You do not use left as a vector and it is not replied as output also. Then omit the "(i)":
function y = ratioimg(u)
[L2,num2] = bwlabel(u,8); %連通區域標記
y = zeros(size(u)); % Better than creating B2 and copy it to y
for i=1:num2
[r,c] = find(L2==i);
left = min(c);
right = max(c);
up = min(r);
down = max(r);
d = (down-up) / (right-left);
if d > 0.8 & d < 1
% [x,y]=find(L2==i); % Done above already
y = y + bwselect(u, c, r, 8); %%%把滿足長寬比在0.8到2的區域留下
end
end
end
