Creating Individual Matrix based on Label Matrix from DataMatrix - matlab

Question

Let label be a matrix of size N x 1 (type double) and data be a matrix of size N x M (type double). The entries in the Label matrix looks like [ 1; 23; 135; ....; 6] which conveys that the
First row in the data matrix belongs to label 1
Second row in the data matrix belongs to label 2 and label 3
Third row in the data matrix belongs to label 1, label 3 and label 5 and so on
I would like to create a cell array say Individual{i} which stores all those rows from the data matrix which belongs to label i as given by the label matrix.
The resultant Individual{i} matrix will be size N_i x M.
Is there any efficient way to do the thing rather than looping row by row of data and label matrix?

Answer 1

I would turn your matrix label into a Boolean matrix L:
L = [ 1 0 0 0 0 0 ;
0 1 1 0 0 0 ;
1 0 1 0 1 0 ;
...
0 0 0 0 0 1 ];
for your example. You can use a sparse matrix if N or the number of labels is very large.
Then I think what you call N_i is sum(L(:, i)) and L' * data would compute the sum of all the rows in data with label L.
What do you want to do with the data once it's reached the Individual cell array? There's almost certainly a better way to do it...

Answer 2

Given the correct variables: N, M, data, label as you described, here's a sample code that creates the desired cell array Individual:
%# convert labels to binary-encoded format (as suggested by #Tom)
maxLabels = 9; %# maximum label number possible
L = false(N,maxLabels);
for i=1:N
%# extract digits of label
digits = sscanf(num2str(label(i)),'%1d');
%# all digits should be valid label indices
%assert( all(digits>=1) && all(digits<=maxLabels) );
%# mark this row as belong to designated labels
L(i,digits) = true;
end
%# distribute data rows according to labels
individual = cell(maxLabels,1);
for i=1:maxLabels
individual{i} = data(L(:,i),:);
end