How to calculate the mean for specific rows in a cell using cellfun

Question

Skirt Zhang 2012-12-8

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https://ww2.mathworks.cn/matlabcentral/answers/56066-how-to-calculate-the-mean-for-specific-rows-in-a-cell-using-cellfun

A cell A{1}=a1 A{2}=a2 ... A{5}=a5

a1=[1 1 1 1 ; 2 2 2 2; 3 3 3 3; 4 4 4 4;]

a2 =

   [ 2     2     2     2;
     3     3     3     3;
     4     4     4     4;
     5     5     5     5;]
...

I want to get the two mean values for the first two rows and the last two rows seperately. Can I realize this using the following function instead of a loop? cellfun(@nanmean,A, 'UniformOutput',false);

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Answer 1

Azzi Abdelmalek 2012-12-8

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编辑：Azzi Abdelmalek 2012-12-8

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A={rand(4);rand(4);rand(4);rand(4);rand(4);rand(4)}
n=numel(A)
c=arrayfun(@(x) mean(mean(A{x})),[1 2 n-1 n],'un',0)

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Azzi Abdelmalek 2012-12-9

Skirt, in your question you said 2 first rows and two last rows, From your post, A is a cell array of n elements, each element is a matrix. Can you explain clearly what is the expected result (look at my example)

Azzi Abdelmalek 2012-12-9

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If you wat first 5 and last 5, just adapt the above code

A={rand(4);rand(4);rand(4);rand(4);rand(4);rand(4)}
n=numel(A)
nr=5
c=arrayfun(@(x) mean(mean(A{x})),[1:nr n-nr+1:n],'un',0)

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Answer 2

Matt J 2012-12-8

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Can I realize this using the following function instead of a loop? cellfun(@nanmean,A, 'UniformOutput',false);

You can, but there will be no advantage in performance over the for loop.

Incidentally, if all the A{i} are the same size, as in your example, it's puzzling that you wouldn't maintain them as a 3D array A(:,:,i). Then you could just do

nanmean(A([1,end],:,:),2)

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Answer 3

José-Luis 2012-12-9

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编辑：José-Luis 2012-12-9

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It is not really clear how you want to split your data, so I added two variables, rowsFromStart and rowsFromEnd so you can specify it yourself. It is also not very clear if you want one mean for each row or the mean of all rows. Finally, a loop would probably be faster.

 A={rand(10,100);rand(10,100);rand(10,100);rand(10,100);rand(10,100);rand(10,100)}
rowsFromStart = 5;
rowsFromEnd = 5;
c=cellfun(@(x) [mean(mean(x(1:rowsFromStart-1,:),2));mean(mean(x(end:end-rowsFromEnd+1,:),2))],A,'un',0)