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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} {} npv (@var{r}, @var{p}, @var{i}) |
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22 ## Returns the net present value of a series of irregular (i.e., not |
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23 ## necessarily identical) payments @var{p} which occur at the ends of @var{n} |
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24 ## consecutive periods. @var{r} specifies the one-period interest rates and |
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25 ## can either be a scalar (constant rates) or a vector of the same |
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26 ## length as @var{p}. |
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27 ## |
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28 ## The optional argument @var{i} may be used to specify an initial |
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29 ## investment. |
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30 ## |
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31 ## Note that the rate @var{r} is specified as a fraction (i.e., 0.05, |
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32 ## not 5 percent). |
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33 ## @end deftypefn |
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34 ## |
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35 ## @seealso{irr and pv} |
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36 |
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37 ## Author: KH <Kurt.Hornik@wu-wien.ac.at> |
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38 ## Description: Net present value of a series of payments |
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39 |
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40 function v = npv (r, p, i) |
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41 |
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42 if ((nargin < 2) || (nargin > 3)) |
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43 usage ("npv (r, p, i"); |
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44 endif |
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45 |
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46 if (! (isvector (p))) |
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47 error ("npv: p has to be a vector"); |
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48 else |
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49 n = length (p); |
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50 p = reshape (p, 1, n); |
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51 endif |
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52 |
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53 if (any (any (r <= -1))) |
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54 error ("npv: all interest rates must be > -1"); |
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55 endif |
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56 if (isscalar (r)) |
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57 d = 1 ./ (1 + r) .^ (0 : n); |
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58 elseif (isvector (r) && (length (r) == n)) |
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59 d = [1, (1 ./ cumprod (reshape (1 + r, 1, n)))]; |
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60 else |
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61 error ("npv: r must be a scalar or a vector of the same length as p"); |
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62 endif |
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63 |
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64 if (nargin == 3) |
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65 if (! isscalar (i)) |
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66 error ("npv: I_0 must be a scalar"); |
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67 endif |
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68 else |
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69 i = 0; |
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70 endif |
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71 |
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72 p = [i, p]; |
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73 v = sum (d .* p); |
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74 |
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75 endfunction |
