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arraysmatlabmatrixstatisticsmean
EllesBellz
EllesBellz

Reputation: 31

Mean of a 4D array across selected dimensions

I am using the mean function in MATLAB on a 4D matrix.

The matrix is a 32x2x20x7 array and I wish to find the mean of each row, of all columns and elements of 3rd dimension, for each 4th dimension.

So basically mean(data(b,:,:,c)) [pseudo-code] for each b, c.

However, when I do this it spits me out separate means for each 3rd dimension, do you know how I can get it to give me one mean for the above equation - so it would be (32x7=)224 means.

Upvotes: 2

Views: 2761

Answers (3)

chappjc
chappjc

Reputation: 30579

Note that with two calls to mean, there will be small numerical differences in the result depending on the order of operations. As such, I suggest doing this with one call to mean to avoid such concerns:

squeeze(mean(reshape(data,size(data,1),[],size(data,4)),2))

Or if you dislike squeeze (some people do!):

mean(permute(reshape(data,size(data,1),[],size(data,4)),[1 3 2]),3)

Both commands use reshape to combine the second and third dimensions of data, so that a single call to mean on the new larger second dimension will perform all of the required computations.

Upvotes: 1

Luis Mendo
Luis Mendo

Reputation: 112659

You could do it without loops:

data = rand(32,2,20,7); %// example data
squeeze(mean(mean(data,3),2))

The key is to use a second argument to mean, which specifies across which dimension the mean is taken (in your case: dimensions 2 and 3). squeeze just removes singleton dimensions.

Upvotes: 3

Guddu
Guddu

Reputation: 2437

this should work

a=rand(32,2,20,7);

for i=1:32
    for j=1:7
        c=a(i,:,:,j);
        mean(c(:))
    end
end

Upvotes: 2

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