matrix addition according to position vector

Question

Rabb 2023-2-6

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1907420-matrix-addition-according-to-position-vector

回答： Voss 2023-2-6

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]

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Answer 1

Voss 2023-2-6

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https://ww2.mathworks.cn/matlabcentral/answers/1907420-matrix-addition-according-to-position-vector#answer_1164840

在 MATLAB Online 中打开

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

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Answer 2

Vilém Frynta 2023-2-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1907420-matrix-addition-according-to-position-vector#answer_1164835

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).