How to create equivalence class from an array in matlab - arrays

Question

I am working on matlab and have an array:
a =
2 1 5 3 2 1 2 1
You can see there may be one value multiple times. I want a function that will give me an array for each of those value that contains the index(s) of that value in the array as output.
Using the above example, the output would be:
(1 5 7)
(2 6 8)
(3)
(4)
(1 5 7) are the indexes of 2 in the input array. Same happens for 1,5 and 3.
This can be done using for loops etc. I just want to know if there is some in-built function for this in matlab.
**** EDIT ****
There may be two columns as following.
2 1 5 3 2 1 2 1
3 4 3 2 4 4 3 4
In that case output will be
(1 7)
(2 6 8)
(3)
(4)
(5)

Answer 1

Use the third output of unique to get unique labels for each column of a, and then apply accumarray with a custom function:
[~, ~, kk] = unique(a.', 'rows', 'stable'); %'
result = accumarray(kk, (1:numel(kk)).', [], #(x) {sort(x).'});
This works for any number of rows. For your two-row example
a = [2 1 5 3 2 1 2 1
3 4 3 2 4 4 3 4];
the result is
result{1} =
1 7
result{2} =
2 6 8
result{3} =
3
result{4} =
4
result{5} =
5
If element order is not important, you can simplify the code a little:
[~, ~, kk] = unique(a.', 'rows'); %'
result = accumarray(kk, (1:numel(kk)).', [], #(x) {x.'});
which gives
result{1} =
2 8 6
result{2} =
7 1
result{3} =
5
result{4} =
4
result{5} =
3

Answer 2

I don't think there is a buit-in function to do that, but you can do it in one line with unique and arrayfun:
Res = arrayfun(#(x) find(x==a), unique (x, 'stable'), 'UniformOutput', false);
Best,

Answer 3

Extend Ratbert's nice answer to arrive at a more general one which addresses the "edited" request:
[~, ~, J]= unique(a.', 'rows');
Res = cellfun(#str2num,accumarray(J,[1:size(a,2)]',[], #(x)num2str(x),'uni',0);
Use of conversion from double to char and back is clunky, but it works with Octave 3.6.4, and should also work on MATLAB. In MATLAB a more elegant answer with accumarray is likely possible.
Edit: The following is a more elegant answer with accumarray (see the MATLAB documentation for more details) - equivalent to LuisMendo's answer.
[~, ~, J]= unique(a.', 'rows');
Res = accumarray(J,[1:numel(J)]',[],#(x){x});