obtaining the average value for a data set

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Hello, I have a data set which contains a value for a number of stations during some years (station, date, value). Data for some days is missing. I need to create a column with annual average value per station. I appreciate your help.

station date value average(yearly per station)

1 2001/1/1 2 2

1 2001/1/3 1 2

1 2001/1/5 3 2

2 2001/1/1 2 2.5

2 2001/1/4 3 2.5

1 2002/1/2 2 1.5

1 2002/1/6 1 1.5

2 2002/1/1 2 2.5

2 2002/1/2 3 2.5 . .

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Azzi Abdelmalek 2012-9-4

编辑：Azzi Abdelmalek 2012-9-4

can you specify the size of your data array, cell or .., how should the result looks like?

FATEMEH 2012-9-4

sorry for the misspecification. The last column is the column that should be created(average). My data has the form of matrix. I have 3 columns for year month and day. To be precise, a typical row has the form of: ( I separated the elements by *)

station* year* month* day*value

1 * 2001 * 1 * 1 * 2

and I want to add a column with yearly average per station:

station* year* month* day*value* average(yearly per station)

1 * 2001 * 1 * 1 * 2 *2

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Answer 1

Matt Tearle 2012-9-4

编辑：Matt Tearle 2012-9-4

在 MATLAB Online 中打开

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It looks like the fourth column is what you want to calculate. If so, this will do the job, assuming that you have the first three columns as station, date, and value (numeric vectors except date which is a cell array of strings)

% get the years from the date strings
dv = datevec(date);
year = dv(:,1);
% get all combinations of station & year
[stlist,yrlist] = meshgrid(unique(station),unique(year));
% allocate space for the calculated average
avg = zeros(size(station));
% loop over every station/year combination
for k = 1:numel(stlist)
    % find the location of this combination in the data set
    idx = (year == yrlist(k)) & (station == stlist(k));
    % calculate the mean
    avg(idx) = mean(value(idx));
end

This could probably be simplified with prior knowledge of the station numbers and years.

EDIT TO ADD: Given your comment above, ignore the first couple of lines -- you already have year.

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Sean de Wolski 2012-9-6

Why would you want to avoid the loop? The loop will likely be very fast and is easy to read/understand.

Matt Tearle 2012-9-6

Possibly, although I doubt it will be neat. More importantly, why do you want to avoid loops?

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obtaining the average value for a data set

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