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## 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: nper (r, p, a [, l] [, method])
##
## Computes the number of regular payments of p necessary to amortize a
## loan of amount a and interest r.
##
## With the optional scalar argument l, one can specify an additional
## lump-sum payment of l made at the end of the amortization time. With
## the optional string argument `method', one can specify whether
## payments are made at the end ("e", default) or at the beginning ("b")
## of each period.
##
## Note that the rate r is not specified in percent, i.e., one has to
## write 0.05 rather than 5 %.
##
## See also: pv, pmt, rate; npv.
## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description: Number of payments needed for amortizing a loan
function n = nper (r, p, a, l, m)
if ((nargin < 3) || (nargin > 5))
usage ("nper (r, p, a [, l] [, method])");
endif
if !(is_scalar (r) && (r > -1))
error ("nper: r must be a scalar > -1");
elseif !is_scalar (p)
error ("nper: p must be a scalar");
elseif !is_scalar (a)
error ("nper: a must be a scalar");
endif
if (nargin == 5)
if !isstr (m)
error ("nper: `method' must be a string");
endif
elseif (nargin == 4)
if isstr (l)
m = l;
l = 0;
else
m = "e";
endif
else
m = "e";
l = 0;
endif
if strcmp (m, "b")
p = p * (1 + r);
endif
q = (p - r * a) / (p - r * l);
if (q > 0)
n = - log (q) / log (1 + r);
else
n = Inf;
endif
endfunction
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