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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, 59 Temple Place - Suite 330, Boston, MA |
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18 ## 02111-1307, USA. |
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19 |
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20 ## -*- texinfo -*- |
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21 ## @deftypefn {Function File} {} pmt (@var{r}, @var{n}, @var{a}, @var{l}, @var{method}) |
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22 ## Return the amount of periodic payment necessary to amortize a loan |
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23 ## of amount a with interest rate @var{r} in @var{n} periods. |
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24 ## |
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25 ## The optional argument @var{l} may be used to specify a terminal |
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26 ## lump-sum payment. |
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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 ## @end deftypefn |
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32 ## |
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33 ## @seealso{pv, nper, and rate} |
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34 |
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35 ## Author: KH <Kurt.Hornik@ci.tuwien.ac.at> |
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36 ## Description: Amount of periodic payment needed to amortize a loan |
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37 |
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38 function p = pmt (r, n, a, l, m) |
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39 |
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40 if (nargin < 3 || nargin > 5) |
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41 usage ("pmt (r, n, a, l, method)"); |
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42 endif |
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43 |
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44 if (! (isscalar (r) && r > -1)) |
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45 error ("pmt: rate must be a scalar > -1"); |
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46 elseif (! (isscalar (n) && n > 0)) |
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47 error ("pmt: n must be a positive scalar"); |
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48 elseif (! (isscalar (a) && a > 0)) |
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49 error ("pmt: a must be a positive scalar"); |
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50 endif |
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51 |
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52 if (nargin == 5) |
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53 if (! isstr (m)) |
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54 error ("pmt: `method' must be a string"); |
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55 endif |
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56 elseif (nargin == 4) |
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57 if (isstr (l)) |
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58 m = l; |
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59 l = 0; |
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60 else |
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61 m = "e"; |
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62 endif |
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63 else |
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64 l = 0; |
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65 m = "e"; |
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66 endif |
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67 |
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68 p = r * (a - l * (1 + r)^(-n)) / (1 - (1 + r)^(-n)); |
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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 |
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75 endfunction |
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76 |
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77 |
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78 |
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79 |
