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2303
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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 |
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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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68
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20 function retval = list_primes (n) |
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21 |
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22 ## usage: list_primes (n) |
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23 ## |
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24 ## List the first n primes. If n is unspecified, the first 30 primes |
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25 ## are listed. |
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26 ## |
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27 ## The algorithm used is from page 218 of the TeXbook. |
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28 |
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29 if (nargin > 0) |
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30 if (! is_scalar (n)) |
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31 error ("list_primes: argument must be a scalar"); |
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32 endif |
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33 endif |
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34 |
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35 if (nargin == 0) |
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36 n = 30; |
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37 endif |
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38 |
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39 if (n == 1) |
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40 retval = 2; |
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41 return; |
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42 endif |
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43 |
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44 if (n == 2) |
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45 retval = [2; 3]; |
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46 return; |
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47 endif |
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48 |
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49 retval = zeros (1, n); |
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50 retval (1) = 2; |
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51 retval (2) = 3; |
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52 |
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53 n = n - 2; |
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54 i = 3; |
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55 p = 5; |
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56 while (n > 0) |
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57 |
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58 is_prime = 1; |
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59 is_unknown = 1; |
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60 d = 3; |
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61 while (is_unknown) |
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62 a = fix (p / d); |
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63 if (a <= d) |
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64 is_unknown = 0; |
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65 endif |
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66 if (a * d == p) |
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67 is_prime = 0; |
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68 is_unknown = 0; |
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69 endif |
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70 d = d + 2; |
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71 endwhile |
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72 |
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73 if (is_prime) |
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74 retval (i++) = p; |
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75 n--; |
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76 endif |
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77 p = p + 2; |
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78 |
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79 endwhile |
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80 |
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81 endfunction |
