How do I get the maximum of repetitive elements in certain portions of a matrix?

I have a 2000x2 matrix, and "for each 10x2 segment of this matrix", I need to calculate the maximum of corresponding values (second column) of repetitive values (first column) in the matrix. Like if the 3rd 10x2 segment of the matrix is this:

 ...
[2 20;
2 30;
2 40;
7 100;
7 110;
7 120;
7 130;
7 140;
15 240;
15 260]
 ...

I want to get this:

 ...
[2 40;
7 140;
15 260]
 ...

And so on. I have written the following but it gives me the maximum of repetitive elements through the "whole matrix":

[uv,~,idx] = unique(A(:,1));
B = [uv accumarray(idx,A(:,2),[],@max)];

But again, I need to do this "for each separate 10x2 segments of the matrix", and then store the results in a 'whatever x 2' sized matrix! Does anyone have any ideas how I could do this?

Answer 1

[Star Strider]

The only way I can see to do this is with a loop that selects and analyses each 10-row segment.

This works:

A = [2 20;
    2 30;
    2 40;
    7 100;
    7 110;
    7 120;
    7 130;
    7 140;
    15 240;
    15 260
    4 20;
    4 30;
    4 40;
    10 100;
    10 110;
    10 120;
    16 130;
    16 140;
    25 240;
    25 260    
    4 20;
    4 30;
    4 40;
    10 100;
    10 110;
    10 120;
    16 130;
    16 140;
    25 240;
    25 260];

for k1 = 1:10:size(A,1)
    T = A(k1:k1+9,:);
    [uv,~,idx] = unique(T(:,1));
    B{ceil(k1/10)} = [uv accumarray(idx,T(:,2),[],@max)];   % Output Cell Array               
end

It seems to do what you want.

[Antonio]

@Star Strider Thanks for your answer, but is there anyway to get the output as a final 'nx2' matrix? I think cell2mat doesn't work cuz dimensions of matrices being concatenated would not be consistent, but I need to use the results in some other processes that's why I need a matrix!

[Star Strider]

My pleasure.

If you want an (Nx2) matrix, change the orientation of ‘B’ to be a column array, then after the loop use cell2mat to convert it to a matrix:

B{ceil(k1/10),:} = [uv accumarray(idx,T(:,2),[],@max)]; % Output Cell Array

then after the loop:

Bmtx = cell2mat(B);

That will work. (I tested it.)

If you need the original cell array results, you still have them.

[Antonio]

@Star Strider Great, thanks a bunch!

[Star Strider]

As always, my pleasure!