average each block of class in a matrix

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alessio tugnolo 2020-3-16

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编辑： Guillaume 2020-3-16

Hi, I have a large matrix where the rows are labelled according to different classes. I would like to obtain an average row of each class. How can I do it?

Thank you!

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Answer 1

Guillaume 2020-3-16

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编辑：Guillaume 2020-3-16

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It's unclear how your input data is actually stored in matlab so please clarify if the below doesn't apply.

I'm assuming that you have a two column matrix with one column storing the the class and the other the column storing the values you want to average by class. In this case, this is trivially achieved with groupsummary:

classcolumn = 1;  %change as appropriate
valuecolumn = 2;  %can be more than one column. Each column will be average by class
classmean = groupsummary(yourmatrix, classcolumn, 'mean', valuecolumn);

Other simple ways this can be achieved: splitapply or accumarray:

classcolumn = 1;
valuecolumn = 2; 
group = findgroups(yourmatrix(:, classcolumn));
classmean = splitapply(@mean, yourmatrix(:, valuecolumn), group);
%or
classmean = accumarray(group, yourmatrix(:, valuecolumn), [], @mean);  %this one can only work on one column though

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Answer 2

Sriram Tadavarty 2020-3-16

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Hi Alessio,

The mean function will help in this case. Here is the documentation page of mean function having different exampleshttps://www.mathworks.com/help/matlab/ref/mean.html

% Consider a random matrix
a = randn(1000,10000); % Implies 1000 rows and each row containing 10000 columns
% Calculate the mean of each column
meCol = mean(a);
% Calculate the mean of each row
meRow = mean(a,2); % This is your intended case

Hope this helps.

Regards,

Sriram

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alessio tugnolo 2020-3-16

Thank you so much Sriram!

I have a situation like that:

I would obtain an average row of each time. Consider that each class haven't the same number of objects.

Thank you!

Sriram Tadavarty 2020-3-16

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Hi Alessio,

In this case, you can run a fop loop on number of rows and take mean for each row.

% Rows variable is a cell array with different number of elements in it. For example
Rows = {[1 4 5],[1 9 10 100]};
for i = 1:numel(Rows)
    avg(i) = mean(Rows{i});
end
% avg returns [3.3333 30.0000] each element corresponding to each row

Hope this helps

Sriram

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