# Problem with matrix indexing in for loop

2 views (last 30 days)
Parag on 29 Jun 2014
Commented: Image Analyst on 30 Jun 2014
HI guys
In this code, I am getting problem when I am trying to find index of object . In for loop if I comment out img=img+obj(x_f,y_f) then Index x_f and y_f shows from 1 to 512 but when I remove comment then indexes changed. I don't know how it is happening in for loop. Can anyone please help to sort out problem. I will appreciate your help
obj=zeros(512);
obj(230:250,230:250)=5;
%theta = pi/2*180;
figure;
subplot(2,2,1);
imshow(obj);
drawnow;
N=512;
x=[-N/2:N/2-1];
y=[-N/2:N/2-1];
theta=pi/180;
img=zeros(512);
for i=1:180
x_f=round( x .* cos(theta*i) + y.* sin(theta * i)+256);
y_f=round( y .* cos(theta*i)- x .* sin(theta * i)+256);
img=img+obj(x_f,y_f);
% subplot(2,2,2);
% imshow(img);
% drawnow;
end

per isakson on 29 Jun 2014
Edited: per isakson on 29 Jun 2014
Here are some links on debugging in Matlab
BTW:
>> x_f(1:5)
ans =
-4 -3 -2 -1 0
Try
>> dbstop if error

Image Analyst on 29 Jun 2014
x_f and y_f will not change depending on whether the img line is in there or commented out. However the values do go negative and you can't have negative indexes into img. What are you trying to do anyway? Draw sine waves into an image???
Image Analyst on 30 Jun 2014
I would be surprised if that produced a rotated image in labview.

Parag on 29 Jun 2014
I could not able to figure out the behavior of matlab in for loop. When I do that similar indexing without using for loop it work for me.
per isakson on 29 Jun 2014

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