In this line, you have the entire array Asim_door (divided by A_fg) on the right side, trying to assign it to a subset on the left side. That's the problem.
Asim_door(fgsim>=1 & Esim4>=n_fg)=(Asim_door./A_fg); %fg installed, Esim above n_fg then A_door/A_fg flow area available
You have to use the same indices on both sides.
So if A_fg is a scalar:
idx = fgsim>=1 & Esim4>=n_fg;
Asim_door(idx) = Asim_door(idx)./A_fg; %fg installed, Esim above n_fg then A_door/A_fg flow area available
Or if A_fg is an array the same size as Asim_door:
idx = fgsim>=1 & Esim4>=n_fg;
Asim_door(idx) = Asim_door(idx)./A_fg(idx); %fg installed, Esim above n_fg then A_door/A_fg flow area available
What's the difference between doing it this way and an if statement?
Doing it this way (using logical indexing) is more concise and likely to be faster than using an if statement. With an if, you'd need to loop over the elements, e.g., if A_fg, n_fg and fgsim are arrays the same size as Asim_door:
for ii = 1:numel(Esim4)
if fgsim(ii) >= 1 && Esim4(ii) >= n_fg(ii)
Asim_door(ii) = Asim_door(ii)./A_fg(ii);
end
end
and if any of them are scalars, remove the (ii) indexing for those (and you could likely simplify the logic, e.g., if fg_sim is a scalar, you do not need to perform the loop at all if fg_sim < 1).
Note that the logical indexing expression
idx = fgsim>=1 & Esim4>=n_fg;
works the same regardless of whether n_fg and/or fgsim are scalars because of how & works:
true & [false false true]
[true true true] & [false false true]
