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1 function retval = list_primes (n) |
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2 |
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3 # usage: list_primes (n) |
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4 # |
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5 # List the first n primes. If n is unspecified, the first 30 primes |
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6 # are listed. |
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7 # |
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8 # The algorithm used is from page 218 of the TeXbook. |
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9 |
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10 if (nargin > 0) |
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11 if (! is_scalar (n)) |
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12 error ("list_primes: argument must be a scalar"); |
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13 endif |
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14 endif |
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15 |
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16 if (nargin == 0) |
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17 n = 30; |
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18 endif |
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19 |
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20 if (n == 1) |
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21 retval = 2; |
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22 return; |
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23 endif |
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24 |
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25 if (n == 2) |
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26 retval = [2; 3]; |
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27 return; |
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28 endif |
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29 |
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30 retval = zeros (1, n); |
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31 retval (1) = 2; |
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32 retval (2) = 3; |
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33 |
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34 n = n - 2; |
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35 i = 3; |
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36 p = 5; |
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37 while (n > 0) |
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38 |
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39 is_prime = 1; |
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40 is_unknown = 1; |
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41 d = 3; |
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42 while (is_unknown) |
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43 a = fix (p / d); |
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44 if (a <= d) |
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45 is_unknown = 0; |
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46 endif |
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47 if (a * d == p) |
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48 is_prime = 0; |
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49 is_unknown = 0; |
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50 endif |
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51 d = d + 2; |
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52 endwhile |
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53 |
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54 if (is_prime) |
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55 retval (i++) = p; |
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56 n--; |
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57 endif |
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58 p = p + 2; |
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59 |
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60 endwhile |
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61 |
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62 endfunction |
