# Blockwise matrix addition without using more than 2 dimensions and cycles

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Bence Cseppento 2022년 11월 21일
댓글: Bence Cseppento 2022년 11월 23일
Dear All,
I am looking for a solution of blockwise matrix addition. To give a simple example, let us have
A = [1,2,3,4,5,6,7,8; 9,10,11,12,13,14,15,16];
I would like to get the result:
B = [1+5,2+6,3+7,4+8; 9+13,10+14,11+15,12+16];
The evident solution which comes to mind would be:
sum(reshape(A,2,4,2), 3);
However, I am using a third-party tool with special variables for which 3D arrays are not implemented. My next idea was to use:
B = zeros(2,4);
for i = 1:4
B(:,i) = sum(A(:, i:4:end), 2);
end
If possible I am looking for functions that can solve this without using a cycle as in my application the number of columns will reach hundreds and thousands. I thank for in advance for anyone's help.

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### 채택된 답변

the cyclist 2022년 11월 22일
This is awkward, but it does what you want. I think there is probably a better way.
% Inputs
A = [1,2,3,4,5,6,7,8; 9,10,11,12,13,14,15,16];
groupSize = 4;
% Output
B = movsum(A,[0,groupSize],2) - movsum(A,[0,groupSize-1],2) + A;
B = B(:,1:groupSize)
B = 2×4
6 8 10 12 22 24 26 28
##### 댓글 수: 1표시숨기기 없음
Bence Cseppento 2022년 11월 23일
Thanks for your effort, it does look good but I'll have to test it for some other cases.
In the mean time I managed to solve my problem by managing a conversion between types and using the original 3D version, but I did not know about the movsum() function before so you have given me food for thought, thank you!

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### 추가 답변(1개)

the cyclist 2022년 11월 21일
A = [1,2,3,4,5,6,7,8; 9,10,11,12,13,14,15,16];
B = A(:,1:end/2) + A(:,(end/2)+1:end)
B = 2×4
6 8 10 12 22 24 26 28
##### 댓글 수: 1표시숨기기 없음
Bence Cseppento 2022년 11월 21일
Thank you, I was a bit over-complicating it because of the context of my problem which I have not shared. However, I am still looking for a more general solution, e.g. I want to get a 2x20 matrix from a 2x2000 matrix, by summing the (:,1:20), (:,21:40) ... (:,1981:2000) submatrices without the use of a cycle.

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