MATLAB: Sum over rows in a matrix for specific numbers in the last column

columnsum

I have a matrix of data that looks like (my origninal data set is a 1450×6 matrix, but I think this matrix illustrates the problem)
A=[0.45 0.11 0.00 -0.32 1;
-0.06 -0.18  0.62  0.20 1;
-0.20  0.50  0.80  0.10 2;
-0.50  0.20  0.10  0.60 2;
-0.30  0.10 -0.05  0.20 2;
-0.45  0.11  0.00 -0.32 3;
-0.60  0.20  0.10  0.40 3;
 0.80 -0.20 -0.20  0.50 4;
-0.30  0.10 -0.05  0.20 4;
 0.45 0.11 0.00 -0.32   1;
-0.06 -0.18  0.62  0.20 1]
In this matrix I need to sum all rows containing a 1 in the last column, then the rows containing a 2 in the last column and so on with preserving the order of the data. To be more explicit the first sum is supposed to be row 1+2, second sum is row (3+4+5), third sum is row (6+7), fourth sum is row (8+9), and then the fifth sum which starts again with ones is row (10+11).
How can this be solved in MATLAB?
Many thanks for your help and support!

Best Answer

An emulation of splitapply for older Matlab versions:
group  = cumsum(diff([0; A(:, end)]) ~= 0);
nRowR  = size(A, 2) - 1;
R      = zeros(group(end), nRowR);
for iG = 1:group(end)
  R(iG, :) = sum(A(group == iG, 1:nRowR));
end
Unfortunately I did not get a smart version with accumarray, because val can be a vector only. But at least:
group = cumsum(diff([0; A(:, end)]) ~= 0);
nRowR = size(A, 2) - 1;
R     = zeros(group(end), nRowR);
for iR = 1:nRowR
  R(:, iR) = accumarray(group, A(:, iR));  % Default fun: @sum
end
@All: Is there no way to let accumarray operate on a matrix? If so, could we rename it to accumvector?
I expect the first approach to be remarkably faster.
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