Creating a monthly average from a matrix

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Kate
Kate 2013-8-20
My data, which has 30 years of monthly data, over a 2 degree grid of the globe:
>> whos KoppenA_prate
Name Size Bytes Class Attributes
KoppenA_prate 90x180x360 23328000 single
Not every pixel has data, just the pixels that fit into Koppen classification A
>> sum(A(:))
ans =
559
My question: Any ideas on how I could write a loop to create an average monthly value over the 30 years?
Thanks a bunch!
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Kate
Kate 2013-8-21
I would like to create a vector of precip data, for each month averaged over 30 years.
Kate
Kate 2013-8-21
This is as far as I've gotten, but my for loop isn't working:
%Create monthly averages over 30 years
Jan=[1:12:349];
Feb=[2:12:350];
Mar=[3:12:351];
Apr=[4:12:352];
May=[5:12:353];
Jun=[6:12:354];
Jul=[7:12:355];
Aug=[8:12:356];
Sep=[9:12:357];
Oct=[10:12:358];
Nov=[11:12:359];
Dec=[12:12:360];
for ii=length(num_mons)
if num_mons==Jan;
Jan_dat=[Jan,KoppenA_prate(3)];
elseif num_mons==Feb;
Feb_dat=[Feb,KoppenA_prate(3)];
end
end

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Cedric
Cedric 2013-8-21
编辑：Cedric 2013-8-21
Just
avgAllYears = mean(KoppenA_prate, 3) ;
where 3 is the dimension along which the mean must be computed. With that you get a 90x180 array of means over all times.
EDIT:
siz = size(KoppenA_prate) ;
avg = zeros(siz(1), siz(2), 12) ;
for mId = 1 : 12
avg(:,:,mId) = mean(KoppenA_prate(:,:,mId:12:end), 3) ;
end
EDIT2: or, more efficient but more complicated
siz = size(KoppenA_prate) ;
buf = mean(reshape(KoppenA_prate, siz(1), 12*siz(2), []), 3) ;
avg = reshape(buf, siz(1), siz(2), []) ;
Note: both produce a 90x180x12 array of monthly averages, were e.g. avg(:,:,5) is the average of 5th months over all years.
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Kate
Kate 2013-8-21
Thanks Cedric, that's great. How could I get a single, say Jan, value over the whole grid? I need to plot average yearly precip.
Thanks for all of your help!

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