# How to calculate a cumulative sum in a loop?

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Sandy on 20 Nov 2013
Commented: Walter Roberson on 21 Nov 2013
I have a loop that looks like this:
for a = 1: 9
for b = 1 : 1000
for c = 1 : 1000
image = myimage_bands(b, c, a);
new_image = image * 5;
end
end
end
I am trying to find the cumulative sum of my variable 'new_image', as it goes through the loop. I've tried cumsum, but it doesn't work.
Any ideas are appreciated. Thanks!
Azzi Abdelmalek on 20 Nov 2013
Edited: Azzi Abdelmalek on 20 Nov 2013
There is no cumulative sum in your code, can you explain what you want?

Walter Roberson on 21 Nov 2013
Before the loop,
image_sum = [];
inside the loop after you have calculated new_image,
if isempty(image_sum)
image_sum = new_image;
else
image_sum(end+1) = image_sum(end) + new_image;
end
Sandy on 21 Nov 2013
Thank you! I have another question though. How would I change the code if my variable new_image was a matrix (6 x 6)?
Walter Roberson on 21 Nov 2013
image_sum(:,:,end+1) = image_sum(:,:,end) + new_image;