Massive matrix, tridiagonal, ones()

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Anastasia Zistatsis
Anastasia Zistatsis on 15 Feb 2021
I need to transform this matrix, taking the three diagonal lines that have numbers and make them their own matrices. My matrix, however, is 124x124. I do not want to write a 1x123 matrix out that has 122 of the same number and 1 zero. Is there a way use ones() for this?
  2 Comments
Adam Danz
Adam Danz on 15 Feb 2021
What 3 diagonal lines?
The description of the data and the description of the goal are not clear.
A demo using a smaller matrix would likely help.
Anastasia Zistatsis
Anastasia Zistatsis on 15 Feb 2021
Edited: Adam Danz on 15 Feb 2021
[2 13 0 0 0 0;
1 2 13 0 0 0;
0 1 2 13 0 0;
0 0 1 2 13 0;
0 0 0 1 2 13];
These are the three diagonals I was talking about: [ 13 13 13 13 13 0], zeros(1,5)*2, and [0 1 1 1 1]
For a much larger matrix, is there a function in matlab that can make it easier to type out the first and third vectors shown above?

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Answers (1)

Jan
Jan on 15 Feb 2021
A = [2 13 0 0 0 0;
1 2 13 0 0 0;
0 1 2 13 0 0;
0 0 1 2 13 0;
0 0 0 1 2 13];
d1 = diag(A, 0) % [2 2 2 2 2],'
d2 = diag(A, 1) % [13 13 13 13 13].'
d3 = diag(A, -1) % [1 1 1 1].'
I do not understand, how you want to get "[13 13 13 13 13 0]" (especially the trailing 0), "zeros(1,5)*2" (zeros times anything is still zeros?), and [0 1 1 1 1] (again the leading zero)?
  1 Comment
Anastasia Zistatsis
Anastasia Zistatsis on 16 Feb 2021
It's part of the answer matrix that I should have put in my question! Diag() helps though, thank you!

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