function [R] = calc_error(M)
% Calculates the LINEAR mistake at each cell of F and G.
% The difference between the equation and the actual cell value


s = size(M) ; rows = s(1); cols = s(2);
h = 1/(rows-1);
R = zeros(s);

% Error at F cells
%==================
for i= 2:2:rows - 1
    for j = 2:2:cols - 1
        R(i,j) = M(i,j) - (1/(2*h))*(M(i,j+1) - M(i,j-1) + M(i+1,j) - M(i-1,j)) ;
    end
end

% Error at G cells
%====================
for i= 3:2:rows - 2
    for j = 3:2:cols - 2
        R(i,j) = M(i,j) - (1/(2*h))*(M(i+1,j) - M(i-1,j) - (M(i,j+1) - M(i,j-1))) ;
    end
end