# I assign A = B; but I could not use A and B interchangeably

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Khoa Nguyen on 6 Sep 2019
Commented: Matt J on 6 Sep 2019
Hi everyone,
I'm trying to make a 3D median filter for a 3D CT image, here is my code:
function I_fted = abc(img, mar)
I_fted = img;
% For-loop to loop through each pixel in x,y,z directions and filtration
for y = 1+mar:size(img,1)-mar %loop through the vertical direction
for x = 1+mar:size(img,2)-mar %loop through the horizontal direction
for z = 1+mar:size(img,3)-mar %loop through the slices
grid = img(y-mar:y+mar, x-mar:x+mar,z-mar:z+mar); %create a cubic grid [mar x mar x mar]
I_fted(y,x,z) = median(grid(:)); %calculate the median of the pixels inside cubic grid
end
end
end
end
I did assign I_fted = img. However, when I use the below commands, the result images are different for each case. I don't know why
% Case 1:
grid = img(y-mar:y+mar, x-mar:x+mar,z-mar:z+mar); %this give different result to the below
% Case 2:
grid = I_fted(y-mar:y+mar, x-mar:x+mar,z-mar:z+mar);

#### 1 Comment

Walter Roberson on 6 Sep 2019
At what point do they become different? You are changing I_fted
By the way, have you considered using nlfilter()

James Tursa on 6 Sep 2019
You are changing I_fted inside the loop. If you subsequently use it in another calculation within the loop, it would not be surprising that the results are different from what you would get using img, which was not changed within the loop.

Khoa Nguyen on 6 Sep 2019
Thanks a lot, it makes sense
Matt J on 6 Sep 2019
@Khoa,
Since you seem to be satisfied with James' answer, you should Accept-click it.