Creating a tridiagonal matrix

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Aaron Atkinson
Aaron Atkinson 2019-11-11
评论： John D'Errico 2022-12-10
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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David Goodmanson
David Goodmanson 2019-11-11
Hi Aaron
check out the 'diag' function
Alex Treat
Alex Treat 2020-10-30
coughs you were in the mec 103 class at CSU...

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Stephen23
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
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
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Arth Patel
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.

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Jihen
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
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John D'Errico
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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