## Copyright (C) 1995, 1996, 1997 Kurt Hornik ## ## This program 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. ## ## This program 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 this file. If not, write to the Free Software Foundation, ## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. ## usage: npv (r, p [, i]) ## ## Returns the net present value of a series of irregular (i.e., not ## necessarily identical) payments p which occur at the ends of n ## consecutive periods. r specifies the one-period interest rates and ## can either be a scalar (constant rates) or a vector of the same ## length as p. ## ## With the optional scalar argument i, one can specify an initial ## investment. ## ## Note that rates are not specified in percent, i.e., one has to write ## 0.05 rather than 5 %. ## ## See also: irr; pv. ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at> ## Description: Net present value of a series of payments function v = npv (r, p, i) if ((nargin < 2) || (nargin > 3)) usage ("npv (r, p [, i]"); endif if !(is_vector (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 is_scalar (r) d = 1 ./ (1 + r) .^ (0 : n); elseif (is_vector (r) && (length (r) == n)) d = [1, 1 ./ cumprod (reshape (1 + r, 1, n))]; else error (["npv: r has be a scalar ", ... "or a vector of the same length as p"]); endif if (nargin == 3) if !is_scalar (i) error ("npv: I_0 must be a scalar"); endif else i = 0; endif p = [i, p]; v = sum (d .* p); endfunction

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