# How do I calculate monthly mean of a 3-D matrix?

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Levente Samu on 12 Sep 2020
Commented: jonas on 13 Sep 2020
Hello all,
I have a 111x151x14243 matrix (lat, lon, time), called cmprecip, containing daily total precipitation data and I have a 14243x1 datetime array, called cmtime, in 'dd.MM:yyyy' format. I would like to calculate monthly mean from the daily data. So the output would be a 111x151x468 matrix. As a rookie in Matlab, my initial thought was to concatenate the matrix and the array and then use 'retime' just as I did before with 2-D arrays, but I was not able to make it work.
Is there a way to solve this issue? Thank you guys for your help in advance!
Levente

jonas on 12 Sep 2020
Using retime is one option, you would just have to shape your array into a timetable first, which is a bit of a hassle.
I think this should work as well
%some data
A = 1:10000;
A = reshape(A,10,10,100);
t = (linspace(datetime(2000,1,1),datetime(2005,1,1),100))';
%find unique month ids
[U,~,G] = unique([year(t),month(t)],'rows');
%preallocate output variable
out = nan(size(A,1),size(A,2),size(U,1));
%loop over months
for i = 1:size(U,1)
f = A(:,:,G==i);
out(:,:,i) = mean(f,3);
end
##### 2 CommentsShowHide 1 older comment
jonas on 13 Sep 2020
My pleasure!

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