Creating a tridiagonal matrix
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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?
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Stephen23
2019-11-11
编辑:Stephen23
2022-3-20
"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:
1- See @giannit's comment: https://www.mathworks.com/matlabcentral/answers/490368-creating-a-tridiagonal-matrix#comment_1027546
>> 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 个评论
Arth Patel
2020-9-29
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
2020-10-30
"It's not clear to me how you're indexing a matrix using just one argument."
The second example uses linear indexing:
更多回答(1 个)
Jihen
2022-12-10
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 个评论
John D'Errico
2022-12-10
This does not actually answer the question, creating instead a matrix with 5 diagonals, so a penta-diagonal matrix.
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