HOW TO GROUP THE ELEMENTS OF AN ARRAY

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

Answer 1

[dpb]

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));

Answer 2

[Prakash S R]

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, :);

Answer 3

[Image Analyst]

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.