function [msg,A,B,C,D] = abcdchck(A,B,C,D)
%ABCDCHK Checks dimensional consistency of A,B,C,D matrices.
%   ERROR(ABCDCHK(A,B,C,D)) checks that the dimensions of A,B,C,D
%   are consistent for a linear, time-invariant system model.
%   An error occurs if the nonzero dimensions are not consistent.
%
%   [MSG,A,B,C,D] = ABCDCHK(A,B,C,D) also alters the dimensions
%   any 0-by-0 empty matrices to make them consistent with the others.

%   Copyright 1984-2000 The MathWorks, Inc.
%   $Revision: 1.20 $  $Date: 2000/01/31 04:12:15 $

if nargin < 4, D = []; end
if nargin < 3, C = []; end
if nargin < 2, B = []; end
if nargin < 1, A = []; end

[ma,na] = size(A);
[mb,nb] = size(B);
[mc,nc] = size(C);
[md,nd] = size(D);

if mc==0 & nc==0 & (md==0 | na==0)
   mc = md; nc = na; C = zeros(mc,nc);
end
if mb==0 & nb==0 & (ma==0 | nd==0)
   mb = ma; nb = nd; B = zeros(mb,nb);
end
if md==0 & nd==0 & (mc==0 | nb==0)
   md = mc; nd = nb; D = zeros(md,nd);
end
if ma==0 & na==0 & (mb==0 | nc==0)
   ma = mb; na = nc; A = zeros(ma,na);
end

if ma~=na & nargin>=1
   msg = 'The A matrix must be square';
elseif ma~=mb & nargin>=2
   msg = 'The A and B matrices must have the same number of rows.';
elseif na~=nc & nargin>=3
   msg = 'The A and C matrices must have the same number of columns.';
elseif md~=mc & nargin>=4
   msg = 'The C and D matrices must have the same number of rows.';
elseif nd~=nb & nargin>=4
   msg = 'The B and D matrices must have the same number of columns.';
else
   msg = '';
end
