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

How to get alternative signs for the numbers on the diagonal of a matrix

[John D'Errico]

What have you tried? For example, a simple loop would suffice.

Or, you could get trickier, and use the diag function.

[John D'Errico]

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

Answer 1

[Image Analyst]

One way, of many:

K = 2 * eye(5)
[rows, columns] = size(K)
K(1:2*rows+2:end) = -K(1:2*rows+2:end)

[kingsley]

Thank you so much !!!

Answer 2

[John D'Errico]

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.

[kingsley]

right!!! Thank you so much!