function [V,Beta,R] = qr_left (A)
%QR_LEFT left-looking Householder QR factorization.
% Example:
%  [V,Beta,R] = qr_left (A)
% See also: cs_demo

%   Copyright 2006-2007, Timothy A. Davis.
%   http://www.cise.ufl.edu/research/sparse

[m n] = size (A) ;
V = zeros (m,n) ;
Beta = zeros (1,n) ;
R = zeros (m,n) ;
for k = 1:n
    x = A (:,k) ;
    for i = 1:k-1
	v = V (i:m,i) ;
	beta = Beta (i) ;
	x (i:m) = x (i:m) - v * (beta * (v' * x (i:m))) ;
    end
    [v,beta,s] = gallery ('house', x (k:m), 2) ;
    V (k:m,k) = v ;
    Beta (k) = beta ;
    R (1:(k-1),k) = x (1:(k-1)) ;
    R (k,k) = s ;
end
