# HOW TO GROUP THE ELEMENTS OF AN ARRAY

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simone zappalà on 10 May 2022
Answered: Image Analyst on 10 May 2022
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

dpb on 10 May 2022
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));

### More Answers (2)

Prakash S R on 10 May 2022
Edited: Prakash S R on 10 May 2022
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 on 10 May 2022
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. 