MATLAB: Calculate the annual discharge over several years

annual mean

Hello,
I have a time series dataset that consists of daily discharge (Q) data from 1915-2006. My matrix: the first column is the year, the second is the month, the third is the day and the fourth column is the data of interest (Q). I need to calculate the mean annual discharge for each year and then arrange the data from the smallest Q to largest Q but knowing to which year that Q belongs.
  • Year Month day Q
  • 1915 10 1 2.13E+02
  • 1915 10 2 2.89E+02
  • 1915 10 3 8.10E+02
  • …
  • …
  • 2006 12 31 2.20E+04
I am very new at Matlab and even though I found some similar examples online I can't figure this out. Any help will be really appreciated.
Thanks!

Best Answer

EDIT after you edited the question: it seems that there are 4 columns and not 2, and the discharge is in column4. I am updating my answer accordingly.
Here is an example
 data = [1915,  7,  1, 6; ...
         1915, 10,  3, 8; ...
         1915, 12,  8, 7; ...
         1916,  4, 22, 3; ...
         1916,  7, 13, 2] ;
 % - Get unique years and assign IDs starting at 1.
 years  = unique( data(:,1) ) ;
 yearId = data(:,1) - min( data(:,1) ) + 1 ;
 % - Compute Q mean per year.
 Qmean  = accumarray( yearId, data(:,4), [], @mean ) ;
 % - Sort Qmean and get corresponding years.
 [Qmean_sorted, ix] = sort( Qmean ) ;
 years_sorted = years(ix) ;
Running this, you get
 >> Qmean 
 Qmean =
        7.0000
        2.5000
 >> Qmean_sorted
 Qmean_sorted =
        2.5000
        7.0000
 >> years_sorted
 years_sorted =
        1916
        1915
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