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1 ## Copyright (C) 1993, 1995, 1996, 1997, 2000, 2002, 2005, 2007 |
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2 ## John W. Eaton |
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3 ## |
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4 ## This file is part of Octave. |
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5 ## |
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6 ## Octave is free software; you can redistribute it and/or modify it |
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7 ## under the terms of the GNU General Public License as published by |
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8 ## the Free Software Foundation; either version 3 of the License, or (at |
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9 ## your option) any later version. |
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10 ## |
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11 ## Octave is distributed in the hope that it will be useful, but |
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12 ## WITHOUT ANY WARRANTY; without even the implied warranty of |
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13 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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14 ## General Public License for more details. |
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15 ## |
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16 ## You should have received a copy of the GNU General Public License |
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17 ## along with Octave; see the file COPYING. If not, see |
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18 ## <http://www.gnu.org/licenses/>. |
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19 |
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20 ## -*- texinfo -*- |
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21 ## @deftypefn {Function File} {} list_primes (@var{n}) |
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22 ## List the first @var{n} primes. If @var{n} is unspecified, the first |
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23 ## 30 primes are listed. |
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24 ## |
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25 ## The algorithm used is from page 218 of the |
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26 ## @iftex |
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27 ## @tex |
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28 ## {\TeX}book. |
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29 ## @end tex |
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30 ## @end iftex |
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31 ## @ifinfo |
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32 ## TeXbook. |
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33 ## @end ifinfo |
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34 ## @end deftypefn |
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35 |
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36 ## Author: jwe |
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37 |
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38 function retval = list_primes (n) |
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39 |
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40 if (nargin > 0) |
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41 if (! isscalar (n)) |
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42 error ("list_primes: argument must be a scalar"); |
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43 endif |
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44 endif |
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45 |
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46 if (nargin == 0) |
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47 n = 30; |
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48 endif |
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49 |
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50 if (n == 1) |
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51 retval = 2; |
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52 return; |
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53 endif |
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54 |
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55 if (n == 2) |
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56 retval = [2; 3]; |
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57 return; |
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58 endif |
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59 |
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60 retval = zeros (1, n); |
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61 retval (1) = 2; |
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62 retval (2) = 3; |
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63 |
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64 n = n - 2; |
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65 i = 3; |
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66 p = 5; |
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67 while (n > 0) |
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68 |
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69 is_prime = 1; |
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70 is_unknown = 1; |
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71 d = 3; |
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72 while (is_unknown) |
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73 a = fix (p / d); |
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74 if (a <= d) |
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75 is_unknown = 0; |
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76 endif |
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77 if (a * d == p) |
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78 is_prime = 0; |
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79 is_unknown = 0; |
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80 endif |
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81 d = d + 2; |
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82 endwhile |
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83 |
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84 if (is_prime) |
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85 retval (i++) = p; |
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86 n--; |
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87 endif |
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88 p = p + 2; |
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89 |
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90 endwhile |
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91 |
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92 endfunction |
