MATLAB: Matlab find unique column-combinations in matrix and respective index

combinationsindexingunique

I have a large matrix with with multiple rows and a limited (but larger than 1) number of columns containing values between 0 and 9 and would like to find an efficient way to identify unique row-wise combinations and their indices to then build sums (somehwat like a pivot logic). Here is an example of what I am trying to achieve:
a =
 1     2     3
 2     2     3
 3     2     1
 1     2     3
 3     2     1
uniqueCombs =
 1     2     3
 2     2     3
 3     2     1
numOccurrences =
 2
 1
 2
indizies:
[1;4]
[2]
[3;5]
From matrix a, I want to first identify the unique combinations (row-wise), then count the number occurrences / identify the row-index of the respective combination.
I have achieved this through generating strings with num2str and strcat, but this method appears to be very slow. Along these thoughts I have tried to find a way to form a new unique number through concatenating the values horizontally, but Matlab does not seem to support this (e.g. from [1;2;3] build 123). Sums won't work because they would remove the possibility to identify unique combinations. Any suggestions on how to best achieve this? Thanks!

Best Answer

More or less the same as Jan's, using accumarray instead of splitapply (I'm still old school!):
A = [ 1     2     3
      2     2     3
      3     2     1
      1     2     3
      3     2     1];
[B, ~, ib] = unique(A, 'rows');
numoccurences = accumarray(ib, 1);
indices = accumarray(ib, find(ib), [], @(rows){rows});  %the find(ib) simply generates (1:size(a,1))'
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