# 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?

4 views (last 30 days)
Srh Fwl on 13 Apr 2024
Commented: Voss on 14 Apr 2024
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.

the cyclist on 13 Apr 2024
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));

Voss on 13 Apr 2024
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 on 14 Apr 2024
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 on 14 Apr 2024
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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