Discipulus, in all of this really mind-boggling code, why aren't you eliminating the numbers which end with 2, 4, 5, 6, 8, and 0 before doing any math or other munging on the number? Wouldn't eliminating those numbers right at the start, before any fancy munging, save time? Those numbers (with the exception of '2' and '5') will never be prime, so why are you even munging them?

why aren't you eliminating the numbers which end with 2, 4, 5, 6, 8, and 0 before doing any math

If you want you can, but how do you spot those numbers? you are using a regex that, as you can guess, is slower than math checks. Theese numbers will be catched with the modulo operator anyway, as choroba explained.
Also the regex in the original code is also not correct: you skip 2 and 5.

perl -e "map {print qq($_ ) unless $_ =~/(2|4|5|6|8|0)$/ } @ARGV" 2 3 
+5 7 11 13 17 19 23 29 37
3 7 11 13 17 19 23 29 37
[download]

i used a different regex (and precompiled in the bench program using qr'\d[245680]$'):

perl -e "map {print qq($_ ) unless $_ =~/\d[245680]$/ } @ARGV" 2 3 5 7
+ 11 13 17 19 23 29 37
2 3 5 7 11 13 17 19 23 29 37
[download]

About saving time skipping numbers ending with some fixed digit, as said, the regex solution is slower: with this code:
Read more... (2 kB)
You will see a significant decreese of speed if you skip numbers using a regex:

1..1
ok 1 - same
                     Rate    primes_skipper   primes_original primes_o
+pt_till_11
primes_skipper     33.5/s                --              -35%         
+      -94%
primes_original    51.4/s               53%                --         
+      -90%
primes_opt_till_11  519/s             1448%              909%         
+        --
[download]


HtH
L*

There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.

Here are some of the problems.

• Some made assumptions I knew the regex was slowing down my code. I had no idea the regex was the problem.
• Some made assumptions of my prowess in math. If it isn't addition, subtraction, multiplication, or division; I won't think to use it. So, using sqrt is confusing.
• Only aaron_baugher commented directly on my code and showed me where I overdid it with the check for divisibility by 9. Everyone else wrote their own versions without commenting on the code I provided first.
• No one commented their code! I'm still stuck back at choroba's first prime example at the square root test. Adding the use of square roots still bothers me for some reason.

Like with many car shows my hubby watches where the show glosses over a lot, it is like you took the logo off the car, trashed the car, and put the logo on another car. I still don't know what is so wrong with the original car (function) that the only thing worth saving was the logo (function name). So take one part out at a time, explain why it is broken, then install the new part telling the audience why the new part is better. Pretty please?

No matter how hysterical I get, my problems are not time sensitive. So, relax, have a cookie, and a very nice day!
Lady Aleena

Has choroba not already answered that last month?

I didn't understand choroba's reason for doing math on numbers which need no math to prove they are not prime.

No matter how hysterical I get, my problems are not time sensitive. So, relax, have a cookie, and a very nice day!
Lady Aleena