## Copyright (C) 1995, 1996, 1997 Kurt Hornik
##
## This file is part of Octave.
##
## Octave 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 2, or (at your option)
## any later version.
##
## Octave 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 Octave; see the file COPYING. If not, write to the Free
## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301, USA.
## -*- texinfo -*-
## @deftypefn {Function File} {} npv (@var{r}, @var{p}, @var{i})
## Returns the net present value of a series of irregular (i.e., not
## necessarily identical) payments @var{p} which occur at the ends of @var{n}
## consecutive periods. @var{r} specifies the one-period interest rates and
## can either be a scalar (constant rates) or a vector of the same
## length as @var{p}.
##
## The optional argument @var{i} may be used to specify an initial
## investment.
##
## Note that the rate @var{r} is specified as a fraction (i.e., 0.05,
## not 5 percent).
## @seealso{irr, pv}
## @end deftypefn
## Author: KH
## Description: Net present value of a series of payments
function v = npv (r, p, ii)
if ((nargin < 2) || (nargin > 3))
print_usage ();
endif
if (! (isvector (p)))
error ("npv: p has to be a vector");
else
n = length (p);
p = reshape (p, 1, n);
endif
if (any (any (r <= -1)))
error ("npv: all interest rates must be > -1");
endif
if (isscalar (r))
d = 1 ./ (1 + r) .^ (0 : n);
elseif (isvector (r) && (length (r) == n))
d = [1, (1 ./ cumprod (reshape (1 + r, 1, n)))];
else
error ("npv: r must be a scalar or a vector of the same length as p");
endif
if (nargin == 3)
if (! isscalar (ii))
error ("npv: I_0 must be a scalar");
endif
else
ii = 0;
endif
p = [ii, p];
v = sum (d .* p);
endfunction
/*
@GROUP
Finance
@SYNTAX
npv (r, p, ii)
@DOC
@EXAMPLES
@NOTES
@SEE
*/