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annotate scripts/finance/nper.m @ 3191:e4f4b2d26ee9

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[project @ 1998-10-23 05:43:59 by jwe]
author jwe
date Fri, 23 Oct 1998 05:44:01 +0000
parents
children 5e0a0b1cba43
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3191
e4f4b2d26ee9 [project @ 1998-10-23 05:43:59 by jwe]
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1 ## Copyright (C) 1995, 1996, 1997 Kurt Hornik
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2 ##
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3 ## This program is free software; you can redistribute it and/or modify
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4 ## it under the terms of the GNU General Public License as published by
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5 ## the Free Software Foundation; either version 2, or (at your option)
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6 ## any later version.
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7 ##
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8 ## This program is distributed in the hope that it will be useful, but
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9 ## WITHOUT ANY WARRANTY; without even the implied warranty of
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10 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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11 ## General Public License for more details.
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12 ##
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13 ## You should have received a copy of the GNU General Public License
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14 ## along with this file. If not, write to the Free Software Foundation,
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15 ## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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16
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17 ## usage: nper (r, p, a [, l] [, method])
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18 ##
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19 ## Computes the number of regular payments of p necessary to amortize a
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20 ## loan of amount a and interest r.
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21 ##
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22 ## With the optional scalar argument l, one can specify an additional
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23 ## lump-sum payment of l made at the end of the amortization time. With
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24 ## the optional string argument `method', one can specify whether
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25 ## payments are made at the end ("e", default) or at the beginning ("b")
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26 ## of each period.
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27 ##
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28 ## Note that the rate r is not specified in percent, i.e., one has to
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29 ## write 0.05 rather than 5 %.
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30 ##
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31 ## See also: pv, pmt, rate; npv.
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32
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33 ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
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34 ## Description: Number of payments needed for amortizing a loan
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35
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36 function n = nper (r, p, a, l, m)
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37
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38 if ((nargin < 3) || (nargin > 5))
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39 usage ("nper (r, p, a [, l] [, method])");
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40 endif
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41
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42 if !(is_scalar (r) && (r > -1))
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43 error ("nper: r must be a scalar > -1");
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44 elseif !is_scalar (p)
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45 error ("nper: p must be a scalar");
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46 elseif !is_scalar (a)
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47 error ("nper: a must be a scalar");
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48 endif
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49
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50 if (nargin == 5)
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51 if !isstr (m)
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52 error ("nper: `method' must be a string");
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53 endif
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54 elseif (nargin == 4)
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55 if isstr (l)
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56 m = l;
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57 l = 0;
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58 else
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59 m = "e";
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60 endif
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61 else
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62 m = "e";
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63 l = 0;
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64 endif
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65
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66 if strcmp (m, "b")
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67 p = p * (1 + r);
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68 endif
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69
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70 q = (p - r * a) / (p - r * l);
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71
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72 if (q > 0)
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73 n = - log (q) / log (1 + r);
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74 else
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75 n = Inf;
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76 endif
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77
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78 endfunction
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79