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1 #!/usr/bin/env python |
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2 |
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3 """Fun with generators. Corresponding Haskell implementation: |
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4 |
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5 primes = 2 : sieve [3, 5..] |
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6 where sieve (p:ns) = p : sieve [n | n <- ns, mod n p /= 0] |
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7 """ |
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8 |
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9 from itertools import dropwhile, ifilter, islice, count, chain |
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10 |
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11 def primes(): |
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12 """Generate all primes.""" |
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13 def sieve(ns): |
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14 p = ns.next() |
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15 # It is important to yield *here* in order to stop the |
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16 # infinite recursion. |
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17 yield p |
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18 ns = ifilter(lambda n: n % p != 0, ns) |
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19 for n in sieve(ns): |
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20 yield n |
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21 |
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22 odds = ifilter(lambda i: i % 2 == 1, count()) |
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23 return chain([2], sieve(dropwhile(lambda n: n < 3, odds))) |
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24 |
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25 if __name__ == "__main__": |
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26 import sys |
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27 try: |
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28 n = int(sys.argv[1]) |
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29 except (ValueError, IndexError): |
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30 n = 10 |
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31 p = primes() |
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32 print "The first %d primes: %s" % (n, list(islice(p, n))) |
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