector is 2-element and represents an axis.

Answer 1

one of the variables {lines1, max_vertex1} is struct and the other is double. from their names I guess lines1 is struct and max_vertex1 is double. You can obtain the same error when running:
clear
% lines1 is struct
lines1.p1 = 1;
lines1.p2 = 1;
% max_vertex1 is double
max_vertex1 = 5;
old1 = [lines1, max_vertex1] ;
There are several ways to overcome this, for example you can extract the struct fields into double using struct2array:
old1 = [struct2array(lines1), max_vertex1] ;