matrix addition according to position vector

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]

so answer would be

C=[4 0 16 7 9

6 0 18 8 10]

Answer 1

[Voss]

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

Answer 2

[Vilém Frynta]

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).