calculate the mean of several columns

Question

Sobhan 2012-4-4

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评论： Walter Roberson 2015-12-2

Hi all, I am new to Matlab and I am struggling with this problem. I have a matrix with 112 columns and 100 rows. How can I calculate the mean of column 1-7 , 8-14, 15-21,...., 106-112 and put the a new matrix with 16 columns and 100 rows. I appreciate your help Sobhan

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Andrei Bobrov 2012-4-5

eg

A = rand(100,112);

fun = @(block_struct)mean(block_struct.data,2);

out = blockproc(A,[100,7],fun);

Walter Roberson 2015-12-2

KAUST comments "The best answer"

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Answer 1

Wayne King 2012-4-4

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One way is simply with a for loop

A = randn(100,112);
for nn = 0:15    
    B(:,nn+1) = mean(A(:,nn*7+1:(nn+1)*7),2); 
end

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Oleg Komarov 2012-4-4

I bet this is more efficient than the one liner and definitely more readable.

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Answer 2

Oleg Komarov 2012-4-4

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A = rand(100,112);
B = reshape(mean(reshape(A.',7,[])),16,100).';
% Check
isequal(mean(A(2,8:14)),B(2,2))

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Answer 3

Thomas 2012-4-4

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You could reshape your matrix with 7 columns becoming 1 in the new matrix and taking the mean of each column

Eg.:

c=rand(4)     % 4x4 matrix
d=reshape(c,[],2)   % take two colm and form 1 total 2
p=mean(d)        % mean of each columns

In your case

    c=rand(100,112)     % 4x4 matrix
    d=mean(reshape(c,7,[]))  % form 1600 columsns
    final_out=(reshape(d,16,100))    % mean of each columns

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Thomas 2012-4-4

thanks, edited accordingly..

Oleg Komarov 2012-4-4

Now, your mean is across the first 7 rows (first column). You have to transpose and re-transpose back. Honestly, in this cases I think Waynes solution is best in terms or readability and performance.

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