Split big matrix in many submatrices having same size

Question

Mehdi Kooli 2022-11-4

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1843693-split-big-matrix-in-many-submatrices-having-same-size

评论： Jilin Zhang 2022-12-21

Hello,

I have a big matrix 396*2600 and I want to split it to many matrices 30*120 as follow : (let's consider this exemple on a small matrix A ans split it to 3*2 matrices):

A = [1 9 17

2 10 18

3 11 19

4 12 20

5 13 21

6 14 22

7 15 23

8 16 24];

results wanted: A1 = [1 9

2 10

3 11]

A2 = [9 17

10 18

11 19]

A3 = [2 10

3 11

4 12]

A3 = [10 18

11 19

12 20] and so on ...

Remarks: I need a solution without for loop, I'm looking for a Matlab command giving this results.

Thanks

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Mehdi Kooli 2022-11-23

移动：Matt J 2022-11-23

Hello Andrei Bobrov, I saw you answered similar question (https://fr.mathworks.com/matlabcentral/answers/357482-reshaping-a-2d-matrix-into-a-3d-matrix-layers-of-square-matrices-with-specific-order-no-loop?s_tid=srchtitle)

Can you please take a look here , maybe you have the solution.

Mehdi Kooli 2022-12-9

移动：Matt J 2022-12-9

Hello @Andrei Bobrov maybe you have the answer ?

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Answer 1

John D'Errico 2022-11-4

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https://ww2.mathworks.cn/matlabcentral/answers/1843693-split-big-matrix-in-many-submatrices-having-same-size#answer_1091563

编辑：John D'Errico 2022-11-4

在 MATLAB Online 中打开

So many times this gets asked for. DON'T DO IT. Instead, learn to use arrays, of many types. For example, just make it into a 3 dimensional array, where each plane of that array is one of the desired sub-arrays. That requires relatively little more than understanding how to index arrays.

A = [1 9 17
10 18
11 19
12 20
13 21
14 22
15 23
16 24];

Here you want to generate all 3x2 contiguous subarrays. The size of A is

[r,c] = size(A)
r = 8
c = 3

So there will be 6*2 such 3x2 sub-arrays to generate.

Where are the elements of A stored in memory? In what order? Understanding this, and how tools like sub2ind work in MATLAB allows you to build these arrays easily.

ind1 = (1:3)' + [0,r];
ind2 = (1:r-2) + r*[0:c-2]';
B = reshape(A(ind1(:)-1 + ind2(:)'),3,2,[]);

Now it is simple to acces any of those subarrays as we have created, and do so programmatically. The array B is of size:

size(B)
ans = 1×3
     3     2    12

There are 12 such sub-arrays.

B(:,:,1)
ans = 3×2
     1     9
     2    10
     3    11
B(:,:,2)
ans = 3×2
     9    17
    10    18
    11    19
B(:,:,12)
ans = 3×2
    14    22
    15    23
    16    24

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Mehdi Kooli 2022-12-12

@Stephen23 Thanks a lot, it resolves the problem for this example but I need to adapt the code for a big matrix A= 426 x 2904 using a sliding window 60 x 240

I don't understand the value 2 in the code (see comments below)

Here is the code :

rect_l=3; % rows lenght of my sliding window

rect_c=2; % columns lenght of my sliding window

% Matrix example 8 x 3

A = [1 9 17

2 10 18

3 11 19

4 12 20

5 13 21

6 14 22

7 15 23

8 16 24];

[r,c] = size(A);

x1 = (1:rect_l)';

y1 = [0,r];

ind1 = bsxfun(@plus,x1,y1);

x2 = (1:r-2); % Why the value 2 ?

y2 = r*[0:c-2]'; % Why the value 2 ?

ind2 = bsxfun(@plus,x2,y2);

ind3 = bsxfun(@minus,ind1(:),1);

ind4 = ind2(:)';

ind5 = bsxfun(@plus,ind3,ind4);

B = reshape(A(ind5),rect_l,rect_c,[]);

Can you please tell me how to modify the code for the new Matrix size and sliding window.

Jilin Zhang 2022-12-21

A=randi([0 10],426,2904)

[r1, c1]=size(A);

r2=60 %target row #

c2=240 %target column #

i=1

j=1

k=1

while i*r2<r1

while j*c2<c1

B(k,:,:)=A((i-1)*r2+1:i*r2,(j-1)*c2+1:j*c2);

j=1+j

k=1+k

end

i=1+i

end

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Answer 2

Matt J 2022-11-7

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https://ww2.mathworks.cn/matlabcentral/answers/1843693-split-big-matrix-in-many-submatrices-having-same-size#answer_1093303

编辑：Matt J 2022-11-7

What I'm trying to do is to split the matrix to submatrices and then for each submatrix, calculate the median value and store the result in another matrix.

The problem simplifies greatly if you choose a submatrix size with odd dimensions, in which case you can use medfilt2. Otherwise, ordfilt2 would allow you the increased flexibility of using any dimensions.