how to make the main diagonal of K are alternatively 2 and -2’s

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kingsley
kingsley on 15 Feb 2017
Commented: kingsley on 16 Feb 2017
How to get alternative signs for the numbers on the diagonal of a matrix
  2 Comments
John D'Errico
John D'Errico on 15 Feb 2017
Please don't add new questions just to add additional information to an existing question. I have moved the separate question by this individual into a comment:
I want to create a matrix that the main diagonal of K are alternatively 2 and -2’s, the sub- and sup-diagonal of K alternatively 1 and -1’s, and everywhere else 0. The size of K is 2n by 2n.
Here is what I got so far.
x=ones(1,5);
y=ones(1,4);
x2=2*x;
y2=y*-1;
z=diag(x2,0)+diag(y2,+1)
d=diag(y2,-1)
g=z+d

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Answers (2)

Image Analyst
Image Analyst on 15 Feb 2017
One way, of many:
K = 2 * eye(5)
[rows, columns] = size(K)
K(1:2*rows+2:end) = -K(1:2*rows+2:end)

John D'Errico
John D'Errico on 15 Feb 2017
Edited: John D'Errico on 15 Feb 2017
So you want to create a tridiagonal matrix, with the main diagonal alternating +/- 2, and the sub and super diagonals alternating +/- 1?
n = 5;
k = mod(1:n,2)*2 - 1;
A = diag(k*2) + diag(k(1:n-1),-1) + diag(k(1:n-1),1)
A =
2 1 0 0 0
1 -2 -1 0 0
0 -1 2 1 0
0 0 1 -2 -1
0 0 0 -1 2
A loop would have been nearly as easy, but not really necessary if you know the tools in MATLAB.
The best solution (IMHO) is to use spdiags.

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