Presuming FS and Depth are vectors, then you need to subscript them as, for example
if FS(i) < 1 && Depth(i)<20
in all expressions...otherwise FS<1 or Depth>=20 return logical vectors of size(FS) or size(Depth)
You can do the assignments using logical addressing without the loop...note as you've written the loop that the depth_all7 array will be equivalent to
depth_all7=zeros(sum(FS>=1 & Depth<=20,1));
so numel() will be the number of cases that satisfy the condition, not the same size as Fs. If that is the intent, that's fine.
The first conditional case is peculiar -- if LPi is a single value not a predefined array, then it is overwritten each pass through the loop and otherwise never used. One would guess that isn't the intent.
