How to add a cumulative constant to all values on each row of a matrix, starting from the bottom row and ending at the top row?

Question

Srh Fwl 2024-4-13，17:26

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https://ww2.mathworks.cn/matlabcentral/answers/2106501-how-to-add-a-cumulative-constant-to-all-values-on-each-row-of-a-matrix-starting-from-the-bottom-row

评论： Voss 2024-4-14，18:53

I have a matrix that is very large. A small example is:

2 3
2 3
2 3       

I need to add a constant to each row, starting at the bottom row. The additon should be cumulative. For example, if the constant is 0.5, the matrix should become:

5 3.5 4.5
0 3.0 4.0
5 2.5 3.5

I know to use "cumsum" for cumulative addition in Matlab but the indexing required in this problem exceeds my skills. I would be vey grateful for any help.

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Answer 1

the cyclist 2024-4-13，17:36

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https://ww2.mathworks.cn/matlabcentral/answers/2106501-how-to-add-a-cumulative-constant-to-all-values-on-each-row-of-a-matrix-starting-from-the-bottom-row#answer_1440801

在 MATLAB Online 中打开

There are many ways to do this. Here is one way:

% Inputs
in = [1 2 3;
      1 2 3;
      1 2 3];
constant = 0.5;
% Algorithm
[m,n] = size(in);
out = in + kron(constant*(m:-1:1)',ones(1,n));

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Answer 2

Voss 2024-4-13，17:32

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2106501-how-to-add-a-cumulative-constant-to-all-values-on-each-row-of-a-matrix-starting-from-the-bottom-row#answer_1440796

在 MATLAB Online 中打开

c = 0.5;
A = [1,2,3;1,2,3;1,2,3]
A = 3x3
     1     2     3
     1     2     3
     1     2     3
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One way:

N = size(A,1);
result = c*(N:-1:1).'+A
result = 3x3
    2.5000    3.5000    4.5000
    2.0000    3.0000    4.0000
    1.5000    2.5000    3.5000
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Another way:

N = size(A,1);
result = cumsum(c*ones(N,1),'reverse')+A
result = 3x3
    2.5000    3.5000    4.5000
    2.0000    3.0000    4.0000
    1.5000    2.5000    3.5000
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Srh Fwl 2024-4-14，18:45

Thank you so much, Voss. I am currently on the road, but look forward to checking this out in more detail later. I did mean "faster" so will think about why that is an issue for me but not you. Thank you very much for thinking about this!