MATLAB: Keeping track of the Index while using “cell2mat” function

cell2mat

hey guys, I have a Cell Array a and am using cell2mat function for further clustering analysis, lets say:
a = {[1 2 3; 7 8 9], [4 5 6;10 11 12; 13 14 15]};
b = cell2mat(a(:));
rng(2)
k=2;
[idx] = kmeans(b,k);
which_in_cluster = cell(k,1);
for j = 1:k
    which_in_cluster{j} = find(idx == j);
end
Cluster_1 = b(which_in_cluster{1,1},:); % 1st Cluster
Cluster_2 = b(which_in_cluster{2,1},:); % 2nd Cluster
Cluster_1 =
     7     8     9
    10    11    12
    13    14    15
Cluster_2 =
     1     2     3
     4     5     6
I am now looking for a way to know in which Cell number and row these clusters have originally been in a. So in this example:
     Cluster_1 =
     7     8     9   % This has been in Cell number 1, row index 2 of a
    10    11    12   % This has been in Cell number 2, row index 2 of a
    13    14    15   % This has been in Cell number 2, row index 3 of a
Any help is greatly appreciated.

Best Answer

key = cell2mat((arrayfun(@(IDX) [repmat(IDX, size(a{IDX},1), 1), (1:size(a{IDX},1).']) , 1:numel(a), 'uniform', 0));
Now index rows of key by the cluster number to get the cell number and row number.
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