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POMerged iterators produce obscure results
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2012-08-23T16:06:41.243
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2017-05-23T10:29:12.163
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I'm trying to implement prime number generator using <a href="http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes" rel="nofollow noreferrer">Sieve of Eratosthenes</a> algorithm. I do it just to try using recursive <a href="https://stackoverflow.com/a/465043/862380">iterator merging</a> to implement sifter. What I do is this: <pre><code>from itertools import count,islice,groupby from heapq import merge def primes3(): p = 2 yield p sifter = (i*p for i in count(p)) s = next(sifter) for p in count(p+1): if p==s: # this p is sieved out print('s: {}'.format(s)) s = next(sifter) else: yield p # this is prime print('p: {}'.format(p)) sifter = (k for k, g in groupby(merge(sifter,(i*p for i in count(p))))) #add this serie to the sifter: p*p, p*(p+1), p*(p+2), ... print(list(islice(primes3(),10))) </code></pre> The output is: <pre><code>p: 3 s: 4 p: 5 p: 6 p: 7 p: 8 p: 9 p: 10 p: 11 s: 12 [2, 3, 5, 6, 7, 8, 9, 10, 11, 13] </code></pre> The first number to be sieved out is <code>4</code>. But the next is <code>12</code>, not <code>6</code> as it should be. Why is that? I checked it with the following code: <pre><code>>>> sifter = (i*2 for i in count(2)) >>> next(sifter) 4 >>> sifter = (k for k, g in groupby(merge(sifter,(i*3 for i in count(3))))) >>> print(list(islice(sifter,20))) [6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 26, 27, 28, 30, 32, 33, 34] </code></pre> So, as we may see, in test conditions sifter yields the correct results. Where am I making a mistake? I think it must be something trivial that I just don't see.
Tags
<python><algorithm><primes><itertools><sieve-of-eratosthenes>
Title
Merged iterators produce obscure results
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