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?
2 个评论
David Goodmanson
2019-11-11
Hi Aaron
check out the 'diag' function
Alex Treat
2020-10-30
coughs you were in the mec 103 class at CSU...
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"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
7 个评论
Juan Jesús Rocha Cuervo
2020-4-9
How can I stop the output of "M" in this example?
Jonathan Wharrier
2020-5-2
a semi-colon at the end of the line supresses output
This can be done also using toeplitz
A = toeplitz( [-1 2 zeros(1,498)] , [-1 4 zeros(1,498)] ); % size = 2 MB
and since there are many zeros you can save space using sparse
B = toeplitz( sparse([1 1],[1 2],[-1 2],1,498) , sparse([1 1],[1 2],[-1 4],1,498) ); % size = 28 kB
Steven Lord
2020-9-28
If you're creating this as a sparse matrix see spdiags.
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:
Ana Sarai
2025-2-22
Thank you, = )
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