Getting a diag function

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Mark Loui 2021-3-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/776837-getting-a-diag-function

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

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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 2021-3-18

编辑：Jan 2021-3-18

在 MATLAB Online 中打开

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    