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