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By: a guest on Jun 13th, 2012  |  syntax: None  |  size: 1.17 KB  |  hits: 16  |  expires: Never
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  1. Explaining this primes generator function, i cannot understand [python]
  2. def primes(n):
  3.     if n==2: return [2]
  4.     elif n<2: return []
  5.     s=range(3,n+1,2)
  6.     mroot = n ** 0.5
  7.     half=(n+1)/2-1
  8.     i=0
  9.     m=3
  10.     while m <= mroot:
  11.         if s[i]:
  12.             j=(m*m-3)/2
  13.             s[j]=0
  14.             while j<half:
  15.                 s[j]=0
  16.                 j+=m
  17.         i=i+1
  18.         m=2*i+3
  19.     return [2]+[x for x in s if x]
  20.        
  21. def primes(n):
  22.     if n==2: return [2]
  23.        
  24. elif n<2: return []
  25.        
  26. s=range(3,n+1,2)
  27.        
  28. mroot = n ** 0.5
  29.        
  30. half=(n+1)/2-1
  31.        
  32. i=0
  33.     m=3
  34.        
  35. while m <= mroot:
  36.         if s[i]:
  37.        
  38. j=(m*m-3)/2
  39.             s[j]=0
  40.        
  41. while j<half:
  42.                 s[j]=0
  43.                 j+=m
  44.        
  45. i=i+1
  46.         m=2*i+3
  47.        
  48. return [2]+[x for x in s if x]
  49.        
  50. >>> numbers = range(40)
  51. >>> numbers[1] = 0    # 1 isn't prime
  52. >>> for i in numbers:
  53. ...     if i:
  54. ...         for j in range(i + i, len(numbers), i):
  55. ...             numbers[j] = 0
  56. ...
  57. >>> [n for n in numbers if n]
  58. [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]
  59.        
  60. [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10...]
  61.        
  62. [0, 0, 2, 3, 4, 5, 6, 7, 8, 9, 10...]
  63. [0, 0, 2, 3, 0, 5, 0, 7, 0, 9, 0...]
  64. [0, 0, 2, 3, 0, 5, 0, 7, 0, 0, 0...]
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