HOW TO GROUP THE ELEMENTS OF AN ARRAY

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simone zappalà
simone zappalà 2022-5-10
回答： Image Analyst 2022-5-10
I've a 252*51 array, i want to group it three-by-three elements and do mean of these groups one-by-one, creating a new array 84*51 which every row is the mean of three elements of the first array

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dpb
dpb 2022-5-10
One of those cases where the optimal way depends upon being familiar with internal array storage order being column-major and manipulation of orientation/shape to take advantage of vector operations --
nRowToAvg=3;
mnAbynRowToAvg=reshape(mean(reshape(A,nRowToAvg,[])),[],size(A,2));

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Prakash S R
Prakash S R 2022-5-10
编辑：Prakash S R 2022-5-10
A more "arithmetic" approach:
So you want to take a 252x51 matrix X and produce a 84x51 matrix Y, right? I would suggest premultiplying X with a 84x252 matrix A that sums successive rows with weights of 1/3.
Thus, Y = (1/3)*A*X, where A has the form
[1 1 1 0 0 0 0 0 0 0 0....
0 0 0 1 1 1 0 0 0 0 0...
0 0 0 0 0 0 1 1 1 0 0 ...
:
0 0 0 ... 1 1 1]
You can construct A using
aa = [[1 1 1], zeros(1,249)];
Atmp = triu(toeplitz(aa));
A = Atmp(1:3:end, :);
Image Analyst
Image Analyst 2022-5-10
One way is to use blockproc(). It's in the Image Processing Library.
meanFilterFunction = @(theBlockStructure) mean(theBlockStructure.data(:)); % Define mean function.
blockSize = [3, 3]; % Whatever window size you want.
% Get an output image where each pixel is the average of a 3x3 window in
% the original image.
blockyImage = blockproc(grayImage, blockSize, meanFilterFunction);
I'm attaching a script with several ways to use it. Adapt as needed.

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