% 4-8-2008
% Matrix construction

clear all;
close all;

% Size of matrix needed
N=100;

% Initialize matrix
A1=zeros(N,N);

% Loop over every index
for i=1:N
    for j=1:N
        if (i==j) % Main diagonal
            A1(i,j) = -2;
        elseif (i==j-1) % Superdiagonal
            A1(i,j) = 1;
        elseif (i==j+1) % Subdiagonal
            A1(i,j) = 1;
        end
    end
end

% Initialize matrix
A2=zeros(N,N);
% Only loop over diagonal indices
for i=1:N
    A2(i,i) = -2; % Every diagonal entry should be 2
    if (i~=N) % Do not execute if we are at the last column
        A2(i,i+1) = 1;
    end
    if (i~=1) % Do not execture if we are at the first column
        A2(i,i-1) = 1;
    end
end

% Create matrix using matlab functions constructively
% Look up the command diag for information on how this works
A3 = diag(-2*ones(N,1)) + diag(ones(N-1,1),1) + diag(ones(N-1,1),-1);