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1 ## Copyright (C) 1995, 1996, 1997 Kurt Hornik |
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2 ## |
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3 ## This file is part of Octave. |
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4 ## |
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5 ## Octave is free software; you can redistribute it and/or modify it |
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6 ## under the terms of the GNU General Public License as published by |
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7 ## the Free Software Foundation; either version 2, or (at your option) |
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8 ## any later version. |
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9 ## |
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10 ## Octave is distributed in the hope that it will be useful, but |
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11 ## WITHOUT ANY WARRANTY; without even the implied warranty of |
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12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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13 ## General Public License for more details. |
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14 ## |
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15 ## You should have received a copy of the GNU General Public License |
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16 ## along with Octave; see the file COPYING. If not, write to the Free |
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17 ## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA |
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18 ## 02110-1301, USA. |
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19 |
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20 ## -*- texinfo -*- |
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21 ## @deftypefn {Function File} {} nper (@var{r}, @var{p}, @var{a}, @var{l}, @var{method}) |
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22 ## Return the number of regular payments of @var{p} necessary to |
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23 ## amortize @var{a} loan of amount @var{a} and interest @var{r}. |
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24 ## |
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25 ## The optional argument @var{l} may be used to specify an additional |
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26 ## lump-sum payment of @var{l} made at the end of the amortization time. |
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27 ## |
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28 ## The optional argument @var{method} may be used to specify whether |
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29 ## payments are made at the end (@var{"e"}, default) or at the beginning |
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30 ## (@var{"b"}) of each period. |
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31 ## |
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32 ## Note that the rate @var{r} is specified as a fraction (i.e., 0.05, |
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33 ## not 5 percent). |
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34 ## @seealso{pv, pmt, rate, npv} |
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35 ## @end deftypefn |
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36 |
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37 ## Author: KH <Kurt.Hornik@wu-wien.ac.at> |
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38 ## Description: Number of payments needed for amortizing a loan |
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39 |
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40 function n = nper (r, p, a, l, m) |
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41 |
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42 if ((nargin < 3) || (nargin > 5)) |
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43 usage ("nper (r, p, a, l, method)"); |
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44 endif |
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45 |
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46 if (! (isscalar (r) && (r > -1))) |
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47 error ("nper: r must be a scalar > -1"); |
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48 elseif (! isscalar (p)) |
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49 error ("nper: p must be a scalar"); |
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50 elseif (! isscalar (a)) |
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51 error ("nper: a must be a scalar"); |
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52 endif |
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53 |
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54 if (nargin == 5) |
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55 if (! ischar (m)) |
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56 error ("nper: `method' must be a string"); |
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57 endif |
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58 elseif (nargin == 4) |
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59 if (ischar (l)) |
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60 m = l; |
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61 l = 0; |
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62 else |
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63 m = "e"; |
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64 endif |
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65 else |
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66 m = "e"; |
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67 l = 0; |
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68 endif |
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69 |
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70 if (strcmp (m, "b")) |
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71 p = p * (1 + r); |
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72 endif |
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73 |
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74 q = (p - r * a) / (p - r * l); |
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75 |
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76 if (q > 0) |
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77 n = - log (q) / log (1 + r); |
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78 else |
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79 n = Inf; |
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80 endif |
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81 |
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82 endfunction |
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83 |
