Select elements of Matrix?

Hi,

I have a 20x20 matrix, P. I wish to define a matrix Q such that the rows of Q are the LAST 18 rows of P (rows 2-20), and the columns of Q are the last 18 columns of P (columns 2-20).

Answer 1

[Guillaume]

It's basic matrix indexing:

Q = P(2:20, 2:20)

Answer 2

[C.J. Harris]

Like so:

P = magic(20);
Q = P(end-17:end,end-17:end);

Or this, as I assume you mean rows/columns 3 to 20 - otherwise you'll have 19 rows/columns:

P = magic(20);
Q = P(3:20,3:20);

Answer 3

[Azzi Abdelmalek]

P=rand(20) % Example  
n=18
Q=P(end-n+1:end,end-n+1:end)