How do I get all non-unique occurrences?

MATLAB's unique function only returns the non-unique indices to either the 'first' or 'last' occurrences of unique elements/rows of a vector/matrix. Is there any way to get 'all' occurrences short of creating my own recursive unique function? I plan on using this for very large matrices (i.e. unique(__,'rows')). Below is a minor example of the problem in the context of a vector:

A = [9 9 9 3 3 3 3];

[U1,AiF] = unique(A,'first');

[U2,AiL] = unique(A,'last')

% Returns

U1 = [3 9] % U1 is the same as U2

AiF = [4 1]

AiL = [7 3]

% I want something like this:

Ai_all = {[4 5 6 7],[1 2 3]}

Answer 1

[Stephen23]

You could use accumarray to collect the corresponding indices:

>> A = [9,9,9,3,3,3,3];
>> [~,~,idx] = unique(A);
>> idy = 1:numel(A);
>> Z = accumarray(idx(:),idy(:),[],@(n){n});
>> Z{:}
ans =
   4
   5
   6
   7
ans =
   1
   2
   3

[Dominik Mattioli]

Nice, no loops! Do you think that this is applicable to finding unique rows?

A = [1,3;2 4;2 4;5 7;1 3];

~~

UArows = [1 3;2 4;5 7];

iUArows = {[1 5];[2 3];[4]}

??

[Stephen23]

>> A = [1,3;2 4;2 4;5 7;1 3];
>> [~,~,idx] = unique(A,'rows');
>> idy = 1:size(A,1);
>> Z = accumarray(idx(:),idy(:),[],@(n){n});
>> Z{:}
ans =
     5
     1
ans =
     3
     2
ans =
     4