how to select data based on month

Question

Mohammed Kamruzzaman 2015-4-7

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https://ww2.mathworks.cn/matlabcentral/answers/196609-how-to-select-data-based-on-month

评论： Mohammad Abouali 2015-11-4

I have thousand of data points (Nx2, col 1=date and col 2=data)for a year for different dates for different sites. I want to make an average for the data for each month. My data are in excel and and date format is yyyymmdd (attached image).I easily can sort data suing sortrows and then I can calculate the average as below. But this is not an efficient method.Looking for an efficient method. Thanks in advance. M1=mean(data(1:30,2)); M2=mean(data(31:69, 2)); M3=mean(data(70:112, 2)); ..............

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Answer 1

Mohammad Abouali 2015-4-7

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https://ww2.mathworks.cn/matlabcentral/answers/196609-how-to-select-data-based-on-month#answer_174378

编辑：Mohammad Abouali 2015-4-7

在 MATLAB Online 中打开

sample.csv

Here is what you should do:

I have assumed that you store those two columns in a csv file as follow:

% reading the CSV File
data=csvread('sample.csv');
% extracting data into a MATLAB Table variable
T=table();
T.year=floor(data(:,1)/10000);
T.mm=floor((data(:,1)-T.year*10000)/100);
T.dd=data(:,1)-T.year*10000-T.mm*100;
T.value=data(:,2);

Now To get yearly average:

grpstats(T,'year','mean','DataVars',{'value'})
ans = 
              year    GroupCount    mean_value
              ____    __________    __________
      2010    2010    365           0.49589   
      2011    2011    365           0.51351   
      2012    2012    366           0.50263   
      2013    2013    365           0.52623   
      2014    2014    365           0.51586   
      2015    2015    360            0.5009

To get monthly average

grpstats(T,{'year','mm'},'mean','DataVars',{'value'})
ans = 
                 year    mm    GroupCount    mean_value
                 ____    __    __________    __________
      2010_1     2010     1    31             0.5204   
      2010_2     2010     2    28            0.57318   
      2010_3     2010     3    31              0.494   
      2010_4     2010     4    30            0.41664   
      2010_5     2010     5    31            0.49894   
      2010_6     2010     6    30            0.47679   
      2010_7     2010     7    31            0.46417   
       // I have chopped rest of the results.