MATLAB Answers


How can i compute the mean of specific number of table rows?

Asked by Ioannis Tsikriteas on 9 Aug 2017
Latest activity Commented on by Cong Ba
on 10 Aug 2017
Hi, I have a large amount of table data which is divided by date and be more specific i have data that is per 10 minutes and i want to compute the mean of them per hour! In other words, i want ot compute the mean of my data per 10 rows until my data is finished! Is this possible and if yes....which command should i use?


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3 Answers

Answer by Cong Ba
on 9 Aug 2017
 Accepted Answer

Try reshape:
a = randn(100,1); % assume this is the column you have
b = reshape(a,[10,10]); % reshape it so each column has 10 rows
avg = mean(b);


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let's assume you throw the first 2 rows...
a = randn(278984,2); % assume this is your table
a = a(3:end,:); % assme throw out the first two rows
b1 = reshape(a(:,1),[6, 278982/6]); % reshape the first column to [6, 278982/6] so it has 6 rows
avg1 = mean(b1);
b2 = reshape(a(:,2),[6, 278982/6]); % same to the second column
avg2 = mean(b2);
avg1 would be the hourly series you want for the first column of your data. avg2 would be for the second.
When i try the reshape command i get the following message: Error using tabular/reshape (line 150) Undefined function 'reshape' for input arguments of type 'table'.
To be more clear isend a pic of the shape of my data tables
Andrei is using the timetable function which may better suit your need (you essentially have a time series). But if you want to manipulate data in matrix form (my code works for matrix form), you can try this function: table2array

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Answer by Andrei Bobrov
on 9 Aug 2017

n = 278984;
t = minutes(10*(1:n)');
D = randi(255,n,2); % Let D - your data (278984 x 2)
TT = timetable(t,D(:,1),D(:,2));
TT_out = retime(TT,'hourly','mean');


I am sorry but i didn't understand the concept of these commands.
Mine data are already ona a table 278984x2 and has the following shape:
I am searching a way to compute the mean between the fifth and the eleventh row for example in order to have my data per hour, not per 10 minutes
a.x_10_double = [a.x_10_{:}]';
TT = table2timetable(a(:,[1,3]),'RowTimes','x08_Jul_100_10_00');
TT_out = retime(TT,'hourly','mean');
a2 = table(a{:,1},[a{:,2}{:}]','v',{'datetime','x_10_double'})
a2.groups = hour(a2{:,1});
a_out = varfun(@mean,a2,'Group','groups');

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Answer by Peter Perkins
on 10 Aug 2017

Add a grouping variable to your table and use varfun. Something like
n = ceil(height(t)/10);
g = repelem(1:n,10)';
t.Group = g(1:height(t));
t.groupMeans = varfun(@mean,t,'GroupingVariable','Group')


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