How to find cumulative sum in matrix?

If I have A =[10 20 30 ;40 50 60;70 80 90], B = [1 2 3 ;2 3 1;3 2 1]

and i want to find cumulative sum from A in previous position follow seq. in B.

---> C = [(0) (0+10) (0+10+20); (0) (0+50) (0+50+60); (0) (0+90) (0+90+80)]

C = [0 10 30; 0 50 110; 0 90 170]

How to code this? thank you.

Answer 1

[DGM]

Here is one way:

A = [10 20 30; 40 50 60; 70 80 90];
B = [1 2 3; 2 3 1; 3 2 1];

nrows = size(A,1);
C = cumsum(A((1:nrows).' + nrows*(B-1)),2); % use linear indexing
C = [zeros(nrows,1) C(:,1:end-1)]

C = 3×3

0 10 30 0 50 110 0 90 170

[Real A]

Thak you so much ,but I have another question

if A is from-to matrix and B is routing

A =[0 10 20 30; 40 0 50 60; 70 80 0 90; 100 110 120 0],

B = [1 2 3 4;2 4 3 1]

How to find cumulative sum

C = [0 (0+10) (0+10+50) (0+10+50+90); 0 (0+60) (0+60+120) (0+60+120+70)]

C = [0 10 60 150; 0 60 180 250]

[DGM]

See if this is what you're after

A = [0 10 20 30; 40 0 50 60; 70 80 0 90; 100 110 120 0];
B = [1 2 3 4;2 4 3 1];
%     x (1,2)  (2,3)     (3,4)         x (2,4)  (4,3)      (3,1)
%C = [0 (0+10) (0+10+50) (0+10+50+90); 0 (0+60) (0+60+120) (0+60+120+70)]
%C = [0 10 60 150; 0 60 180 250]

% each column of BB is a row,col subscript pair into A
% each page of BB corresponds to each row of B
BB = repelem(B,1,2);
BB = reshape(permute(BB(:,2:end-1),[3 2 1]),2,[],size(B,1));
% convert to linear indices
BB = permute(sub2ind(size(A),BB(1,:,:),BB(2,:,:)),[3 2 1]);

% do sum and pad
C = cumsum(A(BB),2);
C = [zeros(size(C,1),1) C]

C = 2×4

0 10 60 150 0 60 180 250