MATLAB: Average Multiple Matrix Columns Based On Duplicate Entries In A Different ColumnVector

MATLABunique accumarray

I have a column vector consisting of hundreds of unit64 values where duplicates appear and are always grouped together but groups appear only once. Each element of this vector corresponds to a row in a separate 2D matrix of double values. I would like to remove the duplicates from the row vector and then remove the corresponding rows in the matrix, replacing them with a single row where each column is average the removed elements in the column, while keeping the order stable. So with data like this:
      1
      1
      2 
 A=   2
      3
      3
      1 2 3
      2 3 4
      3 4 5
  B = 4 5 6
      5 6 7
      6 7 8
I would want the output to be
      1.5 2.5 3.5
  C = 3.5 4.5 5.5
      5.5 6.5 7.5
Some combination of unique and accumarray seems to be in order but I cannot figure out how to handle the multiple coulmn part of the problem.

Best Answer

So if I understood correctly, the shape of B does not matter and it is to be considered a column vector.
[~, ~, subs] = unique(A, 'stable');
C = accumarray(subs, B(:), [], @mean)
Note that whether or not the groups appear only once does not matter for the above. All the values with the same corresponding A still get averaged.
Also note that if A is already integer values from 1 to n with no gap and in the right order, then the call to unique is unnecessary and you can just pass A instead of subs.
Related Question