Talk:Pernicious numbers: Difference between revisions

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C solution too cryptic?

Hi, the C solution is a bit mysterious to me. The for loop seems to "remove the zeros" if I understand correctly, but then what's so special about 2693408940 ?? How can a simple bitwise & tell if the number of 1 (which is now the length since all zeros have been removed) is prime? A bit of explanation in the introduction of the section or in comment would be welcome--Grondilu (talk) 14:43, 12 March 2014 (UTC)

Edit, hum after some analysis it appears that 2693408940 encodes the prime numbers in its binary form: 10100000100010100010100010101100. From right to left, starting from 0, the one marks the primes. Sill this could use some explanations.--Grondilu (talk) 14:51, 12 March 2014 (UTC)

OK, I understand now. I added some comments in the code. Hope it's ok.--Grondilu (talk) 15:02, 12 March 2014 (UTC)

Er yeah, it was pretty cryptic, and your analysis is absolutely correct. But I changed the routine again for something faster, though I did put in some comments this time. I didn't retain the "0b010011..." literal format since it's a GCC exteinsion, but if you want it back in I'm not really troubled by it. --Ledrug (talk) 19:40, 12 March 2014 (UTC)

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 :Edit, hum after some analysis it appears that 2693408940 encodes the prime numbers in its binary form:  10100000100010100010100010101100.   From right to left, starting from 0, the one marks the primes.  Sill this could use some explanations.--[[User:Grondilu|Grondilu]] ([[User talk:Grondilu|talk]]) 14:51, 12 March 2014 (UTC)
 :OK, I understand now.  I added some comments in the code.  Hope it's ok.--[[User:Grondilu|Grondilu]] ([[User talk:Grondilu|talk]]) 15:02, 12 March 2014 (UTC)
+:: Er yeah, it was pretty cryptic, and your analysis is absolutely correct.  But I changed the routine again for something faster, though I did put in some comments this time.  I didn't retain the "0b010011..." literal format since it's a GCC exteinsion, but if you want it back in I'm not really troubled by it. --[[User:Ledrug|Ledrug]] ([[User talk:Ledrug|talk]]) 19:40, 12 March 2014 (UTC)