How to sum previous columns before specific element ?

Question

Maria 2022-10-7

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评论： Maria 2022-10-8

Hello! I have a row vector E(1,50), and I have a binary matrix A (20,50). In each row of A there is only "1". I want to add values in previous columns of E before each "1" existing in A. As example E(1,6) and A(3,6)

E = [2 3 5 4 1 8 ] 
A = [0 1 0 0 0 0 ;...
     0 0 0 1 0 0 ;...
     0 0 0 0 0 1] 

I have a value k in each time I will verify if it is upper to the sum of previous element of E. For this example A(1,2) =1, so the condition is "if k- (2+3)>0, then the two columns in E."

If it is the case I will fill matrix B same dimension of A.

Now for element B(1,2)=1.

For A(2,4)=1, I will take the sum of 4 previous columns in E (4+5+3+2).

etc...

How can I do this? Thanks in advance.

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Torsten 2022-10-7

So the aim is to create the matrix B that has the same rows as A if the condition holds and will have a zero row if not ?

And what about the value for k ? Is it constant throughout the process or does it change with the row in question ?

Maria 2022-10-8

@Torsten yes "k" is constant and each time i will compare it with the sum of previous columns of E.

Thank you for your reply.

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Answer 1

Image Analyst 2022-10-8

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https://ww2.mathworks.cn/matlabcentral/answers/1819855-how-to-sum-previous-columns-before-specific-element#answer_1069205

在 MATLAB Online 中打开

Try this:

E = [2 3 5 4 1 8 ];
A = [0 1 0 0 0 0 ;...
    0 0 0 1 0 0 ;...
    0 0 0 0 0 1];
[rows, columns] = size(A);
% Initialize B
B = zeros(size(A));
for row = 1 : rows
    % Find the first column where A is 1 for this row.
    first1column = find(A(row, :), 1, 'first');
    % Sum the values of E from column 1 up until this first1column and
    % assign to B in that row and column
    B(row, first1column) = sum(E(1:first1column));
end
% Display B in command window
B
B = 3×6
     0     5     0     0     0     0
     0     0     0    14     0     0
     0     0     0     0     0    23