Comments on: Next Prime in C http://involution.com/2007/07/09/next-prime-in-c/ Tony Perrie's Weblog Tue, 15 May 2012 22:43:17 +0000 hourly 1 http://wordpress.org/?v=3.3.2 By: cans in a bag http://involution.com/2007/07/09/next-prime-in-c/comment-page-1/#comment-23299 cans in a bag Sat, 14 Jul 2007 06:16:31 +0000 http://involution.com/2007/07/09/next-prime-in-c/#comment-23299 MATH! MATH!

]]> By: Administrator http://involution.com/2007/07/09/next-prime-in-c/comment-page-1/#comment-23165 Administrator Tue, 10 Jul 2007 18:53:12 +0000 http://involution.com/2007/07/09/next-prime-in-c/#comment-23165 You don't need to return 7 because it's in the right mod 6 row. Also, the compares are just checking for divisibility outside of the mod 6 rows. So, it's more efficient to do that than to actually run through every divisor. You don’t need to return 7 because it’s in the right mod 6 row. Also, the compares are just checking for divisibility outside of the mod 6 rows. So, it’s more efficient to do that than to actually run through every divisor.

]]> By: Andy http://involution.com/2007/07/09/next-prime-in-c/comment-page-1/#comment-23164 Andy Tue, 10 Jul 2007 18:45:37 +0000 http://involution.com/2007/07/09/next-prime-in-c/#comment-23164 Ummm wordpress sucks. Reposting without a less than sign: I haven't thought much about this whole mod-6 thing and how much it buys you, but the beginning of the function is sloppy. You check for n<7 but then you don't handle the default case. You need a "return 7" in that block. I still maintain that you should not calculate that sqrt more than once. Even if it's not proven that there's a prime between every two squares, you can calculate max_divisor once, keep track of when you exceed max_divisor^2, and increment it at that point. As a side note, I wonder if there's a special function to only calculate the floor of the sqrt . that would be pretty easy to write, and really useful for situations like this where you don't care Ummm wordpress sucks. Reposting without a less than sign:

I haven’t thought much about this whole mod-6 thing and how much it buys you, but the beginning of the function is sloppy. You check for n<7 but then you don’t handle the default case. You need a “return 7″ in that block.

I still maintain that you should not calculate that sqrt more than once. Even if it’s not proven that there’s a prime between every two squares, you can calculate max_divisor once, keep track of when you exceed max_divisor^2, and increment it at that point.

As a side note, I wonder if there’s a special function to only calculate the floor of the sqrt . that would be pretty easy to write, and really useful for situations like this where you don’t care

]]> By: Andy http://involution.com/2007/07/09/next-prime-in-c/comment-page-1/#comment-23163 Andy Tue, 10 Jul 2007 18:43:26 +0000 http://involution.com/2007/07/09/next-prime-in-c/#comment-23163 I haven't thought much about this whole mod-6 thing and how much it buys you, but the beginning of the function is sloppy. You check for n I haven’t thought much about this whole mod-6 thing and how much it buys you, but the beginning of the function is sloppy. You check for n

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