% Script MkSparseEx.m
% use spdiags to make Poisson matrix
close all;

n = 5;
e = ones(n^2,1);   % make a vector for the diagonal
A = spdiags([-e -e 4*e -e -e],[-n -1 0 1 n],n^2,n^2);
B = gallery('poisson',5);
spy(A-B)  % not quite right; need to fix up A
for k = n:n:n^2-n
    A(k,k+1) = 0; A(k+1,k) = 0;
end
figure; spy(A-B)
disp(' If 0, then A and B are the same: ')
norm(A-B,inf)
disp(' If 1, A is sparse: ')
issparse(A)
disp(' If 0, full(A) is not sparse: ')
issparse(full(A))