How can I find the yearly average with repeating years?

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msstarr
msstarr 2015-5-8
评论： msstarr 2015-5-8
I have 55170x1 datetime matrix and 55170x1 water_level numeric matrix. The datetime is in years, repeating the same year through many consecutive cells before moving on chronologically. I want to find the mean water level for each year.

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采纳的回答

Mohammad Abouali
Mohammad Abouali 2015-5-8
编辑：Mohammad Abouali 2015-5-8
use grpstats()
meanYearly=grpstats(waterLevel,year);
EDIT: I really recommend using grpstats, but if somehow you don't want to use it, or if you don't have statistics toolbox then alternatively you can compute mean yearly as follow:
meanYearly= arrayfun( @(yy) mean(waterLevel(year==yy)), unique(year));

更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2015-5-8
编辑：Andrei Bobrov 2015-5-8
years = sort(randi([2013,2015],10,1));
water_level = randi([20 50],10,1);
[a,~,c] = unique(years);
out = [a,accumarray(c,water_level,[],@mean)];
with loop for..end
[n,ii] = sort(years(:));
water_level2 = water_level(ii);
yrs = years([true;diff(n)~=0]);
m = numel(yrs);
out2 = zeros(m,2);
for jj = 1:m
out2(jj,:) = [yrs(jj), mean(water_level2(yrs(jj) == years))];
end
Image Analyst
Image Analyst 2015-5-8
Well here's one way, though accumarray can take a bit of study to understand what it's doing.
% Create random years in the range 2007 - 2015
years = randi([2007,2015], 100, 1)
% Create water levels in the range 2007 - 2015
water_level = rand(length(years), 1)
% Get the sums in each year.
yearlySums = accumarray(years - min(years) + 1, water_level)
% Count the number of each year.
counts = histcounts(years, [2007:2015+1])
% Divide to get the average in each year.
yearlyAverages = yearlySums ./ counts'
Make sure you know which are row vectors and which are column vectors if you modify this - make them all the same.
  2 个评论
msstarr
msstarr 2015-5-8
编辑：msstarr 2015-5-8
I am not getting the correct answer. Sorry, I am a beginning with using matlab but I do sort of understand your method. Each year isn't consistent with the same amount of values (for example: 2015 has 75 water level points and 2014 only has 36).
Image Analyst
Image Analyst 2015-5-8
Not sure what that means. My code does create values because you did not attach your data so I had to, if I wanted to test my code.
I was afraid you wouldn't understand - accumarray is not some slam dunk obvious function to understand, especially for beginners. So in that case, just do a for loop . It's the dumb, brute force method , but at least it's intuitive and easy to understand and write. If you can't even do a for loop yet, then read this and let us know if you still need help after that.

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