Creation of double columns whose max size is 2 x 2 from a matrix whose size is n>2 x n>2 ( e.g. with n max = 800)

I need your help. Here is an example of what I want to do:

   B = [1 2 3; 4 5 6; 7 8 9]
   [m,n] = size(B);
   B(m-2:m-1, n-2:n-1)
   B(m-1:m  , n-2:n-1)
   B(m-2:m-1, n-1:n)
   B(m-1:m  , n-1:n)

Input:

B =

     1     2     3
     4     5     6
     7     8     9

Output:

ans =

     1     2
     4     5

ans =

     4     5
     7     8

ans =

     2     3
     5     6

ans =

     5     6
     8     9

Can you please show me how I can do this or give me an example to do it automatically, for example for a larger matrix such as B = magic(10) or more?

Thank you!

[James Tursa]

These can easily be created, but what do you intend to do with these sub-matrices downstream in your code? There may be a way to do this downstream processing without physically creating all of these sub-matrices.

[Gobert]

Can you please indicate how exactly that can be done?

Answer 1

[Harish Ramachandran]

B = rand(2,4);
count = 1;
[xdim ydim] = size(B);
for i=1:xdim-1
for j=1:ydim-1
C{count} = B(i:i+1,j:j+1);
count = count+1;
end
end

In this case, the resultant output will be a cell array with each cell block (C{1}, C{2}...) a distinct 2x2 sub component of the input matrix.

[Gobert]

Thanks, Harish, but this creates a 1-by-5 cell array of empty matrices, which was not expected?

[Harish Ramachandran]

I am not sure what input you are providing? On running the above code with B = magic (3) - the following is my output which I believe is expected.

>> B

B =

       8     1     6
       3     5     7
       4     9     2

>> C{1}

ans =

       8     1
       3     5

>> C{2}

ans =

       1     6
       5     7

>> C{3}

ans =

       3     5
       4     9

>> C{4}

ans =

       5     7
       9     2

[Harish Ramachandran]

Also, I have edited the code above to make sure it works for non square matrix.

[Gobert]

Thanks, Harish!