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?

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Srh Fwl
Srh Fwl on 13 Apr 2024 at 17:26
Commented: Voss on 14 Apr 2024 at 18:53
I have a matrix that is very large. A small example is:
1 2 3
1 2 3
1 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:
2.5 3.5 4.5
2.0 3.0 4.0
1.5 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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Accepted Answer

the cyclist
the cyclist on 13 Apr 2024 at 17:36
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));

More Answers (1)

Voss
Voss on 13 Apr 2024 at 17:32
Ran in:
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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  5 Comments
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Srh Fwl
Srh Fwl on 14 Apr 2024 at 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!
Voss
Voss on 14 Apr 2024 at 18:53
No problem. I was mostly just curious what you meant by "better". Thanks for clarifying you meant faster. Anyway, the time difference is not much.

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