function o = lag(o, p)

% Returns a lagged time series
%
% INPUTS
% - o [dseries]
% - p [integer] Number of lags
%
% OUTPUTS
% - o [dseries]
%
% EXAMPLE
% Define a dseries object as follows:
%
% >> o = dseries(transpose(1:5))
%
% then o.lag(1) returns
%
%       | lag(Variable_1,1)
%    1Y | NaN
%    2Y | 1
%    3Y | 2
%    4Y | 3
%    5Y | 4

% Copyright © 2013-2023 Dynare Team
%
% This file is part of Dynare.
%
% Dynare is free software: you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% Dynare is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with Dynare.  If not, see <https://www.gnu.org/licenses/>.

% Set default number of lags
if nargin<2
    p = 1;
end

o = copy(o);
o.lag_(p);

return % --*-- Unit tests --*--

%@test:1
try
    data = transpose(0:1:50);
    ts = dseries(data,'1950Q1');
    a = ts.lag;
    b = ts.lag.lag;
    c = lag(ts,2);
    t(1) = true;
catch
    t(1) = false;
end

if t(1)
    DATA = [NaN(1,ts.vobs); transpose(0:1:49)];
    t(2) = dassert(a.data,DATA,1e-15);
    DATA = [NaN(2,ts.vobs); transpose(0:1:48)];
    t(3) = dassert(b.data,DATA,1e-15);
    t(4) = dassert(b.data,c.data,1e-15);
end

T = all(t);
%@eof:1
