Question

How can I randomly permute row keeping those with the same value adjacent to each other? I know that I can use randperm on the rows to randomly permute all rows, but I do not know how to keep the same-valued rows adjacent. Thanks.

A = [0 0 0;
     0 0 0;
     1 1 1;
     1 1 1;
     1 1 1;
     2 2 2;
     2 2 2];   

 permute_A = [0 0 0;
     0 0 0;
     2 2 2;
     2 2 2;
     1 1 1;
     1 1 1;
     1 1 1];

Answer 1

You have to identify the clusters and then permute them:

Finding the clusters can be done by finding the changes in their differences:

diffA = diff(A);
clusters_start = [1 ; find(any(diffA,2)~=0)+1];

Then the cluster ends are easily found by:

clusters_end = [clusters_start(2:end)-1 ;size(A,1)];
clusters_length = clusters_end-clusters_start+1;

now you know the number of clusters and you can permute them:

Nclusters = numel(clusters_start);
perm_idx = randperm(Nclusters );
clusters_start = clusters_start(perm_idx);
clusters_end = clusters_end(perm_idx);
clusters_length = clusters_length(perm_idx);

and put them in a new matrix:

newA = NaN(size(A));
for ii=1:Nclusters
    newA(sum(clusters_length(1:ii-1))+(1:clusters_length(ii)),:) = A(clusters_start(ii):clusters_end(ii),:);
end