Here's a nice version for if you need to check a lot of large numbers to see if they're prime: 
use strict;
use warnings;

my ($c, $i) = 0;

for ($i = 36544645654; $c < 100; $i++) {
    $c++, print "$i\n" if prime($i);
}

BEGIN {
    my @primes = (2);
    sub prime {
        my $n = $_[0]; my $p;
        my $rt = int sqrt $n;
        for $p (@primes) {
            return 0 if $n % $p == 0;
            return 1 if $p >= $rt;
        }
        for $p (($primes[-1]+1)..$rt) {
            if (prime($p)) {
                push @primes, $p;
                return 0 if $n % $p == 0;
            }
        }
        return 1;
    }
}

[download]
This version remembers the primes you've found so far, and only checks necessary numbers to see if they're prime. Yes, I know you could also start by knocking out all the /2, /3, /5, etc numbers in your range, but this doesn't improve speed very much, and adds complexity to your code.

In reply to Re: Primes. Again. by TedPride
in thread Primes. Again. by Andrew_Levenson

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