Getting a diag function

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Mark Loui
Mark Loui 2021-3-18
编辑: Matt J 2021-3-18
Hi there im stuck with this problem and really would like help here
n=length(x) %Let says is 20
x=rand(n,1);
y=rand(n,1);
RHS_empty=ones(ny,1);
iter_h=0;
for i=1:n-1
h(i)=x(i+1)-x(i);
iter_h=iter_h+1;
Ma_Sp=diag(1 h(i)) 2(h(i+1)*h(i))
end
I like to create a diagonal matrix with the interval with the specific things,
The diagonal matrix size is depending on the h(i), meaning h19, i like to get the matrix to be 20x20 with 1 at the first lane and h1 2h2*h1 h2 2h3*h2 hn 2h(i+1)*h(i) for the second coloum and it goes on for the third coloum and the fourth all the way to the 20th column, i am stuck on this matter and would appreciate your help
  3 个评论
Mark Loui
Mark Loui 2021-3-18
ermm are you familar with Natural Splines Boundary Condition, my main goal is to create a matrix as such. If not
It means i like to create a matrix that is diagonal
in matlab code wise is
A=[1 0 0 0 ;0 h1 2(h2+h1) h2 2(h3+h2) ;0 0 h2 2(h3+h2); 0 0 0 h4] This is what i meant by the entire sentence
So if n=19, there should be a 20x20 matrix fill with the listed diagonal matrix above, this is what i mean. Hope this clears things up
Jan
Jan 2021-3-18
编辑:Jan 2021-3-18
The shown matrix A is not diagonal. Diagonal matrices contaoin non-zero elements on the diagonal only. You can create them with the command diag() .
There is a large, if not infitine number of ways, to fill a 20x20 matrix with a 4x4 matrix.
Maybe you are looking for the kron() command:
x = [1,2;3,4]
x =
1 2
3 4
kron(eye(3), x)
ans =
1 2 0 0 0 0
3 4 0 0 0 0
0 0 1 2 0 0
0 0 3 4 0 0
0 0 0 0 1 2
0 0 0 0 3 4

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