Note that you change the result outside the loop:
for i=1:u
t(i)=atan(Dy(i)/Dx(i))*200/pi;
end % <-- remove this
if Dx(i)>0 & Dy(i)>0
t(i)=t(i) % Omit this one :-)
elseif Dx(i)<0 & Dy(i)>0
t(i)=t(i)+200
elseif Dx(i)<0 & Dy(i)<0
t(i)=t(i)+200;
elseif Dx(i)>0 & Dy(i)<0
t(i)=t(i)+400;
end
% end % <-- insert an "end" here
When processed after the loop, only the last value of "i" is accessed.
Logical indexing will be more efficient:
t = atan(Dy ./ Dx) * (200/pi); % elementwise: ./
index = (Dx<0 & Dy>0);
t(index) = t(index) + 200;
and so on.
