# Problem with matrix indexing in for loop

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Parag il 29 Giu 2014
Commentato: Image Analyst il 30 Giu 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
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### Risposte (3)

per isakson il 29 Giu 2014
Modificato: per isakson il 29 Giu 2014
Here are some links on debugging in Matlab
BTW:
>> x_f(1:5)
ans =
-4 -3 -2 -1 0
Try
>> dbstop if error
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Image Analyst il 29 Giu 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???
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Parag il 29 Giu 2014
I did it using same logic in labview but here I am getting problems.Thanks for suggestion
Image Analyst il 30 Giu 2014
I would be surprised if that produced a rotated image in labview.

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Parag il 29 Giu 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.
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per isakson il 29 Giu 2014

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