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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 ## -*- texinfo -*- |
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18 ## @deftypefn {Function File} {} nper (@var{r}, @var{p}, @var{a}, @var{l}, @var{method}) |
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19 ## Return the number of regular payments of @var{p} necessary to |
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20 ## amortize @var{a} loan of amount @var{a} and interest @var{r}. |
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21 ## |
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22 ## The optional argument @var{l} may be used to specify an additional |
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23 ## lump-sum payment of @var{l} made at the end of the amortization time. |
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24 ## |
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25 ## The optional argument @var{method} may be used to specify whether |
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26 ## payments are made at the end (@var{"e"}, default) or at the beginning |
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27 ## (@var{"b"}) of each period. |
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28 ## |
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29 ## Note that the rate @var{r} is specified as a fraction (i.e., 0.05, |
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30 ## not 5 percent). |
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31 ## @end deftypefn |
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32 ## @seealso{pv, pmt, rate, and npv} |
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33 |
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34 ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at> |
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35 ## Description: Number of payments needed for amortizing a loan |
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36 |
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37 function n = nper (r, p, a, l, m) |
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38 |
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39 if ((nargin < 3) || (nargin > 5)) |
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40 usage ("nper (r, p, a, l, method)"); |
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41 endif |
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42 |
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43 if (! (is_scalar (r) && (r > -1))) |
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44 error ("nper: r must be a scalar > -1"); |
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45 elseif (! is_scalar (p)) |
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46 error ("nper: p must be a scalar"); |
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47 elseif (! is_scalar (a)) |
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48 error ("nper: a must be a scalar"); |
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49 endif |
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50 |
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51 if (nargin == 5) |
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52 if (! isstr (m)) |
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53 error ("nper: `method' must be a string"); |
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54 endif |
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55 elseif (nargin == 4) |
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56 if (isstr (l)) |
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57 m = l; |
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58 l = 0; |
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59 else |
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60 m = "e"; |
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61 endif |
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62 else |
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63 m = "e"; |
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64 l = 0; |
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65 endif |
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66 |
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67 if (strcmp (m, "b")) |
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68 p = p * (1 + r); |
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69 endif |
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70 |
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71 q = (p - r * a) / (p - r * l); |
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72 |
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73 if (q > 0) |
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74 n = - log (q) / log (1 + r); |
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75 else |
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76 n = Inf; |
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77 endif |
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78 |
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79 endfunction |
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80 |
