Creating a tridiagonal matrix

1,180 views (last 30 days)
Aaron Atkinson
Aaron Atkinson on 11 Nov 2019
Commented: John D'Errico on 10 Dec 2022
I am currently trying to create a 500*500 matrix in matlab with diagonals a=-1, b=4, c=2. My teacher has said that the best way to go about it is using loops, but is there a coded in function to use?
  2 Comments
David Goodmanson
David Goodmanson on 11 Nov 2019
Hi Aaron
check out the 'diag' function
Alex Treat
Alex Treat on 30 Oct 2020
coughs you were in the mec 103 class at CSU...

Sign in to comment.

Accepted Answer

Stephen23
Stephen23 on 11 Nov 2019
Edited: Stephen23 on 20 Mar 2022
"My teacher has said that the best way to go about it is using loops"
Why on earth would they say that? Here are some non-loop aproaches:
2- Use diag :
>> N = 10;
>> a = -1;
>> b = 4;
>> c = 2;
>> M = diag(a*ones(1,N)) + diag(b*ones(1,N-1),1) + diag(c*ones(1,N-1),-1)
M =
-1 4 0 0 0 0 0 0 0 0
2 -1 4 0 0 0 0 0 0 0
0 2 -1 4 0 0 0 0 0 0
0 0 2 -1 4 0 0 0 0 0
0 0 0 2 -1 4 0 0 0 0
0 0 0 0 2 -1 4 0 0 0
0 0 0 0 0 2 -1 4 0 0
0 0 0 0 0 0 2 -1 4 0
0 0 0 0 0 0 0 2 -1 4
0 0 0 0 0 0 0 0 2 -1
3- indexing is reasonably simple:
>> M = zeros(N,N);
>> M( 1:1+N:N*N) = a;
>> M(N+1:1+N:N*N) = b;
>> M( 2:1+N:N*N-N) = c
M =
-1 4 0 0 0 0 0 0 0 0
2 -1 4 0 0 0 0 0 0 0
0 2 -1 4 0 0 0 0 0 0
0 0 2 -1 4 0 0 0 0 0
0 0 0 2 -1 4 0 0 0 0
0 0 0 0 2 -1 4 0 0 0
0 0 0 0 0 2 -1 4 0 0
0 0 0 0 0 0 2 -1 4 0
0 0 0 0 0 0 0 2 -1 4
0 0 0 0 0 0 0 0 2 -1
  6 Comments
Arth Patel
Arth Patel on 29 Sep 2020
Can you please explain the second method a bit ? It's not clear to me how you're indexing a matrix using just one argument.
Stephen23
Stephen23 on 30 Oct 2020
"It's not clear to me how you're indexing a matrix using just one argument."
The second example uses linear indexing:

Sign in to comment.

More Answers (1)

Jihen
Jihen on 10 Dec 2022
function[A]=remplissage(n)
R1=(-4)*ones(n-1,1);
R2=ones(n-2,1);
A=6*eye(n)+diag(R1,-1)+diag(R1,1)+diag(R2,2)+diag(R2,-2);
end
  1 Comment
John D'Errico
John D'Errico on 10 Dec 2022
This does not actually answer the question, creating instead a matrix with 5 diagonals, so a penta-diagonal matrix.

Sign in to comment.

Categories

Find more on Sparse Matrices in Help Center and File Exchange

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!