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1 # Copyright (C) 1996 John W. Eaton |
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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 the |
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7 # Free Software Foundation; either version 2, or (at your option) any |
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8 # later version. |
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9 # |
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10 # Octave is distributed in the hope that it will be useful, but WITHOUT |
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11 # ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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12 # FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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13 # 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 02111-1307, USA. |
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18 |
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19 function retval = list_primes (n) |
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20 |
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21 # usage: list_primes (n) |
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22 # |
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23 # List the first n primes. If n is unspecified, the first 30 primes |
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24 # are listed. |
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25 # |
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26 # The algorithm used is from page 218 of the TeXbook. |
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27 |
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28 if (nargin > 0) |
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29 if (! is_scalar (n)) |
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30 error ("list_primes: argument must be a scalar"); |
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31 endif |
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32 endif |
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33 |
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34 if (nargin == 0) |
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35 n = 30; |
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36 endif |
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37 |
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38 if (n == 1) |
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39 retval = 2; |
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40 return; |
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41 endif |
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42 |
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43 if (n == 2) |
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44 retval = [2; 3]; |
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45 return; |
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46 endif |
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47 |
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48 retval = zeros (1, n); |
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49 retval (1) = 2; |
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50 retval (2) = 3; |
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51 |
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52 n = n - 2; |
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53 i = 3; |
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54 p = 5; |
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55 while (n > 0) |
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56 |
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57 is_prime = 1; |
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58 is_unknown = 1; |
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59 d = 3; |
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60 while (is_unknown) |
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61 a = fix (p / d); |
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62 if (a <= d) |
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63 is_unknown = 0; |
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64 endif |
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65 if (a * d == p) |
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66 is_prime = 0; |
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67 is_unknown = 0; |
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68 endif |
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69 d = d + 2; |
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70 endwhile |
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71 |
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72 if (is_prime) |
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73 retval (i++) = p; |
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74 n--; |
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75 endif |
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76 p = p + 2; |
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77 |
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78 endwhile |
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79 |
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80 endfunction |
