MATLAB: How to reduce this matrix

reduce matrix; algorith; repeated values

How to reduce this matrix?
Suppose that M=[1 0.1 1.1; 1 0.1 1.1; 3 0.1 1.2; 2 0.2 3; 1 2 4; 2 2 4; 2 3 5];
based on the second column, for repeated values of the second column, sum the elements of column 1 when the elements of column 3 are equals. If the elements of column 3 are different, do a weighted average using column 1.
For the matrix M above, the output should be:
M_out = [5 0.1 1.16; 2 0.2 3; 3 2 4; 2 3 5];

Best Answer

M=[1 0.1 1.1; 1 0.1 1.1; 3 0.1 1.2; 2 0.2 3; 1 2 4; 2 2 4; 2 3 5]
[ii,jj,kk]=unique(M(:,2))
a=accumarray(kk,M(:,1))
b=accumarray(kk,1:numel(kk),[],@(x) sum(M(x,1).*M(x,3))/sum(M(x,1)));
M_out =[a ii b]
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