# matrix addition according to position vector

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Rabb on 6 Feb 2023
Answered: Voss on 6 Feb 2023
Hello, i have matrix A wich is 2xn dimension and vector b that is 1xn. I want to sort and sum A acording to vector b. for explamle
A=[1 3 5 7 9 11;
2 4 6 8 10 12]
b=[1 1 3 4 5 3]
C=[4 0 16 7 9
6 0 18 8 10]

Voss on 6 Feb 2023
A=[1 3 5 7 9 11;
2 4 6 8 10 12];
b=[1 1 3 4 5 3];
Maybe something like this:
C = zeros(size(A));
ub = unique(b);
for ii = 1:numel(ub)
C(:,ub(ii)) = sum(A(:,b == ub(ii)),2);
end
disp(C);
4 0 16 7 9 0 6 0 18 8 10 0
Another way:
[bg,gidx] = findgroups(b);
C = zeros(size(A));
C(:,gidx) = splitapply(@(x)sum(x,2),A,bg);
disp(C);
4 0 16 7 9 0 6 0 18 8 10 0

Vilém Frynta on 6 Feb 2023
This looks like homework.
It seems like you know what kind of functions you should work with. I'd recommend to study the documention of these functions and try to think it out. Feel free to share your progress.
If you are beginning with Matlab, it's good idea just to try (and learn from mistakes and experiments).