Shuffle row order within every N rows in a matrix

I would like to shuffle my matrix's rows, but within each miniblock of 8 rows.

So for example, say I have the following 16x5 matrix:

    [1	2	4	1	1
    1	2	4	2	1
    1	2	4	1	2
    1	2	4	2	2
    1	2	4	1	1
    1	2	4	2	1
    1	2	4	1	2
    1	2	4	2	2
    1	2	1	1	1
    1	2	1	2	1
    1	2	1	1	2
    1	2	1	2	2
    1	2	1	1	1
    1	2	1	2	1
    1	2	1	1	2
    1	2	1	2	2];

How can I shuffle it so that the first 8 rows are all shuffled with eachother (maintaining column structure), and the second set of 8 rows are shuffled with eachother? Producing something like this:

    [1	2	4	1	1
    1	2	4	2	2
    1	2	4	2	1
    1	2	4	1	1
    1	2	4	1	2
    1	2	4	1	2
    1	2	4	2	2
    1	2	4	2	1
    1	2	1	1	1
    1	2	1	1	1
    1	2	1	1	2
    1	2	1	2	1
    1	2	1	2	2
    1	2	1	2	1
    1	2	1	1	2
    1	2	1	2	2];

In reality, my matrix is 384x5, so would be nice to be able to automate this somehow.

[Voss]

In reality, is the miniblock size still 8, or is it 192?

[Luca Ferro]

why there is always a plot twist after people start answering lol :)

[Benjy Barnett]

All edits were done before people answered :)

Answer 1

[Voss]

Here's one way:

I'll use a different matrix that makes it easier to see that the process works:

data = repmat((1:8).'+(0:4),2,1);
disp(data);

     1     2     3     4     5
     2     3     4     5     6
     3     4     5     6     7
     4     5     6     7     8
     5     6     7     8     9
     6     7     8     9    10
     7     8     9    10    11
     8     9    10    11    12
     1     2     3     4     5
     2     3     4     5     6
     3     4     5     6     7
     4     5     6     7     8
     5     6     7     8     9
     6     7     8     9    10
     7     8     9    10    11
     8     9    10    11    12

n_rows = 8;
N = size(data,1); % N must be a multiple of n_rows for this to work

n_shuffles = N/n_rows;
idx = zeros(n_rows,n_shuffles);
for ii = 1:n_shuffles
    idx(:,ii) = randperm(n_rows);
end
new_data = data(idx(:),:);

disp(new_data);

     3     4     5     6     7
     5     6     7     8     9
     2     3     4     5     6
     6     7     8     9    10
     4     5     6     7     8
     8     9    10    11    12
     1     2     3     4     5
     7     8     9    10    11
     3     4     5     6     7
     5     6     7     8     9
     8     9    10    11    12
     7     8     9    10    11
     1     2     3     4     5
     4     5     6     7     8
     2     3     4     5     6
     6     7     8     9    10

[Rik]

If your matrix is not a multiple of n_rows, you need edits like in my answer.

Answer 2

[Luca Ferro]

i think this would do the trick:

[rows,~]=size(m);

if mod(rows,2)==0 %detects if even or odd number of rows

half=rows/2;

topHalf=m(1:half,:) %split matrix in half

bottomHalf=m(1+half :end,:)

else

half=(rows+1)/2; %the middle row will be left unshuffled

topHalf=m(1:half-1,:) %split matrix in half

bottomHalf=m(1+half :end,:)

end

topShuffle=randperm(half);

bottomShuffle=randperm(half);

m=[topHalf(topShuffle,:);bottomHalf(bottomShuffle,:)] %merges halves to recreate full

Answer 3

[Rik]

Just use a loop with randperm:

data=[
    1	2	4	1	1
    1	2	4	2	1
    1	2	4	1	2
    1	2	4	2	2
    1	2	4	1	1
    1	2	4	2	1
    1	2	4	1	2
    1	2	4	2	2
    1	2	1	1	1
    1	2	1	2	1
    1	2	1	1	2
    1	2	1	2	2
    1	2	1	1	1
    1	2	1	2	1
    1	2	1	1	2
    1	2	1	2	2];
k=4; % You want 8, but that is a big example

rng(0) % make the result the same each run for this example

sz = size(data,1);
for base_row=1:k:sz
    base_row
    block_indices = randperm( min(sz-base_row,k) );
    
    ind1 = base_row + (1:numel(block_indices))
    ind2 = base_row + block_indices
    
    data(ind2,:) = data(ind1,:);
end

base_row = 1

ind1 = 1×4

2 3 4 5

ind2 = 1×4

4 2 3 5

base_row = 5

ind1 = 1×4

6 7 8 9

ind2 = 1×4

7 8 9 6

base_row = 9

ind1 = 1×4

10 11 12 13

ind2 = 1×4

12 10 11 13

base_row = 13

ind1 = 1×3

14 15 16

ind2 = 1×3

15 16 14